Stationary Vortices in Southern Midlatitudes on Mars

Mars' differentially-heated, seasonally-varying, stratified shallow atmosphere exhibits pronounced mean zonal north-south temperature gradients. From balance constraints, such thermal contrasts support intense, O(100 m s^-1), and deep, extratropical westerly zonal jets. Mars also exhibits continental-scale orographic complexes. The Hellas region is a O(3000 km) wide x O(5~km) deep semi-circular impact basin in southern midlatitudes, roughly 180^o east of the massive Tharsis highlands. Linear and nonlinear climate models have predicted large amplitude stationary planetary (i.e., Rossby) modes to occur in the extratropics between late autumn and early spring. These modes are excited by large O(10 km) east-west excursions in surface relief embedded within zones of enhanced potential vorticity gradients accompanying the westerly polar vortex. Mars Global Surveyor (MGS) atmospheric sounding instruments have recently detected the signature of such waves. Simulations with the NASA Ames Mars global circulation model indicate that a steady, coherent, large-scale vortex can form within the Hellas region. The 3D flow shows substantial low-level flow convergence and ascent within the cyclone's center that gives way to upper-level flow divergence and the formation of an anticyclone aloft (a circulation pattern similar to that found in terrestrial summer subtropical monsoons). The circulation appears driven fundamentally by intense spin-up and spin-down of mean potential vorticity (i.e., vortex stretching and compression) associated with the rapidly varying topographic relief on the westward and eastward rims of the basin. We will present and contrast climate model simulations with recently acquired MGS RS, TES and MHSA data.