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Exact solutions of the vorticity equation on the sphere as a manifold
Ismael Pérez-García
Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica s/n, Ciudad Universitaria, 04510 México, D.F.
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    "textoCompleto" => "<span class="elsevierStyleSections"><span id="sec0005" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">1</span><span class="elsevierStyleSectionTitle" id="sect0020">Introduction</span><p id="par0005" class="elsevierStylePara elsevierViewall">Let <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#61;&#123;<span class="elsevierStyleItalic">x</span> &#8712; <span class="elsevierStyleItalic">R</span><span class="elsevierStyleSup">3</span>&#58; &#124; <span class="elsevierStyleItalic">x</span> &#124;&#61; 1&#125; denote the unit sphere it <span class="elsevierStyleItalic">R</span><span class="elsevierStyleSup">3</span>&#46; The large-scale dynamics of tire atmosphere on the rotating sphere <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> can approximately be governed by the non-linear barotropic vorticity equation &#40;BVE&#41;&#44; which can be written in the non-di-mensional from as&#58;<elsevierMultimedia ident="eq0005"></elsevierMultimedia>where &#936;&#40;<span class="elsevierStyleItalic">&#955;&#44;&#956;</span>&#41; denotes the stream function&#44; &#956;&#61;sin&#966;&#61;cos&#952;&#44;&#8722;&#960;&#8804;&#955;&#8804;&#960;&#44;&#8722;&#960;2&#8804;&#966;&#8804;&#960;2&#44;0&#60;&#952;&#60;&#960;&#44;&#955; the longitude&#44; <span class="elsevierStyleItalic">&#966;</span> the latitude&#44; and <span class="elsevierStyleItalic">&#952;</span> the colatitude&#46; &#916; is the Laplace-Beltrami operator on a sphere and <span class="elsevierStyleItalic">J</span>&#40;&#936;&#44; <span class="elsevierStyleItalic">h</span>&#41; is the Jacobian&#46;</p><p id="par0010" class="elsevierStylePara elsevierViewall">The following is a solution for Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0005">&#40;1&#41;</a> on the sphere proposed by <a class="elsevierStyleCrossRef" href="#bib0180">Thompson &#40;1982&#41;</a>&#58;<elsevierMultimedia ident="eq0010"></elsevierMultimedia></p><p id="par0015" class="elsevierStylePara elsevierViewall">where &#40;<span class="elsevierStyleItalic">&#955;</span>&#8217;&#44; <span class="elsevierStyleItalic">&#956;</span>&#8217;&#41; are the spherical coordinates relative to a rotated pole <span class="elsevierStyleItalic">N&#8217;</span> with coordinates &#40;&#955;<span class="elsevierStyleInf">0</span>&#44; &#956;<span class="elsevierStyleInf">0</span>&#41; with respect to the original system&#44; and <span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span> is an eigenfunction of the operator Laplace-Beltrami with eigenvalue <span class="elsevierStyleItalic">&#967;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span><a class="elsevierStyleCrossRef" href="#bib0190">Verkley&#40;1984&#41;</a> generalized Thompson&#39;s solution and demonstrated that <span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span> could be a set of eigenfunctions that contains more than only spherical harmonics&#46; Then Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0010">&#40;2&#41;</a> describes a configuration in which the structure <span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span> moves through the zonal flow <span class="elsevierStyleItalic">-&#969;&#956;</span> with constant velocity <span class="elsevierStyleItalic">c</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span> and without changing size and shape&#46; The pole of the primed system <span class="elsevierStyleItalic">N&#8217;</span> that moves along a latitude at a constant angular velocity <span class="elsevierStyleItalic">c</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span> is given by<elsevierMultimedia ident="eq0015"></elsevierMultimedia>where <span class="elsevierStyleItalic">&#967;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span> is an eigenvalue for the spectral problem &#916;<span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span><span class="elsevierStyleItalic">&#61; &#8722;</span>&#967;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span><span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span>&#46; In particular&#44; for spherical harmonics <span class="elsevierStyleItalic">Y</span> &#40;<span class="elsevierStyleItalic">&#955;&#8217;&#44; &#956;&#8217;</span>&#41; of degree <span class="elsevierStyleItalic">n</span> corresponding to the eigenvalue <span class="elsevierStyleItalic">&#967;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span> &#61; <span class="elsevierStyleItalic">&#967;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">n</span></span><span class="elsevierStyleItalic">&#61; n</span>&#40;<span class="elsevierStyleItalic">n &#43;</span> 1&#41;&#44; Eq&#46;<a class="elsevierStyleCrossRef" href="#eq0010">&#40;2&#41;</a> is a Rossby-Haurwitz &#40;RH&#41; wave&#46; RH waves have proven to be very useful to describe the large-scale wave structure of atmospheric circulation in middle latitudes &#40;<a class="elsevierStyleCrossRef" href="#bib0125">Rossby&#44; 1939</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0050">Haurwitz&#44; 1940</a>&#41;&#46; The solution modon is constructed to divide the sphere <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> into two regions &#40;<a class="elsevierStyleCrossRef" href="#bib0185">Tribbia&#44; 1984</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0190">Verkley&#44; 1984</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0195">1987</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0200">1990</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0095">Neven&#44; 1992</a>&#41;&#58; an inner region <span class="elsevierStyleItalic">S</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">i</span></span> centered around the pole <span class="elsevierStyleItalic">N</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">&#8217;</span></span><span class="elsevierStyleItalic">&#44;</span> and an outer region <span class="elsevierStyleItalic">S</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">o</span></span> separated from the inner region by a boundary circle in which &#936;&#44; <span class="elsevierStyleItalic">q</span> and its normal derivative &#936;<span class="elsevierStyleItalic">&#8217;</span> are continuous&#46; Modons are considered appropriate to describe some types of atmospheric blocking events &#40;<a class="elsevierStyleCrossRef" href="#bib0200">Verkley&#44; 1990</a>&#41;&#46;</p><p id="par0020" class="elsevierStylePara elsevierViewall">Hydrodynamic equations on manifolds were studied by <a class="elsevierStyleCrossRef" href="#bib0045">Ebin and Marsden &#40;1970&#41;</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0165">Szeptycki &#40;1973a</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0170">b</a>&#41;&#44; <a class="elsevierStyleCrossRef" href="#bib0020">Avez and Bamberger &#40;1978&#41;</a>&#44; Ghidaglia <span class="elsevierStyleItalic">et al&#46;</span> &#40;1988&#41;&#44; Temam &#40;1987&#41; and Ilyin &#40;1993&#41;&#46; The existence&#44; unicity and regularity of the solution for the evolution equation &#40;Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0005">&#40;1&#41;</a>&#41; on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> were proven by <a class="elsevierStyleCrossRef" href="#bib0165">Szeptycki &#40;1973a</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0170">b</a>&#41;&#44; <a class="elsevierStyleCrossRef" href="#bib0020">Avez and Bamberger &#40;1978&#41;</a>&#44; Ilyin &#40;1993&#41; and <a class="elsevierStyleCrossRef" href="#bib0155">Skiba &#40;2012&#41;</a>&#46; <a class="elsevierStyleCrossRef" href="#bib0045">Ebin and Marsden &#40;1970&#41;</a> dealt with the motion of an incompressible fluid on manifolds under a differential geometric point of view&#46; Problems from the transition map between the charts are transferred to those of finding geodesics on the group of all volume-preserving diffeomorphisms&#44; to which the methods of global analysis and infinite-dimensional geometry can be applied&#46;</p><p id="par0025" class="elsevierStylePara elsevierViewall">In this paper we study the manifolds <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> in terms of the stream function &#936; for an RH wave which is sufficiently smooth and for Wu-Verkley waves and modons which are weakly differentiable of higher orders&#46; Section 2 deals with the compact differentiable manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and the way in which functions are constructed on this manifold&#46; Section 3 shows the types of solutions that will be considered&#46; Another aim of this paper is to deepen the understanding of the BVE solution on the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and its usage for deriving the properties of solutions to the manifold &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41;&#46; The paper concludes with a summary in section 4&#46;</p></span><span id="sec0010" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">2</span><span class="elsevierStyleSectionTitle" id="sect0025">Structure of functions on the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span></span><p id="par0030" class="elsevierStylePara elsevierViewall">In this section we review some basic facts concerning to the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#46; We should recall that the unit sphere <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is a compact and connected differentiable manifold&#46; Indeed&#44; because <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is compact it is not possible to cover it with only one chart&#46; A chart of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is then a pair &#40;&#937;&#44;<span class="elsevierStyleItalic">&#966;</span>&#41; where &#937; is an open subset of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; and &#966; is a homeomorphism of &#937; onto some open subset <span class="elsevierStyleItalic">of R</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span><span class="elsevierStyleItalic">&#46;</span> Let us consider the two charts &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44;<span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#44; &#40;&#937;<span class="elsevierStyleInf">&#954;</span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf">&#954;</span>&#41;&#125; of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">p</span></span> for <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> where every chart corresponds to a geographical coordinate group&#46; It is possible to define a coordinate chart that covers most of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> by using the standard spherical coordinate map&#46; Let <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span> denote the coordinate function&#44; which maps from &#40;<span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf">1</span>&#44; <span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf">2</span>&#44; <span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf">3</span>&#41; to angles &#40;<span class="elsevierStyleItalic">&#955;&#44; &#952;</span>&#41; or to &#40;<span class="elsevierStyleItalic">&#955;&#44; &#956;</span>&#41;&#46; The