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 
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 )
slope centered at 57 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 
180 .
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
200 . The surface stress does not
exceed 15 
- a value close but below the current
estimates for the dust lifting threshold.