  We present results of simulations with the Max Planck Institute general
circulation model (MPI-MGCM) implementing a hydrological cycle scheme. The
simulations reveal a seasonal water "pump" mechanism responsible for the upward
transport of water vapor. This mechanism occurs in high latitudes above
60$^\circ$ of the southern hemisphere at perihelion, when the upward branch of
the meridional circulation is particularly strong. A combination of the mean
vertical flux with variations induced by solar tides facilitates penetration of
water across the "bottleneck" at approximately 60 km. The meridional
circulation then transports water across the globe to the northern hemisphere.
Since the intensity of the meridional cell is tightly controlled by airborne
dust, the water abundance in the thermosphere strongly increases during dust
storms.