domain of &#966;&#953;&#8722;1 is the open set defined by <span class="elsevierStyleItalic">&#955;</span> &#8712; &#40;&#8722;&#960;&#44; &#960;&#41; and <span class="elsevierStyleItalic">&#952;</span> &#8712; &#40;0&#44; &#960;&#41; &#40;this excludes the poles&#41;&#46; The inverse map &#966;&#953;&#8722;1 yields the parameterization <span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">1</span></span> &#61; cos <span class="elsevierStyleItalic">&#955;</span> sin <span class="elsevierStyleItalic">&#952;</span>&#44; <span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">2</span></span> &#61; sin <span class="elsevierStyleItalic">&#955;</span> sin <span class="elsevierStyleItalic">&#952;</span>&#44; <span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">3</span></span> &#61; cos <span class="elsevierStyleItalic">&#952;</span> and its variation &#966;&#954;&#8722;1 yields the parameterization &#966;&#954;&#8722;1 &#40;<span class="elsevierStyleItalic">&#955;&#8217;&#44; &#952;</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">&#8217;</span></span>&#41;&#61;&#40;cos <span class="elsevierStyleItalic">&#955;&#8217;</span> sin <span class="elsevierStyleItalic">&#952;&#8217;&#44;</span> cos <span class="elsevierStyleItalic">&#952;&#8217;&#44; sin &#955;&#8217; sin &#952;&#8217;</span>&#41;<span class="elsevierStyleItalic">&#46;</span> The domain of &#966;&#954;&#8722;1 in the open set defined by <span class="elsevierStyleItalic">&#955;&#8217;</span> &#8712; &#40;&#8211;&#960;&#44; &#960;&#41; and <span class="elsevierStyleItalic">&#952;&#8217;</span> &#8712; &#40;0&#44; &#960;&#41;&#46; The charts &#937;&#953;&#44;&#966;&#953; and &#937;&#954;&#44;&#966;&#954; correspond to poles N and N<span class="elsevierStyleItalic">&#8217;</span> on the sphere <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#46; N<span class="elsevierStyleItalic">&#8217;</span> might be taken as the point &#40;&#955;0&#61;&#8722;&#960;2&#966;0&#61;0&#41; in the old system and as the angle <span class="elsevierStyleItalic">&#955;&#8217;</span> in this new north pole&#44; so that the new international date line is the half circle &#915;&#954;&#61;&#123;p&#8712;S2&#58;&#8722;&#960;2&#60;&#955;&#40;p&#41;&#60;&#960;2&#44;&#952;&#61;&#960;2&#44;&#125; of the old equator in the <span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf">1</span><span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf">2</span>- plane&#44; on the front where <span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf">1</span> &#8805; 0 &#40;<a class="elsevierStyleCrossRef" href="#bib0120">Richtmyer&#44; 1981</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0135">Skiba&#44; 1989</a>&#59; <a class="elsevierStyleCrossRef" href="#bib1115">P&#233;rez-Garc&#237;a&#44; 2001</a>&#41;&#46; The international date line&#44; for the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; is the half circle &#915;&#953;&#61;&#123;p&#8712;S2&#58;&#8722;&#960;2&#60;&#966;&#40;p&#41;&#60;&#960;2&#44;&#955;&#61;&#177;&#8201;&#960;&#125; in the <span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf">1</span><span class="elsevierStyleItalic">x</span><span class="elsevierStyleInf">3</span> &#8211;plane&#46; The chart covers &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; covers the sphere except for the set &#915;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; and the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41; similarly covers the sphere with the exception of a set &#915;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#46; Hence the two charts &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#44;&#40;&#937;<span class="elsevierStyleInf">&#954;</span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41;&#125; together cover <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and they constitute an atlas&#46;</p><p id="par0035" class="elsevierStylePara elsevierViewall">The local coordinates associated with the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; are functions &#966;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#58; &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44;&#8594;<span class="elsevierStyleItalic">R</span><span class="elsevierStyleSup">2</span>&#44; such that for p&#8712;S2&#44;&#8201;&#966;&#953;&#40;p&#41;&#61;&#40;&#966;&#953;&#44;1&#40;p&#41;&#44;&#966;&#953;&#44;2&#40;p&#41;&#41;&#61;&#40;x&#953;1&#40;p&#41;&#44;&#8201;&#8201;x&#953;2&#40;p&#41;&#41;&#61;&#40;&#955;&#40;p&#41;&#44;&#956;&#40;p&#41;&#41;and &#966;&#954;&#40;p&#41;&#61;&#40;x&#954;1&#40;p&#41;&#44;x&#954;2&#40;p&#41;&#41;&#61;&#40;&#955;&#39;&#40;p&#41;&#59;&#956;&#39;&#40;p&#41;&#41; are local coordinates with respect to the chart &#40;&#937;<span class="elsevierStyleInf">&#954;</span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41; &#40;<a class="elsevierStyleCrossRef" href="#fig0005">Fig&#46; 1</a>&#41;&#46;</p><elsevierMultimedia ident="fig0005"></elsevierMultimedia><p id="par0040" class="elsevierStylePara elsevierViewall">To construct the map &#966;&#8467;&#58;S2&#8594;U&#8834;R&#8467;2&#44;&#8467;&#61;&#953;&#44;&#8201;&#954; a bijection with inverse &#966;&#953;&#8722;1&#58;U&#953;&#8594;S2 defined as &#966;&#953;&#8722;1&#40;x&#953;1&#44;x&#953;2&#41;&#61;&#40;1&#8722;&#40;x&#953;2&#41;2cos&#8201;x&#953;1&#44;&#8201;1&#8722;&#40;x&#953;2&#41;2&#8201;sinx&#953;1&#44;x&#953;2&#41;&#44; and the &#966;&#954;&#8722;1&#40;x&#954;1&#44;x&#954;2&#41;&#61;&#40;1&#8722;&#40;x&#954;2&#41;2cos&#8201;x&#954;1&#44;x&#954;2&#44;1&#8722;&#40;x&#954;2&#41;2sinx&#954;1&#41;&#44; it is seen that every U<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#8467;</span></span> is open&#46; Hence each &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#8467;</span></span> is an open subset of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and &#937;&#953;&#8746;&#937;&#954; cover <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#46;</p><p id="par0045" class="elsevierStylePara elsevierViewall">Given two charts of the atlas &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#44; &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41;&#125; with &#937;&#954;&#8201;&#8745;&#8201;&#937;&#953;&#8800;&#248;&#44; the transition maps &#966;&#953;&#954;&#61;&#966;&#954;&#8728;&#966;&#953;&#8722;1&#58;&#8201;U&#953;&#954;&#8594;U&#954;&#953; outline open sets of <span class="elsevierStyleItalic">R</span><span class="elsevierStyleSup">2</span> into <span class="elsevierStyleItalic">R</span><span class="elsevierStyleSup">2</span><span class="elsevierStyleItalic">&#44;</span> where U&#953;&#954;&#61;&#966;&#953;&#937;&#953;&#8201;&#8745;&#8201;&#937;&#954; and U&#954;&#953;&#61;&#966;&#954;&#937;&#954;&#8201;&#8745;&#8201;&#937;&#953;&#46; This determines a differentiable structure for <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; and &#966;&#953;&#954;&#61;&#966;&#954;  &#8728;  &#966;&#953;&#8722;1&#58; is a diffeomorphism&#46; It is then said that the atlas is of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">k</span></span> if the transition functions are of <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">k</span></span><span class="elsevierStyleItalic">&#46;</span></p><p id="par0050" class="elsevierStylePara elsevierViewall">Let <span class="elsevierStyleItalic">x</span> be any point of <span class="elsevierStyleItalic">U</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;&#954;</span></span>&#44; and &#40;&#966;&#953;&#954;1&#40;x&#41;&#44;&#966;&#953;&#954;2&#40;x&#41;&#41; the coordinate of <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;&#954;</span></span> &#40;<span class="elsevierStyleItalic">x</span>&#41;&#59; then &#966;&#953;&#954;i&#40;x&#41; is a continuous function on two variables&#46; Now&#44; if <span class="elsevierStyleItalic">p</span> &#8712; &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span> &#8745; &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span> such that <span class="elsevierStyleItalic">x</span> &#61; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span> &#40;<span class="elsevierStyleItalic">p</span>&#41;&#44; and since <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#40;<span class="elsevierStyleItalic">p</span>&#41; &#8712; <span class="elsevierStyleItalic">U</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;&#954;</span></span>&#44; we have the relations<elsevierMultimedia ident="eq0020"></elsevierMultimedia><elsevierMultimedia ident="eq0025"></elsevierMultimedia></p><p id="par0055" class="elsevierStylePara elsevierViewall">This is the transformation formula betwen the two local coordinate systems &#40;x&#953;1&#44;x&#953;2&#41; and &#40;x&#954;1&#44;x&#954;2&#41; defined on &#937;&#954;&#8201;&#8745;&#8201;&#937;&#953;&#46; To obtain the relations between the unprimed and primed coordinate of any point <span class="elsevierStyleItalic">Q</span> on the sphere&#44; <a class="elsevierStyleCrossRef" href="#bib0190">Verkley &#40;1984&#41;</a> examined the spherical triangle <span class="elsevierStyleItalic">NQN&#8217;</span> and the application of the cosine rules to this triangle&#44; deriving explicit expressions for the transformation between the two coordinate systems as given by &#40;4&#41; and &#40;5&#41;&#46;</p><p id="par0060" class="elsevierStylePara elsevierViewall">Let &#937;&#953;&#8201;&#8745;&#8201;&#937;&#954;&#8800;&#248; and set <span class="elsevierStyleItalic">J &#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;&#954;</span></span> as the Jacobian matrix of map <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;&#954;</span></span>&#44; so we can verify that<elsevierMultimedia ident="eq0030"></elsevierMultimedia></p><p id="par0065" class="elsevierStylePara elsevierViewall">Then det <span class="elsevierStyleItalic">J &#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;&#954;</span></span> &#62; 0&#46; Hence&#44; it is said that if manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is oriented for every pair &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span> of intersecting local coordinate neighbourhoods&#44; det <span class="elsevierStyleItalic">J &#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;&#954;</span></span> &#62; 0&#46;</p><p id="par0070" class="elsevierStylePara elsevierViewall">Indeed we can regard the coordinate as a device to decide which of many functions <span class="elsevierStyleItalic">&#968;</span> on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> are to be differentiable&#46; Since &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span> is just a set&#44; it makes no sense to ask that <span class="elsevierStyleItalic">&#968;</span>&#58; &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#8594; <span class="elsevierStyleItalic">R</span> be differentiable &#40;<a class="elsevierStyleCrossRef" href="#bib0085">Matsushima&#44; 1972</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0080">Loomis and Sterberg&#44; 1990</a>&#41;&#46; However&#44; we can consider the map &#936;&#953;&#61;&#968;&#8201;&#8728;&#8201;&#966;&#953;&#8722;1&#58;&#966;&#953;&#8201;&#40;&#937;&#953;&#41;&#8594;R Then &#968;&#8201;&#8728;&#8201;&#966;&#953;&#8722;1 is a function defined on an open &#966;&#953;&#40;&#937;&#953;&#41;&#8834;R2&#44; and we know what it means for such a function to be differentiable or smooth &#40;see <a class="elsevierStyleCrossRef" href="#fig0005">Fig&#46; 1</a>&#41;&#46; Consider now what