Climatology of transient eddies and dust storms in the southern hemisphere of Mars

This paper examines the behavior of transient eddies in the southern hemisphere of Mars and illustrates how the eddies shape the annual cycle and spatial distribution of dust storms. The investigation of transient eddies is based on off-nadir measurements of brightness temperature by the Mars Climate Sounder, which are the most direct and reliable source of information about eddy activity adjacent to the surface. Basic properties of the transient eddies are determined through least-squares analysis of brightness temperatures spanning 8.6 Mars years, yielding a far more complete description of eddy activity in the southern hemisphere than was available previously. The space-time spectrum is dominated by eddies with zonal wavenumbers 1–4 and periods shorter than 5 sols. The wave-4 component is much more important than was previously recognized. An annual cycle comprising three distinct periods of eddy activity is observed each year. Shallow (in temperature) baroclinic waves are present at $L_s=0$–$75°$ and $125$–$200°$. In the intervening period, the baroclinic waves are replaced by a deep, 3.3-sol, barotropic wave that has a relatively small amplitude near the surface. The transient eddies strongly influence the annual cycle of discrete dust storms cataloged in the Mars Dust Activity Database and the Mars Dust Storm Sequence Dataset. Dust storms are rare and their net area is negligible during the barotropic phase of eddy activity ($L_s=75$–$125°$), when the amplitude near the surface is small. Conversely, the wind stress routinely exceeds the threshold required for dust lifting during the two baroclinic phases of eddy activity, when the amplitude near the surface is much larger, resulting in a conspicuous increase in the net area of the dust storms. Dust storms occur frequently in a corridor extending from Aonia Terra to Margaritifer Terra, owing to the presence of a persistent storm track of eddy activity in the western hemisphere. The density of dust storms in this corridor varies in response to the seasonal evolution in the strength and latitude of the storm track. Further insight is gained by consolidating results from previous studies. This investigation improves our understanding of the Martian climate in the half year centered on the southern winter solstice, when the atmosphere is relatively cool and clear.