1990 Volume 63 Issue 2 Pages 156-178
The seasonal variations of global wind fields in the tropics mainly at 850 mb and their relation to the cloud distribution revealed from outgoing longwave radiation (OLR) are studied based on 5-day mean data.
Some abrupt changes in the location and/or regional extent of the equatorial westerlies are recognized at 850 mb during the progression of seasons. They occur in early March, late March, mid-April, mid-May, mid-June, late July, early September, the beginning of October, late October, mid-November and late December. The OLR fields also change dramatically at these periods. Therefore, eleven natural seasons can be recognized according to these periods.
Seasonal composite maps of wind and OLR fields are constructed and features of these seasons are described. Differences in the characteristics of seasonal transition processes on each continent are pointed out. In individual regions, the seasonal changes of wind and OLR fields do not always occur simultaneously. Schematic illustrations representing the seasonal transition processes of the equatorial westerlies, which are revealed to be different among longitudinal belt, are shown in three types. A map showing the locations in northernmost, southernmost, and the minimum regional extent seasons of equatorial westerlies is presented.