happens when we change coordinates to some other chart&#44; lets say &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41; for convenience&#44; assuming that &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span> &#61; &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; Then it is possible for &#968;&#8201;&#8728;&#8201;&#966;&#953;&#8722;1 to be differentiable but &#968;&#8201;&#8728;&#8201;&#966;&#954;&#8722;1 is not&#46; To compare both&#44; let &#968;&#8201;&#8728;&#8201;&#966;&#953;&#8722;1&#61;&#968;&#8201;&#8728;&#966;&#954;&#8722;1&#8201;&#8728;&#8201;&#40;&#966;&#954;&#8201;&#8728;&#8201;&#966;&#953;&#8722;1&#41; where the map &#966;&#954;&#8728;&#966;&#953;&#8722;1&#58;&#966;&#953;&#40;&#937;&#953;&#41;&#8594;&#966;&#954;&#40;&#937;&#954;&#41; is a bijection between open subsets of <span class="elsevierStyleItalic">R</span><span class="elsevierStyleSup">2</span>&#46; Then a sufficient condition for &#968;&#8201;&#8728;&#8201;&#966;&#953;&#8722;1 to be differentiable if &#966;&#8201;&#8728;&#8201;&#966;&#953;&#8722;1 is&#44; is that &#8217;&#966;&#954;&#8201;&#8728;&#8201;&#966;&#953;&#8722;1 is also differentiable&#46; We often write &#936; for the composite function &#968;&#8201;&#8728;&#8201;&#966;&#953;&#8722;1</p><p id="par0075" class="elsevierStylePara elsevierViewall">Lets take a curve &#964; &#58; &#40;&#8722;1&#44; 1&#41; &#8594; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> with &#964; &#40;0&#41; &#61; <span class="elsevierStyleItalic">p&#46;</span> In a local chart &#964; is given by x&#953;i&#61;&#964;i&#40;t&#41;&#46; On the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; one can define a vector <span class="elsevierStyleBold">U</span> tangent to the parametrized curve &#964; at any point <span class="elsevierStyleItalic">p</span> on the curve&#46; The tangent vector <span class="elsevierStyleBold">U</span> is given by a column vector <span class="elsevierStyleBold">u</span> whose components u&#953;i are d&#964;idt0&#44; &#40;<span class="elsevierStyleItalic">i</span> &#61; 1&#44; 2&#41;&#44; with the initial condition &#964; &#40;0&#41; &#61; <span class="elsevierStyleItalic">p</span>&#46; If we use another coordinate system corresponding to the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41; by x&#954;i then the tangent vector <span class="elsevierStyleBold">U</span> is given by a column vector <span class="elsevierStyleBold">v</span> with components &#965;&#954;i<span class="elsevierStyleItalic">&#46;</span> According to the chain rule&#44; the column vectors <span class="elsevierStyleBold">u</span> and <span class="elsevierStyleBold">v</span> are related by &#965;&#954;i&#61;u&#953;j&#8706;x&#954;i&#8706;x&#953;j&#46; The expression u&#953;i&#8706;&#8706;x&#953;j is the partial differential operator in the direction of the tangent vector&#46;</p><p id="par0080" class="elsevierStylePara elsevierViewall">The space <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span><span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is called the <span class="elsevierStyleItalic">tangent space of S</span><span class="elsevierStyleSup">2</span><span class="elsevierStyleItalic">at p</span>&#44; and <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span><span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is a two-dimensional vector space&#46; For each <span class="elsevierStyleBold">u</span> &#8712; T<span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span><span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> we shall write u&#61;u&#953;i&#8706;&#8706;x&#953;j&#61;u1&#8706;&#8706;&#955;&#43;u2&#8706;&#8706;&#952;&#44; where u&#953;i are the contravariant components of <span class="elsevierStyleBold">u</span>&#46; It is well known that on the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> an inner product is defined at each tangent space <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span><span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span><span class="elsevierStyleItalic">&#46;</span> Now lets present a basis in which we denote the coordinate system corresponding to the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; by &#123;x&#953;i&#125;&#61;&#40;&#955;&#44;&#956;&#41;&#44; and for any <span class="elsevierStyleItalic">&#968;</span>&#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">R</span> define the vectors &#40;&#8706;&#8706;x&#953;j&#41;p by &#40;&#8706;&#8706;x&#953;j&#41;p&#968;&#61;&#40;&#8706;&#968;&#8728;&#966;&#953;&#8722;1&#8706;x&#953;i&#41;&#966;&#953;&#40;p&#41;&#44; so that they are independent since &#40;&#8706;&#8706;x&#953;j&#41;px&#953;j&#61;&#948;ij Let &#40;n&#710;&#61;&#40;1&#8722;&#40;x&#953;2&#41;2cosx&#953;1&#44;1&#8722;&#40;x&#953;2&#41;2sinx&#953;1&#44;x&#953;2&#41; be the outward normal to <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> in <span class="elsevierStyleItalic">R</span><span class="elsevierStyleSup">3</span>&#59; without any loss of generality we may assume that the vectors e&#955;&#61;&#8706;n&#710;&#953;&#8706;x&#953;1&#44;&#8201;&#8201;&#8201;e&#956;&#61;&#8706;n&#710;&#953;&#8706;x&#953;2 form a basis for T<span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span><span class="elsevierStyleItalic"><span class="elsevierStyleSmallCaps">S</span></span><span class="elsevierStyleSup"><span class="elsevierStyleSmallCaps">2</span></span><span class="elsevierStyleSmallCaps">&#46;</span></p><p id="par0085" class="elsevierStylePara elsevierViewall">We will denote the vector space of a vector field on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> by &#915;&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; A tangent vector field on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is a smooth map <span class="elsevierStyleBold">u</span>&#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">T S</span><span class="elsevierStyleSup">2</span> such that&#44; for any <span class="elsevierStyleItalic">x</span> &#8712; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleBold">u</span>&#40;<span class="elsevierStyleItalic">x</span>&#41; &#8712; <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">x</span></span><span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#46; At the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#44; for <span class="elsevierStyleItalic">x</span> &#8712; &#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; the vector functions <span class="elsevierStyleBold">u</span> &#8712; <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">x</span></span><span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span> and <span class="elsevierStyleBold">v</span> &#8712; &#915;&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span>&#41; have components u&#61;up1e&#955;&#43;up2e&#956; and v&#61;&#965;p1e&#955;&#43;&#965;p2e&#956;&#44; respectively&#44; being these up upi&#61;upx&#953;i&#61;u&#955;&#44;u&#956; the components of <span class="elsevierStyleBold">u</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span> as the vectors of the unitary base indicated by <span class="elsevierStyleItalic">e</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#955;</span></span>&#44; and <span class="elsevierStyleItalic">e</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#956;</span></span> in the directions <span class="elsevierStyleItalic">&#955;</span> and <span class="elsevierStyleItalic">&#956;</span>&#44; respectively&#46;</p><p id="par0090" class="elsevierStylePara elsevierViewall">Let us recall that an oriented Riemannian manifold is a pair &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41; where <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is the oriented compact manifold and <span class="elsevierStyleItalic">g</span> a Riemannian metric on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; which assigns a length vgp&#8712;R&#43;&#46; The <span class="elsevierStyleItalic">g</span> on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is a smooth &#40;2&#44; 0&#41;-tensor field on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> such that for any <span class="elsevierStyleItalic">p</span> &#8712; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span>&#58; <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; &#215; <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; &#8594; <span class="elsevierStyleItalic">R</span> is a scalar product on the tangent space <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41;&#44; and in any chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; its components<elsevierMultimedia ident="eq0035"></elsevierMultimedia>form a symmetric matrix&#44; with its inverse denoted by &#40;<span class="elsevierStyleItalic">g</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">ij</span></span>&#41; &#61; &#40;<span class="elsevierStyleItalic">g</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">ij</span></span>&#41;<span class="elsevierStyleSup">-1</span>&#44; and <span class="elsevierStyleItalic">g</span> &#61; <span class="elsevierStyleItalic">det</span>&#40;<span class="elsevierStyleItalic">g</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">ij</span></span>&#41; &#61; 1 &#8211; <span class="elsevierStyleItalic">&#956;</span><span class="elsevierStyleSup">2</span>&#46; The length of a tangent vector <span class="elsevierStyleBold">v</span> &#8712; <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span><span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is defined as usual&#44; v&#61;gpv&#44;v12&#61;v&#183;v12 Moreover&#44; the inner product on <span class="elsevierStyleItalic">T S</span><span class="elsevierStyleSup">2</span> is given by <span class="elsevierStyleBold">u</span> &#46; <span class="elsevierStyleBold">v</span> &#61; <span class="elsevierStyleItalic">g</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">ij</span></span><span class="elsevierStyleItalic">u</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">i</span></span><span class="elsevierStyleItalic">&#957;</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">j</span></span> for <span class="elsevierStyleBold">u</span>&#44; <span class="elsevierStyleBold">v</span> &#8712; <span class="elsevierStyleItalic">TS</span><span class="elsevierStyleSup">2</span>&#46;</p><p id="par0095" class="elsevierStylePara elsevierViewall">Let &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41; be the smooth Riemannian manifolds of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#46; Let us now recall some operators arising in partial differential equations on the sphere as manifold&#46; Given the scalar function <span class="elsevierStyleItalic">&#968;</span> &#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">R</span>&#44; the <span class="elsevierStyleItalic">gradient of &#968;</span>&#44; is given by the vector field <span class="elsevierStyleItalic">grad &#968;</span>&#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span><span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span>&#44; for which<elsevierMultimedia ident="eq0040"></elsevierMultimedia>where e&#710;&#955;&#8201;&#61;11&#8722;&#956;2e&#955; and e&#710;&#956;&#61;1&#8722;&#956;2  e&#956;&#46;</p><p id="par0100" class="elsevierStylePara elsevierViewall">If <span class="elsevierStyleBold">u</span> &#8712; &#1043;&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41;&#44; the divergence of <span class="elsevierStyleBold">u</span> is the function