  Simulations with the Max Planck Institute Martian general circulation model
for Martian years 28 and 34 reveal details of the water "pump" mechanism and
the role of gravity wave (GW) forcing. Water is advected to the upper
atmosphere mainly by upward branches of the meridional circulation: in low
latitudes during equinoxes and over the south pole during solstices. Molecular
diffusion plays little role in water transport in the middle atmosphere and
across the mesopause. GWs modulate the circulation and temperature during
global dust storms, thus changing the timing and intensity of the transport. At
equinoxes, they facilitate water accumulation in the polar warming regions in
the middle atmosphere followed by stronger upwelling over the equator. As
equinoctial storms decay, GWs tend to accelerate the reduction of water in the
thermosphere. GWs delay the onset of the transport during solstitial storms and
change the globally averaged amount of water in the upper atmosphere by 10-25%.
