EPIC SIMULATIONS OF TIME-DEPENDENT, 3-DIMENSIONAL VORTICES WITH APPLICATION TO NEPTUNE GREAT DARK SPOT

We use the EPIC general circulation model, described in the companion paper by Dowling et al. (1998. Icarus 132, 221-238), to simulate large vortices under conditions similar to those found on Neptune. The vort ices are anticyclones with roughly elliptical cross sections and exhib it motions that resemble the behavior of Neptune's Great Dark Spot (GD S), including equatorward drift, oscillations in aspect ratio and orie ntation angle, and tail formation. The vortices also exhibit three-dim ensional motions that may explain the occasional appearance of the GDS as two overlapping ellipses. We find that the meridional drift of the vortices is correlated with the meridional gradient of the background absolute vorticity, beta. This result complements studies of hurrica ne drift. The correlation suggests that the drift rate of GDS-type vor tices on Neptune, which can be monitored over the long term by the Hub ble Space Telescope (HST), is diagnostic of the vorticity gradient on the planet. The best fit to the Voyager GDS drift rate in our simulati ons corresponds to beta approximate to 2 x 10(-12) m(-1) s(-1). This is about 1/3 of the value given by the zonal-wind profile determined b y fitting an even polynomial in latitude to the cloud-tracking data (S romovsky et al. 1993). Refitting the data with spherical harmonics (Le gendre polynomials) yields a value for beta that is about 1/2 of the Sromovsky et al. value, and more in line with our vortex-drift results . We show that vortex shape oscillations occur both in the case beta = 0, corresponding to the analytical model of Kida (1981), and for bet a = 0. Interpreting the shape oscillations is more complicated than i nterpreting meridional drift because shape oscillations are sensitive to the distribution of vorticity in the vortex as well as in the envir onment. Rossby-wave dispersion strongly affects the model vortices tha t drift too close to the equator. The vortices disrupt before reaching the equator, dispersing into waves that propagate in both the souther n and northern hemispheres over the course of a few weeks. (C) 1998 Ac ademic Press.