on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> which on the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; is given by<elsevierMultimedia ident="eq0045"></elsevierMultimedia></p><p id="par0105" class="elsevierStylePara elsevierViewall">A linear connection <span class="elsevierStyleItalic">D</span> on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is a map D&#58; <span class="elsevierStyleItalic">T</span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; &#215; &#1043;&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; &#8594; <span class="elsevierStyleItalic">T</span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; called the <span class="elsevierStyleItalic">covariant derivative</span> and the usual notation for <span class="elsevierStyleItalic">D</span>&#40;<span class="elsevierStyleItalic">U&#44; V</span>&#41; is <span class="elsevierStyleItalic">D</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">U</span></span><span class="elsevierStyleItalic">V&#46;</span> Let &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; be a chart and as one can observe&#44; the vectors &#40;e&#955;&#61;&#8706;&#8706;&#955;&#44;e&#956;&#61;&#8706;&#8706;&#956;&#41; can be nonconstant&#46; An easy notation is set &#8711;i&#61;D&#8706;&#8706;xi &#40;eg&#46; <a class="elsevierStyleCrossRef" href="#bib0055">Hebey&#44; 2000</a>&#41;&#46; There are smooth functions &#915;ijk&#58;&#937;&#953;&#8594;R such that for any <span class="elsevierStyleItalic">i</span>&#44; <span class="elsevierStyleItalic">j&#44;</span> and any <span class="elsevierStyleItalic">p</span> &#8712;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44;<elsevierMultimedia ident="eq0050"></elsevierMultimedia></p><p id="par0110" class="elsevierStylePara elsevierViewall">where &#915;ijk are the Christoffel symbols&#44; defined by &#915;ijk&#61;12&#8721;l&#61;12gkl&#40;&#8706;glj&#8706;x&#953;i&#43;&#8706;gil&#8706;x&#953;j&#43;&#8706;gij&#8706;x&#953;l&#41;&#46;On the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#44; we have &#915;111&#61;0&#44;&#915;112&#61;&#8722;2&#956;1&#8722;&#956;2&#44;&#915;121&#61;cot&#8201;&#952;&#61;&#956;1&#8722;&#956;2&#44;&#915;122&#61;0&#44;&#915;221&#61;0 and &#915;222&#61;0</p><p id="par0115" class="elsevierStylePara elsevierViewall">The fundamental operator which we study is the Laplacian &#916;&#44; then for real or complex valued functions&#44; &#916; is the Laplace-Beltrami operator on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and it is given by<elsevierMultimedia ident="eq0055"></elsevierMultimedia></p><p id="par0120" class="elsevierStylePara elsevierViewall">This operator satisfies some properties&#58; &#916;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span>&#44; is selfadjoint&#44; symmetric and non-negative &#40;Aubin&#44; 1998&#41;&#46; Thus&#44; the operators <span class="elsevierStyleItalic">div&#44; grad</span> and &#916;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span> on the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> have the conventional meaning&#46;</p><p id="par0125" class="elsevierStylePara elsevierViewall">Let &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41; be a compact <span class="elsevierStyleItalic">oriented Riemannian</span> manifold&#44; with metric <span class="elsevierStyleItalic">g</span>&#46; The metric and the orientation are combined to give a volume element <span class="elsevierStyleItalic">d</span>&#965;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span> on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; which can be used to integrate functions on &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41;&#46; In order to apply the integral calculus on the oriented manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; we define a volume element to be a two-form &#969; &#61; <span class="elsevierStyleItalic">d</span>&#965; which is defined on all of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#46; For every chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#8467;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#8467;</span></span>&#41; which is consistently oriented with <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; the coordinate expresion for &#969;&#61;d&#965;l is &#934;ldxl1&#923;dxl2 where &#934;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#8467;</span></span> is a partition of unity subordinate to the covering &#937;l&#44;&#8201;l&#61;&#953;&#44;&#954;</p><p id="par0130" class="elsevierStylePara elsevierViewall">On the Riemannian manifold &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41;&#44; at the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#59; a volume form <span class="elsevierStyleItalic">&#951;</span> &#61; <span class="elsevierStyleItalic">d</span>&#965;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span> defines a Lebesgue measure on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> by d&#965;g&#61;&#951;&#61;gdxl1&#8201;&#923;dxl2 Then<elsevierMultimedia ident="eq0060"></elsevierMultimedia>wheredxl&#61;dxl1dxl2 defines a Lebesgue measure on <span class="elsevierStyleItalic">R</span><span class="elsevierStyleSup">2</span>&#46;</p><p id="par0140" class="elsevierStylePara elsevierViewall">Let C<span class="elsevierStyleSup">&#8734;</span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; denote the set of infinitely differentiable functions of compact support <span class="elsevierStyleItalic">&#968;</span>&#40;<span class="elsevierStyleItalic">x</span>&#41;&#46; At <span class="elsevierStyleItalic">&#956;</span> &#61; &#177; 1 the functions are smooth&#44; together with the periodic boundary condition at <span class="elsevierStyleItalic">&#955;</span> with period 2&#960;&#46; If we define the usual Hilbert space <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup">2</span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; to be the completion of C<span class="elsevierStyleSup">&#8734;</span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; with respect to the inner product<elsevierMultimedia ident="eq0065"></elsevierMultimedia></p><p id="par0145" class="elsevierStylePara elsevierViewall">and norm f&#61;&#123;&#8747;s2f2d&#965;g&#125;12&#44;&#8201;&#8201;g&#42; is the complex conjugate of function <span class="elsevierStyleItalic">g&#46;</span> Let &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g&#41;</span> be the compact <span class="elsevierStyleItalic">Riemannian</span> manifold and <span class="elsevierStyleItalic">d&#965;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span> the Riemannian volume element&#46; Then functional spaces &#40;Sobolev and the Holder spaces&#41; can be defined on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> as well &#40;<a class="elsevierStyleCrossRef" href="#bib0155">Skiba&#44; 2012</a>&#41;&#46; For each <span class="elsevierStyleItalic">p</span> &#8712; <span class="elsevierStyleBold">R</span> with 1&#8804; <span class="elsevierStyleItalic">p</span> &#60; &#8734; we associate a Banach space<elsevierMultimedia ident="eq0070"></elsevierMultimedia></p><p id="par0150" class="elsevierStylePara elsevierViewall">with respect to the norm<elsevierMultimedia ident="eq0075"></elsevierMultimedia></p><p id="par0155" class="elsevierStylePara elsevierViewall">and <span class="elsevierStyleItalic">ess sup</span> &#124; f &#124; &#60; &#8734; if <span class="elsevierStyleItalic">p</span> &#61; &#8734;&#46; <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup">2</span>&#40;<span class="elsevierStyleItalic">TS</span><span class="elsevierStyleSup">2</span>&#41; represent the Hilbert space of the vector fields <span class="elsevierStyleItalic">U</span> &#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">TS</span><span class="elsevierStyleSup">2</span> endowed with the inner product in <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup">2</span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; induced by <span class="elsevierStyleItalic">g</span> in <span class="elsevierStyleItalic">T</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">p</span></span><span class="elsevierStyleItalic">&#40;S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span><span class="elsevierStyleItalic">&#41;</span> &#40;see <a class="elsevierStyleCrossRef" href="#bib0040">D&#237;az and Tello&#44; 1999</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0055">Hebey&#44; 2000</a>&#41;&#46;</p><p id="par0160" class="elsevierStylePara elsevierViewall">We now turn to the eigenvalue problems for &#916;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span>&#58; We usually seek to find all eigenvalues &#947; for which there is an eigenfunction <span class="elsevierStyleItalic">Y</span> such that &#916;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span><span class="elsevierStyleItalic">Y</span> &#61; &#8211;&#947;Y&#46; Then&#44; which information about geometry of &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41; is encoded by the eigenvalues&#63;&#46; The structure of eigenfunctions&#58; <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">p</span></span> norms and relations to RH waves or modons&#46;</p><p id="par0165" class="elsevierStylePara elsevierViewall">Global harmonic analysis is the study of the spectral theory of the Laplacian &#916;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span> on a compact Riemannian manifold &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41;&#44; and its relation to the global geometric structure&#46; Since &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41; is compact&#44; there exists an orthonormal basis &#123;<span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">j</span></span>&#125; of smooth eigenfunctions and the spectrum of &#916;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span> is a discrete set &#123;&#947;<span class="elsevierStyleInf">0</span> &#61; 0 &#60; &#947;<span class="elsevierStyleInf">1</span> &#8804; &#947;<span class="elsevierStyleInf">2</span> &#8804; &#947;<span class="elsevierStyleInf">3</span> &#8804; &#46;&#46;&#46;&#125;&#46; Recent developments show that the non-zero eigenvalues also contain substantial geometric and analytic information&#46; The solution modon constructed by <a class="elsevierStyleCrossRef" href="#bib0185">Tribbia &#40;1984&#41;</a>&#44; Verkley &#40;1984&#44; <a class="elsevierStyleCrossRef" href="#bib0195">1987</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0200">1990</a>&#41; and Neven &#40;1993&#41; proposed the use of eigenfunctions &#123;<span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">j</span></span>&#125; as basic geometric structures&#46;</p><p id="par0170" class="elsevierStylePara elsevierViewall">The space of spherical harmonics of degree <span class="elsevierStyleItalic">n</span> on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; which coincides with the eigenspace of operator &#8211; &#916;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">g</span></span> corresponding to the eigenvalue &#947;<span class="elsevierStyleInf">n</span> &#61;&#967;<span class="elsevierStyleInf">n</span>&#61;<span class="elsevierStyleItalic">n</span>&#40;<span class="elsevierStyleItalic">n</span>&#43;1&#41;&#44; is denoted by <span class="elsevierStyleBold">H</span><span class="elsevierStyleInf"><span class="elsevierStyleBold">n</span></span>&#46; Self-adjoint operators have the property that its