**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,
. 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 .
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 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.