Mountain Waves Observed with AIRS above Islands in the Southern Ocean: Seasonal, Interannual, and Latitudinal Variations

The Atmospheric Infrared Sounder (AIRS) instrument on the Aqua satellite measures stratospheric brightness temperatures at high horizontal resolution, and the data reveal distinct patterns above small islands in the Southern Hemisphere that are attributed to mountain waves generated by flow over the island topography. Using data from over 3000 satellite overpasses of 14 remote islands in the Southern Ocean, we examine latitudinal, seasonal, and interannual variations in the occurrence frequency and strength of island mountain wave events. The data examined in our study include the fall to spring seasons, May through September, and two years, 2003-2004. The results show that the occurrence of the waves in the stratosphere is to first order governed by variations in the winds: Winds at the surface affect the amplitude of the mountain waves in the lower atmosphere, and even more importantly winds in the stratosphere affect the mountain wave vertical wavelengths. These two factors together largely control the frequency of occurrence of the island waves observed in the AIRS data because they determine the visibility of the waves in the AIRS signal above noise. The high horizontal resolution and imaging capability of AIRS further permits daily calculation of vector momentum flux in the stratosphere associated with these island wave events. Clear seasonal variations in the magnitude of the flux in these events are observed. The seasonal cycle is related to both orographic relief and to surface wind, which varies with latitude and season. Interpretation of the results permits a first estimation of the effects of small island mountain waves on the Southern Hemisphere winter season circulation.