Barotropic and Baroclinic Annular Modes of Variability in the Atmospheres of Mars and Titan

Abstract:

The Southern/Northern (barotropic) Annular modes and the Baroclinic Annular modes are the leading patterns of large-scale variability in the mid-latitudes and influence mid-latitude circulation, precipitation, and cloud cover; however, they have never been identified in atmospheres outside of Earth. Using a combination of reanalysis datasets for Mars and a state-of-the-art model for Titan, annular modes of climatic variability are revealed to be prominent and influential in the atmospheres of both Mars and Titan. Their modes are defined, as on Earth, using the zonal-mean wind and eddy kinetic energy and appear, in many ways, quite similar to Earth’s modes. However, they explain larger percentages of variance of their respective fields and exhibit several key differences in the underlying dynamics. While the barotropic mode on Mars resembles the terrestrial one, Titan’s equivalent barotropic-like mode describes vertical—as opposed to meridional—shifts in the jet. Conversely, Mars’s and Titan’s modes in eddy kinetic energy exhibit baroclinic and barotropic characteristics, unlike Earth’s, which is purely baroclinic. Nevertheless, the relative similarity of annular variability on Titan and Mars to that of Earth demonstrates that annular modes may be ubiquitous in terrestrial atmospheres. A description of these modes beyond Earth may invigorate efforts to understand and predict variability on terrestrial planets, and a specific link to Martian dust storm activity is provided.