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M.I. Richardson (Caltech), R.J. Wilson (NOAA/GFDL)
Mars is replete with hemispheric asymmetry. Within the climate system, asymmetry is expressed by the presence of a permanent CO2 ice cap in the south and a residual water ice cap in the north; a cool, long northern summer and a warm, short southern summer; and, far more vigourous atmospheric circulation during southern summer than during northern summer. Geophysically, asymmetry is expressed by the large difference (~5 km) in the mean elevation of the much older southern hemisphere and the younger north. It is generally accepted that the primary cause of the climatic asymmetries is the relatively large eccentricity of the Martian orbit (0.093) and the timing of perihelion just before southern summer solstice. As the argument of perihelion changes with time, it is expected that asymmetric biases in climate will cycle between the hemispheres on time-scales of 105-106 years. In this presentation, we demonstrate that a decidedly endogenic and non-time varying characteristic of Mars - the offset in planetary centre-of-mass from centre-of-figure - imparts a strong hemispherically asymmetric signature on the climate. We show that the topographic dichotomy forces an annual-average tropical circulation which is strongly biased towards the southern summer solstice, winter hemisphere cell. This bias has important implications for interhemispheric transport of water and lifting of dust. We relate the circulation bias to model predictions that in the absence of a residual CO2 ice cap, water ice would be unstable at the southern pole. Hemispheric biases in the atmospheric cycles of water ice and dust may be fundamentally linked to the distribution of Martian volatiles.