Propagation characteristics of extratropical planetary waves observed in the ATSR global sea surface temperature record

This paper examines the characteristics of planetary wave signatures that h ave been found in the Along Track Scanning Radiometer averaged sea surface temperature (ASST) record for 1991-1996. Longitude-time plots for every lat itude between 5 degrees and 50 degrees, north and south, reveal westward pr opagating wave-like patterns at many locations, whose speed decreases with latitude like baroclinic Rossby waves. A two-dimensional Radon transform me thod is used to measure the wave speed and its variation with location and time, which broadly matches the Rossby wave speeds predicted by the most re cent theory and those measured by TOPEX altimetry, although there are some discrepancies. At low latitudes the thermally detected speeds are slower th an expected, a possible consequence of sampling limitations. Wave signature s are dearest between 25 degrees and 40 degrees S, where the meridional tem perature gradient is strongest. Here observed speeds are 20-30% greater tha n theoretical predictions. Planetary wave speed varies considerably with lo ngitude. In general, it increases toward the west of ocean basins, and dist inct differences between ocean basins are evident. The propagation characte ristics of the waves appear to change abruptly at locations consistent with latitudinal variations in seafloor bathymetry, particularly midocean ridge s. In addition, eastward propagating signatures are found in the Southern O cean. The results demonstrate the value of the ASST data set as a tool for studying basin-scale wave processes as a complement to the use of altimetry . By observing the thermal signature of Rossby waves the method has the pot ential to clarify their influence on air sea interaction processes and to c ontribute to climate modeling studies.