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# Stuart-type polar vortices on a rotating sphere

• * Corresponding author: Adrian Constantin
This research was supported by the WWTF research grant MA16-009
• Stuart vortices are among the few known smooth explicit solutions of the planar Euler equations with a nonlinear vorticity, and they can be adapted to model inviscid flow on the surface of a fixed sphere. By means of a perturbative approach we show that the method used to investigate Stuart vortices on a fixed sphere provides insight into the dynamics of the large-scale zonal flows on a rotating sphere that model the background flow of polar vortices. Our approach takes advantage of the fact that while a sphere is spinning around its polar axis, every point on the sphere has the same angular velocity but its tangential velocity is proportional to the distance from the polar axis of rotation, so that points move fastest at the Equator and slower as we go towards the poles, both of which remain fixed.

Mathematics Subject Classification: Primary: 86A10; Secondary: 35Q3, 30C20, 35J15.

 Citation: • • Figure 1.  Image showing the hot spot centered on Saturn's South Pole (the core of the southern polar vortex, at the bottom of the image), taken from the observatory in Hawaii with infrared radiation sensitive to temperatures in Saturn's upper troposphere [Image credit: NASA/JPL-CalTech/Space Science Institute]. A similar hot spot is found at Saturn's North Pole

Figure 2.  The spherical coordinate system describing flow on a rotating planet. The coordinate system is fixed with respect to the planet rotating with an angular speed $\Omega'$ about the $z'$ axis of the Cartesian coordinate system $(x',y',z')$. The spherical coordinates are $(r',\theta,\phi)$ where $r' = |\bf{r'}|$ is the distance from the origin at the planet's center, $\theta$ is the polar angle (co-latitude) and $\phi$ is the angle of longitude. The North Pole of the planet is at $\theta = 0$ and the South Pole is at $\theta = \pi$

Figure 3.  The stereographic projection maps the point $(x,y,z)$ on the unit sphere with the North Pole $N$ excised to the intersection point $(X,Y)$ of the equatorial plane with the ray from $N$ to $(x,y,z)$. The point $N$ itself is mapped to the point at infinity on the equatorial plane

Figure 4.  Stereographic projection of a spherical cap onto the equatorial plane. The cap near the South pole ($S$) encloses the vortex region and has boundary co-latitude $\theta_s$

Figure 6.  The eye of the stationary vortex centered on Saturn's South Pole and extending to 88.5$^\circ$S, captured in 2008 by NASA's Cassini spacecraft using thermal radiation, is about 2000 km across and features peak prograde winds of 170 m$\,$s$^{-1}$ [Image credit: NASA/JPL-CalTech/Space Science Institute]

Figure 5.  The projected vortex region $\mathcal{V}$ in the projected plane, with maximum radius $\delta$ and minimum radius $\mu$. The boundary of $\mathcal{V}$ is denoted by $\mathcal{V}_B$

Figure 7.  The eye of the stationary vortex at Saturn's North Pole (with a surrounding hexagonal jet stream), captured in 2017 by NASA's Cassini spacecraft, is more than 2000 km wide and features prograde wind speeds of 200 m$\,$s$^{-1}$ on its outer edge at 88$^\circ$N (decreasing within the eye to zero at the pole) [Image credit: NASA/JPL-CalTech/Space Science Institute]

Figure 8.  Depiction of the streamline pattern (40) inside the polar vortex at the South Pole (the black dot). The velocity and vorticity fields are smooth inside the polar vortex region

Figure 9.  Depiction of the streamline pattern inside the polar vortex at the South Pole (the black dot) for the choice (41) with $A = 1$ and $B^6 = .001$. The velocity and vorticity fields are smooth inside the polar vortex region

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