Temporal and Spatial Behavior of Large-Scale Disturbances in Tropical Cloudiness Deduced from Satellite Brightness Data

The large-scale cloud field in the latitude belt 35°S–35°N is studied using satellite brightness data for the periods May 1967 to October 1967, and November 1967 to Apfil 1968. Time and space-time spectrum and longitude-time section analyses are employed. The cloud field is found to be highly transient and localized, with marked seasonal and regional variations. The variance of cloud activity is remarkably asymmetric about the equator during northern summon. Throughout the year the predominant time scale near 10°N is 5 days. It is associated with two major spatial scales (wavenumbers 10 and 5–6) similar to those of tropospheric easterly waves and the stratospheric Yanai-Maruyama waves, respectively. During the northern winter, pronounced activity near 15°S is associated with wavenumbers 7–15 and a period of 7–12 days. Over the equatorial Indian Ocean eastward-moving disturbances of wavenumber 2 (formally) and period 40 days, and wavenumber 5 and period 9 days, prevail. Well defined space and time scales, consistent with eastward-moving cyclone waves, are found in subtropical latitudes (30°–35°). Intense transient cloud activity is found over warm (>26.5°C) oceans. Moreover, latitudinal profiles of sea surface temperature with a maximum over the equator are associated with the eastward-moving waves, and profiles having a maximum away from the equator are generally associated with the westward-moving waves.