Comparison of Easterly Waves in the Tropical Pacific during Two Contrasting Periods of Sea Surface Temperature Anomalies

Abstract

The structure and properties of tropical easterly waves have been found to vary considerably between different regimes and time periods. This study investigates the possible roles played by the temporal and spatial sea surface temperature (SST) variations on the waves. Time series of tropical western Pacific radiosonde data during two contrasting 8-month periods of SST anomalies, May?December 1972, which had abnormally high SST in the central and eastern Pacific, and May-December 1973 which had below normal SST in the same region, are analyzed. In both periods, the waves have the same periodicity of 4?5 days and a lower tropospheric zonal wavelength on the order of 3300 km, but their vertical phase and amplitude distributions as well as the thermal structures are different. The results are discussed in terms of two possible influences the SST variations may have on the waves: 1) a direct effect in which the warmer SST represents stronger thermal control through cumulus heating and 2) an indirect effect in which the variation of SST changes the large-scale mean wind circulation which, in turn, has a strong impact on the wave vertical structure and the relative importance of energy sources other than cumulus heating. Finally, a schematic model of these influences is proposed which may be applied to both the temporal and the spatial variations of SST.