eigenfunctions with different eigenvalues are orthogonal&#44; which implies that the eigenspaces <span class="elsevierStyleBold">H</span><span class="elsevierStyleInf"><span class="elsevierStyleBold">n</span></span> are orthogonal and have 2<span class="elsevierStyleItalic">n</span>&#43;1 dimensions&#46; On the sphere&#44; the homogeneous harmonic polynomials span the set of all polynomials&#44; which in turn are dense in <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup">2</span>&#46; Our spherical harmonics therefore span <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup">2</span>&#46; If we take a basis within each eigenspace then this collection will give a basis for <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup">2</span> of the sphere&#46; The harmonics spherical term was introduced by Kelvin on potentials studies &#40;<a class="elsevierStyleCrossRef" href="#bib0065">Hobson&#44; 1931</a>&#41; and is understood as the development of a function in terms of this series of spherical harmonics&#46;</p><p id="par0175" class="elsevierStylePara elsevierViewall">The spaces <span class="elsevierStyleBold">H</span><span class="elsevierStyleInf"><span class="elsevierStyleBold">n</span></span> and <span class="elsevierStyleBold">H</span><span class="elsevierStyleInf"><span class="elsevierStyleBold">k</span></span> &#40;<span class="elsevierStyleItalic">n</span> &#8800; <span class="elsevierStyleItalic">k</span>&#41; are mutually orthogonal in <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup">2</span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41;&#46; Then there is the orthogonal projection <span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">n</span></span> &#58; <span class="elsevierStyleItalic">L</span><span class="elsevierStyleInf">2</span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; &#8594; <span class="elsevierStyleBold">H</span><span class="elsevierStyleInf"><span class="elsevierStyleBold">n</span></span>&#44; and so smooth functions &#936; &#8712; <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup">2</span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#41; on the sphere <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span> have a development in spherical harmonics&#44;<elsevierMultimedia ident="eq0080"></elsevierMultimedia>where Yn&#955;&#44;&#956;&#61;&#8721;m&#61;&#8201;&#8722;nn&#936;nm&#8201;Ynm&#955;&#44;&#956; is the homogeneous spherical polynomial of degree <span class="elsevierStyleItalic">n</span> from <span class="elsevierStyleBold">H</span><span class="elsevierStyleInf"><span class="elsevierStyleBold">n</span></span>&#44; and &#936;nm&#61;&#8201;&#60;&#8201;&#936;&#44;Ynm&#62; is the Fourier coefficient of &#936;&#46; The 2<span class="elsevierStyleItalic">n</span> &#43; 1 spherical harmonics<elsevierMultimedia ident="eq0085"></elsevierMultimedia>of degree <span class="elsevierStyleItalic">n</span> and zonal number <span class="elsevierStyleItalic">m</span> &#40;&#8211;<span class="elsevierStyleItalic">n</span> &#8804; <span class="elsevierStyleItalic">m</span> &#8804; <span class="elsevierStyleItalic">n&#41;</span> form an orthonormal basis in <span class="elsevierStyleBold">H</span><span class="elsevierStyleInf"><span class="elsevierStyleBold">n</span></span>&#46; Here the numbers <span class="elsevierStyleItalic">C</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">nm</span></span> are the normalizers in <span class="elsevierStyleItalic">L</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span>&#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span>&#41;&#44; <span class="elsevierStyleItalic">given by</span>Cnm&#61;2n&#43;14&#960;n&#8722;m&#33;n&#43;m&#33;12 and Pnm are the associated Legendre functions given by<elsevierMultimedia ident="eq0090"></elsevierMultimedia></p><p id="par0180" class="elsevierStylePara elsevierViewall">Considering that an oriented compact Riemannian manifold is a pair &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41; where <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is the oriented compact manifold and <span class="elsevierStyleItalic">g</span> a Riemannian metric on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; we can define in it covariant derivatives and various notions of curvature&#46; When a manifold also has a group structure &#40;so that multiplication and inversion are smooth&#41;&#44; a very interesting structure called a Lie group &#40;Bihlo&#44; 2007&#59; <a class="elsevierStyleCrossRef" href="#bib0030">Bihlo and Popoych&#44; 2012</a>&#41; arises&#46; Even if a manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is not a Lie group&#44; there may be an action &#58; G &#215; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> of a Lie group G on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; and under certain conditions <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> can be viewed as a &#8220;quotient&#8221; G&#47;K&#44; where K is a subgroup of G &#40;<a class="elsevierStyleCrossRef" href="#bib0120">Richtmyer&#44; 1981</a>&#41;&#46; When <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8773; G&#47;K as above&#44; certain notions on G can be transported to <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; then we say that <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is a homogeneous space&#46; As an example of the last point we could mention the theory of spherical harmonic expansion on the <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; which is a homogeneous space for the rotation group O&#40;n&#43;1&#41;&#46; The surface spherical harmonics are eigenfunctions for the Laplace-Beltrami operator&#44; which is a rotation invariant &#40;<a class="elsevierStyleCrossRef" href="#bib0060">Helgason&#44; 1984</a>&#41;&#46; Harmonic analysis is concerned with the representation of functions as the superposition of basic waves&#44; the study and generalization of the notions of Fourier series as well as the Fourier transforms&#46;</p><p id="par0185" class="elsevierStylePara elsevierViewall">Elements of harmonic analysis on the sphere can be found at <a class="elsevierStyleCrossRef" href="#bib0160">Stein and Weiss &#40;1971&#41;</a>&#46; After introducing the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and the Riemannian manifolds &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41;&#44; a general type of spaces &#40;Besov and Triebel-Lizorkin spaces&#41; on the sphere may also be introduced &#40;<a class="elsevierStyleCrossRef" href="#bib0090">Narcowich <span class="elsevierStyleItalic">et al&#46;&#44;</span> 2006</a>&#41;&#46; Using the power of a Laplace operator&#44; the Sobolev space on Riemannian manifolds can also be incorporated as a field currently undergoing great development &#40;Aubin&#44; 1998&#59; <a class="elsevierStyleCrossRef" href="#bib0055">Hebey&#44; 2000</a>&#41;&#46;</p></span><span id="sec0015" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3</span><span class="elsevierStyleSectionTitle" id="sect0030">Exact solutions to the barotropic vorticity equation on the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span></span><p id="par0190" class="elsevierStylePara elsevierViewall">Let &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#125;<span class="elsevierStyleItalic"><span class="elsevierStyleSmallCaps">&#44;&#8467;&#61;</span>&#953;</span>&#44; <span class="elsevierStyleItalic">&#954;</span> be an atlas of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and <span class="elsevierStyleItalic">&#968;</span>&#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">R</span> the streamfunction of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span><span class="elsevierStyleItalic">&#46;</span> We can consider that the map &#936;&#61;&#968;&#8728;&#966;&#953;&#8722;1&#58;&#966;&#953;&#937;&#953;&#8594;R and &#968;&#8728;&#966;&#953;&#8722;1 is the streamfunction defined on an open <span class="elsevierStyleItalic">U</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span> &#61; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span> &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; &#8834; <span class="elsevierStyleItalic">R</span><span class="elsevierStyleSup">2</span> and that it is of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span><span class="elsevierStyleItalic">&#46;</span></p><p id="par0195" class="elsevierStylePara elsevierViewall">To simulate the time evolution of a two-dimensional nondivergent and inviscid flow for a rotating sphere&#44; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> is governed by a non-linear barotropic vorticity equation&#44; which can be written in the non-dimensional form as<elsevierMultimedia ident="eq0095"></elsevierMultimedia></p><p id="par0200" class="elsevierStylePara elsevierViewall">where Jc&#44;q&#61;&#8706;c&#8706;&#955;&#8706;q&#8706;&#956;&#8722;&#8706;c&#8706;&#956;&#8706;q&#8706;&#955;&#61;k&#215;&#8711;c&#183;&#8711;q&#61;u&#183;&#8711;q is the jacobian&#44; u  &#61; k&#215;&#8711;c&#61;u&#955;&#44;u&#956;&#61;&#8722;1&#8722;&#956;2&#8706;c&#8706;&#956;&#44;11&#8722;&#956;2&#8706;c&#8706;&#955; is a tangent velocity vector&#44; <span class="elsevierStyleItalic">grad</span>c&#61;&#8711;c&#61;11&#8722;&#956;2&#8706;c&#8706;&#955;&#44;1&#8722;&#956;2&#8706;c&#8706;&#956;&#44;c&#61;&#936;&#44;&#958;&#61;&#916;&#936;&#61;div&#8201;grand&#936;&#44; is the relative vorticity <span class="elsevierStyleItalic">q &#61;</span>&#916;&#936; &#43; 2 <span class="elsevierStyleItalic">&#956;</span> is the absolute vorticity and <span class="elsevierStyleBold">k</span> is a unit outward normal vector&#46; The velocity vector field <span class="elsevierStyleBold">u</span> having the components &#40;<span class="elsevierStyleItalic">u</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#955;</span></span>&#44;<span class="elsevierStyleItalic">u</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#956;</span></span>&#41; is solenoidal&#58; &#8711; &#183; <span class="elsevierStyleBold">u</span> &#61; 0 Throughout decades the nonlinear barotropic vorticity equation has been successfully used to describe low frequencies and large-scale barotropic processes of atmospheric dynamics&#46; Despite the simplicity to this nonlinear equation&#44; it contains the principal elements that describe the complexity of atmospheric behavior &#40;<a class="elsevierStyleCrossRef" href="#bib0130">Simmons <span class="elsevierStyleItalic">et al&#46;</span>&#44; 1983</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0140">Skiba&#44; 1997</a>&#41;&#46; The four types of exact solutions to <a class="elsevierStyleCrossRef" href="#eq0005">Eq&#46; &#40;1&#41;</a> known up to now are described below&#58;<ul class="elsevierStyleList" id="lis0005"><li class="elsevierStyleListItem" id="lsti0005"><span class="elsevierStyleLabel">&#8226;</span><p id="par0205" class="elsevierStylePara elsevierViewall">The zonal flows and Rossby-Haurwitz &#40;RH&#41; waves &#40;Haurwitz&#44; 1949&#41;&#44; called classical solutions&#44; differentiated from the generalized solutions which are not so smooth&#46;</p></li><li class="elsevierStyleListItem" id="lsti0010"><span class="elsevierStyleLabel">&#8226;</span><p id="par0210" class="elsevierStylePara elsevierViewall">The first generalized solutions of Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0095">&#40;6&#41;</a>&#44; kown as modons&#44; were originally constructed