Mesoscale circulations on the Southern impact basin slopes during the polar cap retreat

T. Siili (Finnish Meteorological Institute / GEO), H. Savijärvi (University of Helsinki / MET), J. R. Murphy, R. M. Haberle (NASA Ames / SST)

Towards the end of the southern hemisphere winter the Martian southern polar cap extends at its largest up to 40 tex2html_wrap_inline29 S covering the southern slopes of the Hellas and Argyre basins and substantial parts of the basin floors as well. The two predominant forcings driving the mesoscale circulations here are the topographical slope and the thermal contrast between the ice cap and the exposed regolith. During the cap retreat the changing ice coverage on the slopes changes the contributions of the two forcings and hence the mesoscale circulation types.

We have investigated the circulation types and the transition between them using the University of Helsinki / Finnish Meteorological Institute 2-D Mars mesoscale circulation model (MMCM) with large-scale wind estimates derived from the NASA Ames Mars GCM. The cap retreat has been described by a set of five model runs, in which the ice coverage has been varied from 100 % to 0 % by shifting an ice coverage profile on top of a stationary, steep (0.6 tex2html_wrap_inline29 ) slope centered at 57 tex2html_wrap_inline29 S, rising to the south. The effects of the large-scale wind and of the dust optical thickness have been included in an idealized fashion and the season is tex2html_wrap_inline35 180 tex2html_wrap_inline29 .

Two circulation types are seen: (1) Antarctic-type, where the wind is directed downslope and has high directional constancy throughout the sol in the whole slope region, and (2) slope-type, characterised by anticlockwise turning of the wind vector above the slope and eventual reversal of wind direction during the diurnal cycle. Type (1) is prevalent when the ice coverage is high and clear transition to type (2) does not occur until the slope is practically ice-free. In the Hellas basin this would occur at tex2html_wrap_inline35 200 tex2html_wrap_inline29 . The surface stress does not exceed 15 tex2html_wrap_inline43 - a value close but below the current estimates for the dust lifting threshold.