Midwinter Suppression of Baroclinic Storm Activity on Mars (and Earth) in Assimilated Observations and Models

Baroclinic instability and intense traveling wave activity on Mars is well known to occur in “storm zones” close to the edge of the advancing or retreating polar ice cap. Such activity usually sets in during Martian fall and continues until the onset of the summer season when large-scale instability mostly ceases as the atmosphere is no longer baroclinically unstable. The stormy season is typically characterized by large-scale, zonally-propagating waves with zonal wavenumbers m = 1-3, the lower wavenumber modes typically penetrating to considerable altitude though may also be surface-intensified.

Many observations, however, suggest that this eddy activity does not persist uniformly throughout the autumn, winter and spring seasons, but appears to decrease quite markedly and consistently within 10 sols or so either side of the winter solstice. This midwinter ‘solsticial pause' appears to be a sufficiently consistent feature of each winter season in both hemispheres to be regarded as a significant feature of Martian climatology, and could affect a variety of aspects of Martian meteorology including global heat and momentum transport, occurrence of dust storms etc.

A somewhat similar phenomenon has also been documented for the Earth, especially in relation to seasonal variations in the north Pacific storm tracks. The cause of this phenomenon is still not well established, though suggested mechanisms include the effects of enhanced barotropic shear (the so-called ‘barotropic governor') and interactions with topography over central Asia.

In this presentation we examine evidence for this phenomenon in the assimilated record of Martian climate from the Thermal Emission Spectrometer on board the Mars Global Surveyor mission (MGS-TES) and the Mars Climate Sounder on board Mars Reconnaissance Orbiter (MRO-MCS), in conjunction with the UK version of the LMD-Oxford-OU-IAA Mars GCM. This is further corroborated in other evidence from seasonal variations in the incidence of local and regional dust storms that owe their origin to circumpolar baroclinic storms. We also discuss the extent to which this ‘solsticial pause' phenomenon is reproduced in stand-alone atmospheric models and present results of some simulations to test a number of hypotheses for its dynamical origin on Mars.