by <a class="elsevierStyleCrossRef" href="#bib0185">Tribbia &#40;1984&#41;</a> and Verkley &#40;1984&#44; <a class="elsevierStyleCrossRef" href="#bib0195">1987</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0200">1990</a>&#41; by using two eigenfunctions for the Laplace operator of different degrees&#46;</p></li><li class="elsevierStyleListItem" id="lsti0015"><span class="elsevierStyleLabel">&#8226;</span><p id="par0215" class="elsevierStylePara elsevierViewall">Later on&#44; <a class="elsevierStyleCrossRef" href="#bib0095">Neven &#40;1992&#41;</a> gave generalized solutions in the form of a quadrupole modon&#46;</p></li><li class="elsevierStyleListItem" id="lsti0020"><span class="elsevierStyleLabel">&#8226;</span><p id="par0220" class="elsevierStylePara elsevierViewall"><a class="elsevierStyleCrossRef" href="#bib0205">Wu and Verkley &#40;1993&#41;</a> constructed generalized global solutions composed of two RH waves &#40;<a class="elsevierStyleCrossRef" href="#bib0100">P&#233;rez-Garcia and Skiba&#44; 1999</a>&#41;&#46;</p></li></ul></p><p id="par0225" class="elsevierStylePara elsevierViewall">In the present work&#44; zonal flows&#44; homogeneous spherical polynomials flows&#44; RH waves&#44; and modons on the manifold S<span class="elsevierStyleSup">2</span> are considered&#46;</p><span id="sec0020" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3&#46;1</span><span class="elsevierStyleSectionTitle" id="sect0035">Classical solutions</span><p id="par0230" class="elsevierStylePara elsevierViewall">Let us consider the zonal flows&#44; homogeneous spherical polynomials flows and Rossby-Haurwitz &#40;RH&#41; waves&#46;</p><p id="par0235" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proposition &#40;zonal flow&#41;</span>&#46; Let &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#125;&#44; <span class="elsevierStyleItalic">&#8467; &#61; &#953;&#44; &#954;</span> be an atlas of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span> and the streamfunction <span class="elsevierStyleItalic">&#968;</span> &#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span> &#8594; <span class="elsevierStyleItalic">R</span> of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span><span class="elsevierStyleItalic">&#46;</span> Then the zonal flow map &#936;&#953;&#61;&#968;&#8728;&#966;&#953;&#8722;1&#58;U&#953;&#8834;R&#953;2&#8594;R of <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span> defined as<elsevierMultimedia ident="eq0100"></elsevierMultimedia>is an exact solution of the vorticity Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0095">&#40;6&#41;</a> for any <span class="elsevierStyleItalic">b</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">n</span></span>&#46;</p><p id="par0245" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proof&#46;</span> The demonstration&#44; obtained from Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0095">&#40;6&#41;</a>&#44; is quite trivial&#46;</p><p id="par0250" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proposition &#40;homogeneous polynomials&#41;</span>&#46;</p><p id="par0255" class="elsevierStylePara elsevierViewall">Let &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#125;&#44;<span class="elsevierStyleItalic">&#8467; &#61; &#953;&#44; &#954;</span> be an atlas of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and the streamfunction <span class="elsevierStyleItalic">&#968;</span> &#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">R</span> of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span><span class="elsevierStyleItalic">&#46;</span> Then the homogeneous spherical polynomial map &#936;&#953;&#61;&#968;&#8728;&#966;&#953;&#8722;1&#58;U&#953;&#8834;R&#953;2&#8594;R of degree n &#8805; 2 defined as<elsevierMultimedia ident="eq0105"></elsevierMultimedia>is an exact solution to the vorticity Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0095">&#40;6&#41;</a>&#44; where <span class="elsevierStyleItalic">a</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">m</span></span> can be a complex factor and<elsevierMultimedia ident="eq0110"></elsevierMultimedia></p><p id="par0260" class="elsevierStylePara elsevierViewall">is the angular phase speed&#46;</p><p id="par0265" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proof&#46;</span> Given &#936;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span> &#8712; <span class="elsevierStyleBold">H</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">n</span></span>&#44; we define &#936;&#953;&#955;&#44;&#8201;&#956;&#44;&#8201;&#8201;t&#61;&#8201;&#936;n&#40;&#955;&#8722;ct&#44;&#8201;&#956;&#41;&#61;&#8721;m&#61;&#8722;nnamYnm&#955;&#8722;ct&#44;&#8201;&#956;&#44; then &#8706;&#936;n&#8706;t&#61;&#8722;2&#936;&#8242;&#44; and &#8706;&#936;n&#8706;&#955;&#61;&#936;&#8242;&#44; where &#936;&#8242;&#61;&#8721;m&#61;&#8722;nnimamYnm&#40;&#955;&#8722;ct&#44;&#8201;&#956;&#41;&#46; If&#44; in addition&#44; we have the following expression<elsevierMultimedia ident="eq0245"></elsevierMultimedia> we have <elsevierMultimedia ident="eq0115"></elsevierMultimedia>from BVE &#40;Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0095">&#40;6&#41;</a>&#41;&#46; It follows that c&#8201;&#61;&#8722;2&#967;n&#46;</p><p id="par0270" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proposition &#40;Rossby-Haurwitz waves&#41;</span>&#46; Let &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#125;&#44; <span class="elsevierStyleItalic">&#8467; &#61; &#953;&#44; &#954;</span> be an atlas of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and the streamfunction <span class="elsevierStyleItalic">&#968;</span> &#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">R</span> of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup">&#8734;</span>&#46;Then&#44; the map &#936;&#953;&#61;&#968;&#8728;&#966;&#953;&#8722;1&#58;U&#953;&#8834;R&#953;2&#8594;R of <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup">&#8734;</span> define as<elsevierMultimedia ident="eq0120"></elsevierMultimedia><elsevierMultimedia ident="eq0250"></elsevierMultimedia>with <span class="elsevierStyleItalic">n &#8805;</span> 1 is called Rossby-Haurwitz &#40;RH&#41; waves&#46;</p><p id="par0275" class="elsevierStylePara elsevierViewall">It is an exact solution of the vorticity Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0095">&#40;6&#41;</a> if the angular phase speed of the RH wave<elsevierMultimedia ident="eq0130"></elsevierMultimedia></p><p id="par0280" class="elsevierStylePara elsevierViewall">Here <span class="elsevierStyleItalic">&#969;</span> is the super-rotation velocity and each <span class="elsevierStyleBold">H</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">n</span></span> corresponds to the eigenvalue <span class="elsevierStyleItalic">&#967;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">n</span></span> &#61; <span class="elsevierStyleItalic">n</span>&#40;<span class="elsevierStyleItalic">n</span>&#43;1&#41;&#46;</p><p id="par0285" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proof</span>&#46; Here &#936;<span class="elsevierStyleInf">&#953;</span> is expressed by<elsevierMultimedia ident="eq0135"></elsevierMultimedia>where &#936;n&#955;&#8722;ct&#44;&#8201;&#956;&#61;&#8721;m&#61;&#8722;nnamY&#955;&#8722;ct&#44;&#8201;&#956;&#46; We can notice that<elsevierMultimedia ident="eq0140"></elsevierMultimedia>which implies that<elsevierMultimedia ident="eq0145"></elsevierMultimedia></p><p id="par0290" class="elsevierStylePara elsevierViewall">Furthermore&#58;<elsevierMultimedia ident="eq0150"></elsevierMultimedia>where &#936;&#8242;&#61;&#8721;m&#61;&#8722;nnimamYnm&#40;&#955;&#8722;ct&#44;&#8201;&#956;&#41;&#59; so that from BVE &#40;Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0095">&#40;6&#41;</a>&#41;<elsevierMultimedia ident="eq0155"></elsevierMultimedia></p><p id="par0295" class="elsevierStylePara elsevierViewall">Hence<elsevierMultimedia ident="eq0160"></elsevierMultimedia>so that<elsevierMultimedia ident="eq0165"></elsevierMultimedia>and thus the proposition is proved&#46;</p><p id="par0300" class="elsevierStylePara elsevierViewall">The streamfunction of the stationary RH&#40;2&#44;5&#41; wave<elsevierMultimedia ident="eq0170"></elsevierMultimedia>with the parameters defined by &#40;<span class="elsevierStyleItalic">m</span>&#44; <span class="elsevierStyleItalic">n</span>&#41; &#61; &#40;2&#44; 5&#41;&#44; <span class="elsevierStyleItalic">a</span> &#61; &#46;007 and &#969;&#61;23&#40;&#967;3&#8722;2&#41; is given in <a class="elsevierStyleCrossRef" href="#fig0010">Figure 2</a>&#46;</p><elsevierMultimedia ident="fig0010"></elsevierMultimedia><p id="par0305" class="elsevierStylePara elsevierViewall"><a class="elsevierStyleCrossRef" href="#bib0105">P&#233;rez and Skiba &#40;2001&#41;</a> and <a class="elsevierStyleCrossRef" href="#bib0145">Skiba and P&#233;rez &#40;2006&#41;</a> developed a numerical spectral method for the normal mode instability study of the arbitrary steady flow of an ideal nondivergent fluid on a rotating sphere&#44; and <a class="elsevierStyleCrossRef" href="#bib0145">Skiba and P&#233;rez &#40;2006&#41;</a> tested this method for the RH&#40;2&#44;5&#41; wave&#46; <a class="elsevierStyleCrossRef" href="#bib0115">P&#233;rez-Garc&#237;a &#40;2014&#41;</a> constructed a basic flow regarded as a sum of a zonally symmetric flow &#40;<a class="elsevierStyleCrossRef" href="#eq0100">Eq&#46; 7</a> and a Rossby-Haurwitz wave component &#40;<a class="elsevierStyleCrossRef" href="#eq0120">Eq&#46; 9</a>&#46;</p></span><span id="sec0025" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">3&#46;2</span><span class="elsevierStyleSectionTitle" id="sect0040">Generalized solutions</span><p id="par0310" class="elsevierStylePara elsevierViewall">Denote the spherical distance between two points of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> by <span class="elsevierStyleItalic">d</span>&#40;&#46;&#44;&#46;&#41;&#46; Let <span class="elsevierStyleItalic">N&#8217;</span> be the north pole of the chart coordinate &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span><span class="elsevierStyleSmallCaps">&#41;&#46;</span> Then a disk or inner region <span class="elsevierStyleItalic">S</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">i</span></span> on the sphere is defined as Si&#61;DN&#8242;&#44;&#966;a&#61;&#123;s&#8201;&#8712;&#8201;S2&#124;&#8201;dN&#8242;&#44;s&#60;&#966;a&#125;&#44; such that 0&#60;&#966;a&#8804;&#960;2 The solution modon is constructed &#40;<a class="elsevierStyleCrossRef" href="#bib0185">Tribbia&#44; 1984</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0190">Verkley&#44; 1984</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0195">1987</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0200">1990</a>&#59; Neven&#44; 1993&#41; to divide the sphere <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> into two regions&#58; an inner region <span class="elsevierStyleItalic">S</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">i</span></span> centered around the pole <span class="elsevierStyleItalic">N&#8217;&#44;</span> and an outer region <span class="elsevierStyleItalic">S</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">o</span></span> separated from the inner region by a boundary circle &#8706;DN&#8242;&#44;&#966;a&#61;&#123;s&#8201;&#8712;&#8201;S2&#124;&#8201;dN&#8242;&#44;s&#60;&#966;a&#125;&#44; on which <span