Three-dimensional Simulations of Neptune's Great Dark SPOT

The Great Dark Spot (GDS) seen on Neptune by Voyager was the most time-dependent Jovian vortex yet observed, with oscillations in aspect ratio and orientation, recurring formation of a tail, and steady equatorward drift. We have simulated each of these motions using an isentropic coordinate, primitive equation GCM (the EPIC model), and report here on idealized-model results that form part of a sensitivity study using a hierarchy of increasingly realistic Neptune models. The runs reported here are for a 9.5-layer model covering the pressure range 0-4000 mbar and overlying an adiabatic interior that has a specified zonal-wind profile. Analytical models of uniform potential vorticity spots, either two-dimensional ellipses (Polvani et al, 1990, Science 249) or three-dimensional ellipsoids (Meacham et al, 1994, Dyn.Atm.Oceans 21), exhibit periodic oscillations while retaining an elliptical shape. To connect to these simplified models and to isolate physical processes, we have run a series of experiments with uniform background potential vorticity (PV), uniform stratification, and zero vertical wind shear. Removing the background PV gradient significantly reduces or eliminates the equatorward drift. Unlike the analytical models, our vortices tend to reconfigure themselves to have nonuniform potential vorticity (becoming increasingly anticyclonic towards the center), even when initialized with near-uniform PV. Tails that closely resemble those of the GDS form quasi-periodically; the details governing their formation are not completely understood. Vertically, while each three-dimensional vortex remains coherent, the centers of vorticity on different layers exhibit complex "shaky column" behavior: sometimes leaning, as seen in the 2.5-layer-equivalent model of Achterberg and Ingersoll (1994, J.Atmos.Sci. 51), sometimes kinking, where the center of a higher and a lower layer are both to the same side of a central layer, and generally not retaining their initial vertical alignment. This behavior may be related to the interpretation of the shape of the Great Dark Spot as two or more overlapping ellipses in some of the Voyager II observations. The variety of motions in these simulations also reproduce the motions of other active Jovian vortices, notably DS2 of Neptune and the brown barges on Jupiter.