Recent studies link the observed hydrogen escape in the Martian thermosphere to the water of lower atmospheric origin. However, the cold mesosphere hinders penetration of vapor into the upper atmosphere. We present results of simulations with the Max Planck Institute general circulation model (MPI-MGCM) implementing a state-of-the-art hydrological cycle scheme. The simulations reveal a seasonal water ``pump” mechanism responsible for the upward transport of vapor. It takes place in high latitudes of the southern hemisphere at perihelion, when the upward branch of the meridional circulation is particular 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.