Characteristics of transition seasons in the eastern hemisphere tropics

Based on outgoing long-wave radiation (OLR) and FGGF level Illb wind data, meteorological differences between the two monsoon transition periods in the eastern hemisphere tropics are investigated. In order to set up the major monsoons of Asia, Africa and Australia, the circulation system goes through two transition periods. Associated with these circulation changes which are also related to the semi-annual character of the solar heating, the two transition seasons are convectively active in the near equatorial zone. The rising branch of the thermally direct Hadley circulation is located in this area. Although similarities exist, significant differences are found between spring and autumn in terms of strength and location of rising and sinking branches of the Hadley and the east-west (Walker) circulations. During spring the rising branch of the east-west circulation is east of 90°F and the Hadley-type overturning over the Indian Ocean is weak. In contrast, during autumn the rising branch of the main east-west circulation is over the Indian Ocean and the Hadley overturning is strong there. These are mainly attributed to the asymmetric distribution of land-mass with respect to the equator which in turn relates to the time lag in land/ocean thermal response to the heating cycle. Circulation patterns for April and November are discussed and differences are noted. Major differences between the two seasons are found in OLR, vertical motion and velocity potential fields. In the moisture budget, significant differences are found with respect to source regions and convergence.