class="elsevierStyleItalic">&#968;</span>&#44; <span class="elsevierStyleItalic">q</span> and <span class="elsevierStyleItalic">&#968;&#8217;</span> are continuous&#46;</p><p id="par0315" class="elsevierStylePara elsevierViewall">For <span class="elsevierStyleItalic">S</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">i</span></span> a solution of the Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0010">&#40;2&#41;</a> form is chosen with an eigenfunction <span class="elsevierStyleItalic">Y</span> &#40;<span class="elsevierStyleItalic">&#955;&#8217;</span>&#44; <span class="elsevierStyleItalic">&#956;&#8217;</span>&#41; which has its singularity in the outer region&#46; The same type of solution is chosen for the outer region&#44; but such that <span class="elsevierStyleItalic">Y</span> &#40;<span class="elsevierStyleItalic">&#955;&#8217;</span>&#44; <span class="elsevierStyleItalic">&#956;&#8217;</span>&#41; has its singularity in the inner region&#46; Then both solutions are combined as smoothly as possible on the boundary circle &#8706;DN&#8242;&#44;&#966;a &#40;<a class="elsevierStyleCrossRef" href="#bib0185">Tribbia&#44; 1984</a>&#59; <a class="elsevierStyleCrossRef" href="#bib0190">Verkley&#44; 1984</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0195">1987</a>&#41;&#46;</p><p id="par0320" class="elsevierStylePara elsevierViewall">To construct the <a class="elsevierStyleCrossRef" href="#bib0190">Verkley &#40;1984&#41;</a> modon or the <a class="elsevierStyleCrossRef" href="#bib0095">Neven &#40;1992&#41;</a> cuadrupole modon on the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; it is interpreted as&#58;</p><p id="par0325" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proposition</span>&#46; Let &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic"><span class="elsevierStyleSmallCaps">&#8467;</span></span></span><span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic"><span class="elsevierStyleSmallCaps">&#8467;</span></span></span>&#41;&#125;&#44; <span class="elsevierStyleItalic"><span class="elsevierStyleSmallCaps">&#8467;</span></span> &#61; <span class="elsevierStyleItalic">&#953;</span>&#44;<span class="elsevierStyleItalic">&#954;</span><span class="elsevierStyleSmallCaps">&#44;</span> be an atlas of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; and <span class="elsevierStyleItalic">&#968;</span> &#61; <span class="elsevierStyleItalic">&#968;</span><span class="elsevierStyleInf">1</span> &#43; <span class="elsevierStyleItalic">&#968;</span><span class="elsevierStyleInf">2</span> &#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">R</span> the streamfunction of <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span><span class="elsevierStyleItalic">&#46;</span> Then<elsevierMultimedia ident="eq0175"></elsevierMultimedia></p><p id="par0330" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proof</span>&#46; Let <span class="elsevierStyleItalic">&#968;</span><span class="elsevierStyleInf">1</span> and <span class="elsevierStyleItalic">&#968;</span><span class="elsevierStyleInf">2</span> be two real-value functions of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span> defined on the differential manifolds <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#46; We define their sum by setting &#936;&#953;&#61;&#40;&#968;1&#43;&#968;2&#41;&#8728;&#966;&#953;&#8722;1&#61;&#968;1&#8728;&#966;&#953;&#8722;1&#43;&#968;2&#8728;&#966;&#953;&#8722;1 for any chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44;<span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; Since the sum of two functions of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span> on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> are functions of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span><span class="elsevierStyleItalic">&#44;</span> the proof of this formula can be obtained by the expression<elsevierMultimedia ident="eq0180"></elsevierMultimedia>where &#966;&#953;&#954;&#40;&#955;&#44;&#8201;&#956;&#41;&#61;&#40;&#966;&#953;&#954;1&#8201;&#40;&#955;&#44;&#8201;&#956;&#41;&#44;&#8201;&#966;&#953;&#954;2&#8201;&#40;&#955;&#44;&#8201;&#956;&#41;&#41;&#61;&#40;&#955;&#8242;&#40;&#955;&#44;&#8201;&#956;&#41;&#44;&#956;&#8242;&#40;&#955;&#44;&#8201;&#956;&#41;&#41;</p><p id="par0335" class="elsevierStylePara elsevierViewall">Decompose now the streamfunctions into an eigenfunction part &#40;&#968;1&#8728;&#966;&#954;&#8722;1&#41;&#8201;&#40;&#955;&#8242;&#44;&#8201;&#956;&#8242;&#41;&#61;Y&#957;&#40;&#955;&#8242;&#44;&#8201;&#8201;&#956;&#8242;&#41; and a zonal part &#40;&#968;2&#8728;&#966;&#953;&#8722;1&#41;&#8201;&#40;&#955;&#44;&#8201;&#956;&#41;&#61;&#8722;&#969;&#956;&#43;D where &#8211;<span class="elsevierStyleItalic">&#969;&#956;</span> is a solid-body rotation and <span class="elsevierStyleItalic">D</span> a constant&#46; In chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41; with coordinates &#40;<span class="elsevierStyleItalic">&#955;&#8217;&#44; &#956;&#8217;</span>&#41;&#44; the north pole <span class="elsevierStyleItalic">N&#8217;</span> moves along a circle of constant latitude with constant angular velocity c<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span>&#46; In the primed coordinates&#44; Verkley &#40;1984&#44; <a class="elsevierStyleCrossRef" href="#bib0195">1987</a>&#41; modons have the form<elsevierMultimedia ident="eq0185"></elsevierMultimedia>which consists of a dipole and a monopole component&#58;<elsevierMultimedia ident="eq0190"></elsevierMultimedia></p><p id="par0340" class="elsevierStylePara elsevierViewall">where <span class="elsevierStyleItalic">&#956;</span><span class="elsevierStyleInf">0</span> &#61; sen <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf">0</span><span class="elsevierStyleItalic">&#956;</span><span class="elsevierStyleInf">a</span> &#61; sen <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">a</span></span>&#46; The functions <span class="elsevierStyleItalic">f</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">d</span></span>&#40;<span class="elsevierStyleItalic">&#956;</span>&#41; and <span class="elsevierStyleItalic">f</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">m</span></span>&#40;<span class="elsevierStyleItalic">&#956;</span>&#41; are defined as<elsevierMultimedia ident="eq0195"></elsevierMultimedia>and<elsevierMultimedia ident="eq0200"></elsevierMultimedia></p><p id="par0345" class="elsevierStylePara elsevierViewall">where b&#61;k2&#43;14&#43;2&#945;&#40;&#945;&#43;1&#41;&#8722;2 and<elsevierMultimedia ident="eq0205"></elsevierMultimedia></p><p id="par0350" class="elsevierStylePara elsevierViewall">The fact that<elsevierMultimedia ident="eq0210"></elsevierMultimedia>is a solution to Eq&#46; <a class="elsevierStyleCrossRef" href="#eq0095">&#40;6&#41;</a> is due to the work of <a class="elsevierStyleCrossRef" href="#bib0190">Verkley &#40;1984&#41;</a>&#44; which I will not reproduce in this paper&#46; <span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span> is an eigenfunction of the Laplace-Beltrami operator and <span class="elsevierStyleItalic">&#967;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span> &#61; &#8211;&#957;&#40;&#957; &#43; 1&#41; is the eigenvalue of Y<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#957;</span></span>&#46; The Legendre functions H&#956;&#61;P&#957;m&#956; and H&#956;&#61;Q&#957;m&#956; are solutions to the Legendre differential equation of hypergeometric type&#44; where P&#957;m&#40;&#956;&#41; is a Legendre function of the first kind and Q&#957;m&#40;&#956;&#41; is a Legendre function of the second kind for order <span class="elsevierStyleItalic">m</span> such that &#957; is the complex degree&#46; The explicit expresion for P&#957;m&#956; and Q&#957;m&#956; with &#8211;1 &#60; <span class="elsevierStyleItalic">&#956;</span> &#60; 1 can be found in <a class="elsevierStyleCrossRef" href="#bib0005">Abramowitz and Stegun &#40;1965&#41;</a> or <a class="elsevierStyleCrossRef" href="#bib0190">Verkley &#40;1984&#41;</a>&#46;</p><p id="par0355" class="elsevierStylePara elsevierViewall">By using a grid of 5 &#215; 5&#176; upon the local coordinate associated with the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span>&#44;</span><span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41;&#44; the <a class="elsevierStyleCrossRef" href="#bib0190">Verkley&#44; 1984</a> modon was numerically generated&#46; Using <a class="elsevierStyleCrossRef" href="#eq0020">Eqs&#46; &#40;4&#41;</a> and <a class="elsevierStyleCrossRef" href="#eq0025">&#40;5&#41;</a> a workable Gaussian mesh of &#40;128&#44; 64&#41; points upon the geographical coordinate group &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; was also generated&#46; This mesh was mapped onto the local coordinates system associated to the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span>&#44;</span><span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41;&#46; The values of &#936;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span> were interpolated on the Gaussians points &#40;128&#44; 64&#41; by implementing a nine- point Lagrange interpolation scheme&#46; The resulting function&#44; i&#46;e&#46; the <a class="elsevierStyleCrossRef" href="#bib0190">Verkley&#44; 1984</a> equatorial modon viewed on the geographical coordinate group &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;<span class="elsevierStyleSmallCaps">&#44;</span> is shown in <a class="elsevierStyleCrossRef" href="#fig0015">Figure 3a</a>&#46; This small modon was defined by the following parameters&#58;<elsevierMultimedia ident="eq0215"></elsevierMultimedia></p><elsevierMultimedia ident="fig0015"></elsevierMultimedia><p id="par0360" class="elsevierStylePara elsevierViewall">A numerical spectral model was used to simulate this small-size modon in <a class="elsevierStyleCrossRef" href="#bib0100">P&#233;rez-Garc&#237;a and Skiba &#40;1999&#41;</a>&#44; and in <a class="elsevierStyleCrossRef" href="#bib0150">Skiba and P&#233;rez-Garc&#237;a &#40;2009&#41;</a> a numerical spectral method for normal mode stability study of ideal flows on a rotating sphere was tested for this isolated steady modon constructed by <a class="elsevierStyleCrossRef" href="#bib0190">Verkley &#40;1984&#41;</a>&#46;</p><p id="par0365" class="elsevierStylePara elsevierViewall">Studies done by <a class="elsevierStyleCrossRef" href="#bib0070">Illari &#40;1984&#41;</a> and <a class="elsevierStyleCrossRef" href="#bib0035">Crum and Stevens &#40;1988&#41;</a> noted that the values of isentropic potential vorticity are relatively low and uniform in the blocking region&#46; In our following argument we consider that Verkley&#39;s modon &#40;1990&#41; provides a better and more uniform description of atmospheric blocking&#46; Our interest lays within the fields that build this phenomenon&#46; These solutions are characterized by a region <span class="elsevierStyleItalic">S</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">i</span></span> in which <span class="elsevierStyleItalic">q</span> is constant&#44; and an outer region <span class="elsevierStyleItalic">S</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">o</span></span> separated from the inner region by the boundary circle &#8706;<span class="elsevierStyleItalic">D</span>&#44; on which &#936; and <span class="elsevierStyleItalic">q</span> are both constant&#44; i&#46;e&#46;&#44; &#936; &#61; <span