Climatology of the Residual Mean Circulation of the Martian Atmosphere
and Contributions of Resolved and Unresolved Waves Based on EMARS
Abstract
The objective of this study was to examine both the climatology of the
residual mean circulation, and the roles of resolved wave (RW) and
unresolved wave (UW) forcings over four Mars years, based on the
transformed Eulerian mean equation system using the EMARS reanalysis
dataset. While RW forcing was estimated directly as Eliassen–Palm flux
divergence, the forcing by UWs, including subgrid-scale gravity waves,
was estimated indirectly using the zonal momentum equation. This
indirect method, devised originally for study of Earth’s middle
atmosphere, is applicable to latitudinal regions having angular momentum
isopleths connected from the surface to the top of the atmosphere, which
are usually mid- and high-latitude regions. In low latitudes of the
winter hemisphere, a strong residual mean poleward flow is observed at
an altitude range of 40–80 km, where the latitudinal gradient of the
absolute angular momentum is small. The strong poleward flow crosses the
isopleths of angular momentum in the regions of its northern and
southern ends, indicating the necessity of the wave forcing. Our results
suggest that the structure of the residual mean circulation at mid- and
high-latitude regions is largely determined by UW forcing, particularly
above the altitude of 60 km, whereas the RW contribution is also large
below the altitude of 60 km.