Climatology of the Residual Mean Circulation of the Martian Atmosphere and Contributions of Resolved and Unresolved Waves Based on a Reanalysis Data Set

Abstract

The objective of this study is to examine the climatology of the residual mean circulation, and the roles of wave forcings by both resolved waves (RWs) and unresolved waves (UWs). The analysis is performed using data from the Ensemble Mars Atmosphere Reanalysis System (EMARS) over four Mars Years without global dust storms, based on the transformed Eulerian mean equation theory. While the RW forcing is estimated directly as Eliassen-Palm flux divergence, the forcing by UWs, including subgrid-scale gravity waves, is estimated indirectly using the zonal momentum equation. This indirect method, originally devised for studying the Earth's middle atmosphere, is applicable to latitudinal ranges where angular momentum isopleths are continuous from the surface to the top of the atmosphere, typically mid- and high-latitude regions. In low latitudes of the winter hemisphere, a strong residual mean flow toward the winter pole is observed in a pressure range between ∼20 and ∼0.5 Pa (∼30–60 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 ∼2 Pa level, whereas the RW contribution is also significant below the ∼2 Pa level.