class="elsevierStyleItalic">d</span> and <span class="elsevierStyleItalic">q</span> &#61; <span class="elsevierStyleItalic">b</span>&#46;</p><p id="par0370" class="elsevierStylePara elsevierViewall">In the primed coordinates the <a class="elsevierStyleCrossRef" href="#bib0200">Verkley &#40;1990&#41;</a> modon has the form<elsevierMultimedia ident="eq0220"></elsevierMultimedia></p><p id="par0375" class="elsevierStylePara elsevierViewall">where solid-body rotation terms can be expressed in primed coordinates using <a class="elsevierStyleCrossRefs" href="#eq0020">Eqs&#46; &#40;4-5&#41;</a>&#44; such that in chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; the eigenfuctions at the outer region and inner region are &#916;&#8242;Yo&#61;&#8722;&#967;oYo&#59;&#8201;&#916;&#8242;Yi&#61;ei being &#967;<span class="elsevierStyleSup"><span class="elsevierStyleItalic">o</span></span> and <span class="elsevierStyleItalic">e</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">i</span></span> constants&#46; Certain requirements of continuity must be met to generate these functions over the circle &#8706;<span class="elsevierStyleItalic">D&#46;</span> We require continuity in and the first and second derivative in <span class="elsevierStyleItalic">&#966;</span>&#8217; at <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf">a</span>&#46;</p><p id="par0380" class="elsevierStylePara elsevierViewall">To construct the <a class="elsevierStyleCrossRef" href="#bib0200">Verkley&#44; 1990</a> uniform modon on the manifold S<span class="elsevierStyleInf">2</span>&#44; it is interpreted as&#58;</p><p id="par0385" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proposition</span>&#46; Consider an atlas &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#8467;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#8467;</span></span>&#41;&#125;&#44; <span class="elsevierStyleItalic">&#8467;</span> &#61; <span class="elsevierStyleItalic">&#953;</span><span class="elsevierStyleBold">&#44;</span><span class="elsevierStyleItalic">&#954;</span><span class="elsevierStyleBold">&#44;</span> of <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> and let <span class="elsevierStyleItalic">&#968;</span> &#61; <span class="elsevierStyleItalic">&#968;</span><span class="elsevierStyleInf">1</span> &#43; <span class="elsevierStyleItalic">&#968;</span><span class="elsevierStyleInf">2</span> &#58; S<span class="elsevierStyleSup">2</span> &#8594; R be the stream-function of <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span><span class="elsevierStyleItalic">&#46;</span> Then<elsevierMultimedia ident="eq0225"></elsevierMultimedia></p><p id="par0390" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleBold">Proof</span>&#46; Let <span class="elsevierStyleItalic">&#968;</span><span class="elsevierStyleInf">1</span> and <span class="elsevierStyleItalic">&#968;</span><span class="elsevierStyleInf">2</span> be two real-value functions of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span> defined on the differential manifolds <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#46; We define their sum by setting &#936;&#954;&#968;1&#43;&#968;2&#8728;&#966;&#954;&#8722;1&#61;&#968;1&#8728;&#966;&#954;&#8722;1&#43;&#968;2&#8728;&#966;&#954;&#8722;1 for any chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41;&#46; Since the sum of two functions of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span> on <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> are functions of class <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">r</span></span><span class="elsevierStyleItalic">&#44;</span> the proof of the proposition follows from&#58;<elsevierMultimedia ident="eq0255"></elsevierMultimedia>because &#955;&#44;&#956;&#61;&#966;&#954;&#953;&#955;&#8242;&#44;&#956;&#8242;</p><p id="par0395" class="elsevierStylePara elsevierViewall">According to <a class="elsevierStyleCrossRef" href="#bib0200">Verkley &#40;1990&#41;</a>&#44; to express the modon in a more explicit manner&#44; the functional forms <span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">o</span></span> and <span class="elsevierStyleItalic">Y</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">i</span></span> of the eigenfuntions are&#58;<elsevierMultimedia ident="eq0230"></elsevierMultimedia>where <span class="elsevierStyleItalic">A</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">s</span></span> and <span class="elsevierStyleItalic">B</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">s</span></span> are constant&#46; The special functions S&#963;m&#952;&#8242;&#61;P&#963;m&#8722;cos&#8201;&#952;&#8242; and <span class="elsevierStyleItalic">T</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">m</span></span>&#40;<span class="elsevierStyleItalic">&#952;</span>&#8217;&#41; were given by <a class="elsevierStyleCrossRef" href="#bib0200">Verkley &#40;1990&#41;</a>&#46;</p><p id="par0400" class="elsevierStylePara elsevierViewall">We have also reproduced numerically the uniform modon constructed by <a class="elsevierStyleCrossRef" href="#bib0200">Verkley &#40;1990&#41;</a> using the parameters &#966;a&#61;5&#960;12&#44;&#8201;&#8201;&#963;&#61;8&#46;06&#59;&#8201;&#8201;&#969;o&#61;0&#46;028&#44; and when the modon center is in the point &#955;o&#61;180&#176;&#44;&#966;o&#61;&#960;4 &#40;<a class="elsevierStyleCrossRef" href="#fig0015">Fig 3b</a>&#41;&#46;</p></span></span><span id="sec0030" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleLabel">4</span><span class="elsevierStyleSectionTitle" id="sect0045">Conclusions</span><p id="par0405" class="elsevierStylePara elsevierViewall">The exact solutions of the barotropic vorticity equation on the rotating unit sphere as a compact differentiable manifold without boundary&#44; which are zonal flows&#44; homogeneous spherical polynomial flows&#44; Rossby-Haurwitz waves and generalized solutions named modons&#44; were represented in this paper&#46; A concrete notion of local chart&#44; change of charts&#44; and atlas for the manifolds <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> was developed&#46; An atlas &#123;&#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#44; &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41;&#125; was constructed for <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; where every chart corresponds to a geographical coordinate group that covers most of S2&#46; The transition maps &#966;&#953;&#954;&#61;&#966;&#954;&#8728;&#966;&#953;&#8722;1&#58;&#966;&#953;&#937;&#953;&#8745;&#937;&#954;&#8594;&#966;&#954;&#937;&#953;&#8745;&#937;&#954; and &#966;&#954;&#953;&#61;&#966;&#953;&#8728;&#966;&#954;&#8722;1&#58;&#966;&#954;&#937;&#954;&#8745;&#937;&#953;&#8594;&#966;&#953;&#937;&#954;&#8745;&#937;&#953; were also constructed&#44; and the exact solutions of the barotropic vorticity equation in a manifold context were studied&#46; This work also formulates on the manifolds <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> in terms of the stream function <span class="elsevierStyleItalic">&#968;</span>&#58; <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> &#8594; <span class="elsevierStyleItalic">R</span>&#44; for RH waves which are sufficiently smooth&#44; and for Wu-Verkley waves and modons which are differentiably weak&#46; RH waves as solutions &#968;&#8728;&#966;&#953;&#8722;1 of the barotropic vorticity equation on the manifolds <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> are presented at the local coordinate associated with a chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41;&#46; The way in which the modon solution &#968;&#8728;&#966;&#954;&#8722;1 can be constructed in the chart &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41;<span class="elsevierStyleSmallCaps">&#44;</span> where <span class="elsevierStyleItalic">&#968;</span> is <span class="elsevierStyleItalic">C</span><span class="elsevierStyleSup">2</span>&#44; is investigated too&#46; In the chart coordinate &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#954;</span></span>&#41;<span class="elsevierStyleSmallCaps">&#44;</span> the Verkley &#40;1984&#44; <a class="elsevierStyleCrossRef" href="#bib0195">1987</a>&#41; modons have the form<elsevierMultimedia ident="eq0235"></elsevierMultimedia>To construct the <a class="elsevierStyleCrossRef" href="#bib0200">Verkley &#40;1990&#41;</a> uniform modon on the manifold <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; it is interpreted as<elsevierMultimedia ident="eq0240"></elsevierMultimedia></p><p id="par0410" class="elsevierStylePara elsevierViewall">when the modon center is in the point <span class="elsevierStyleItalic">&#955;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">o</span></span> &#61; 270&#59; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">o</span></span> &#61; 0&#46; However&#44; to contruct the <a class="elsevierStyleCrossRef" href="#bib0190">verkley &#40;1984</a>&#44;<a class="elsevierStyleCrossRef" href="#bib0195">1987</a>&#44; <a class="elsevierStyleCrossRef" href="#bib0200">1990</a>&#41; with <span class="elsevierStyleItalic">N&#8217;</span> a point arbitrary on the sphere <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span> a collection of pairs &#40;&#937;<span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#44; <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleInf"><span class="elsevierStyleItalic">&#953;</span></span>&#41; &#40;i &#62; 2&#41; is needed&#46; Our viewpoint here was to understand the solution of the barotropic vorticity equation on the manifold S<span class="elsevierStyleSup">2</span> and its use to derive properties of the solutions to the Riemannian manifold &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span><span class="elsevierStyleItalic">&#44; g</span>&#41;&#46;</p></span></span>"
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        "resumen" => "<span id="abst0005" class="elsevierStyleSection elsevierViewall"><p id="spar0005" class="elsevierStyleSimplePara elsevierViewall">El prop&#243;sito de este trabajo es representar las soluciones exactas de la ecuaci&#243;n de vorticidad barotr&#243;pica sobre la esfera unitaria <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span> en rotaci&#243;n como una variedad&#44; que son flujos zonales&#44; ondas Rossby-Haurwitz y soluciones generalizadas llamadas modones&#46; Se relacionan los m&#233;todos modernos de la teor&#237;a de funciones con la esfeoa definida como una variedad compacta y diferenciable&#46; Cuando &#233;sta se ha comprendido de forma correcta&#44; se esclarece la noci&#243;n abstracta de mapa local&#44; cambio de mapa y atlas&#46; Uno de los objetivos de este trabajo ns entender mejor lo soluci&#243;n de la ecuaci&#243;n de vorticidad barotr&#243;pica sobre la varieded <span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup"><span class="elsevierStyleItalic">2</span></span> y su utilidad para identificar las propiedades de lac soluciones en la variedad Riemanniana &#40;<span class="elsevierStyleItalic">S</span><span class="elsevierStyleSup">2</span>&#44; <span class="elsevierStyleItalic">g</span>&#41;&#46; Por lo tanto&#44; estar&#225; disponible un tipo m&#225;s general de espacio que tambi&#233;n puede contener informaci&#243;n geom&#233;trica y anal&#237;tica sustancial sobre las soluciones a la ecuaci&#243;n de vorticidad barotr&#243;pica&#46;</p></span>"
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