Dust-storm forcing of Rossby waves on Mars

Off-nadir measurements by the Mars Climate Sounder are an underutilized source of information about planetary-scale waves and their role in the climate. More than 8 Mars years of data is currently available. The data set is used here to investigate a pair of waves whose vertical structure is well aligned with the off-nadir contribution function of the A3 channel. Both waves have a zonal wavenumber of 1 and a large amplitude in the high-latitude, winter-season, westerly jet. One is an eastward-traveling free wave with a period of 15–20 sols; it recurs annually in the north, appearing in most years in a roughly 50-sol span that ends near the winter solstice. The other is a forced Rossby wave that alternates between the north and the south, emerging at the autumnal equinox and subsiding at the vernal equinox. It is stationary with respect to the surface in the south and quasi-stationary in the north. A-type and C-type regional dust storms cause a conspicuous increase in the amplitude of the northern forced wave through their effect on the meridional overturning circulation. The dust-storm enhancement of the overturning circulation is initially confined in longitude, as determined by the spatial distribution of dust in the southern hemisphere at the start of the storm. The amplification of the forced wave is a consequence of this zonal asymmetry. The wave amplitude increases rapidly at the start of each dust storm and returns to its pre-storm level 10–15 sols later when the distribution of dust becomes zonally uniform. A detailed description of the forced wave is obtained from the OpenMARS reanalysis, whose reliability is confirmed by comparisons with the off-nadir measurements. During the early stage of the A and C storms, the wave amplitude exceeds 3 km in geopotential height and the center of the polar vortex is displaced from the pole. In addition, the northern forced wave exhibits a pre-winter-solstice phase reversal whose origin is not known. •Properties of atmospheric waves are derived from 8 years of spacecraft observations.•Wave-1 stationary Rossby waves respond dramatically to forcing by large dust storms.•The wave forcing is a consequence of zonal asymmetry in the dust distribution.•Dust-storm forcing is the main source of year-to-year variability of the Rossby wave.•There is an unexplained pre-solstice phase reversal in the northern forced wave.