Simulations of Time-Dependent Three-Dimensional Vortices with Application to Neptune's Great Dark Spot

R.P. LeBeau (MIT), T.E. Dowling (U. of Louisville)

We use the EPIC atmospheric model, a primitive-equation, isentropic-coordinate GCM, to simulate time-dependent vortices under conditions similar to those found on Neptune. The vortices have roughly elliptical cross-sections and exhibit motions that resemble the behavior of Neptune's Great Dark Spot (GDS), including equatorward drift, nutating oscillations in aspect ratio and orientation angle, and quasi-periodic tail formation. The simulated vortices also exhibit complex, three-dimensional motions that may explain the occasional appearance of the GDS as two overlapping ellipses. We find that the meridional drift of the vortices is strongly correlated with the meridional gradient of the environmental potential vorticity, tex2html_wrap_inline16 . The correlation suggests that the drift rate of GDS-type vortices on Neptune, which can be monitored over the long term by the Hubble Space Telescope, is diagnostic of the vorticity gradient on the planet. The best fit to the Voyager GDS drift rate in our simulations corresponds to tex2html_wrap_inline18 . This is about 1/3 of the value given by the zonal-wind profile of Sromovsky et al. (1993), determined by fitting a polynomial in latitude to the cloud-tracking data. We calculate a new fit to the same data using Legendre polynomials (spherical harmonics), which yields a significantly lower value for tex2html_wrap_inline16 in the mid-latitudes. We show that vortex shape oscillations occur both in cases of zero background potential-vorticity gradient, corresponding to the conditions in analytical Kida-type models of oscillating vortices, and in cases of non-zero background gradient, corresponding to conditions that have not yet been investigated analytically. While the shape oscillations are qualitatively Kida-like, in detail they are distinctly different. We also use the EPIC model to examine the demise of GDS-type vortices that drift too close to the equator.