Wet periods along the East Africa Coast and the extreme wet spell event of October 1997

Abstract

Extreme wet spells affect the East Africa Coast (EAC) during March to June (long rains) and October to December (short rains). While these spells are less frequent during the short rains, some of the most extreme wet spells occur at this time of the year. The present study examined the general characteristics of the wet spells during the short rains. A detailed study of the anomalous wet spell event of October 1997, with record rainfall around Mombasa (4.0°S, 39.6°E), was also carried out. Daily rainfall for 1962-1997 and NCEP2 reanalysis data for 1979-1997 were used to study the characteristics of the wet events. A high spatial coherence is found in the rainfall over the EAC. The circulation features that were common during most of the wet events were: weakening or reversal of the east-west (Walker type) circulation over the Indian Ocean, enhanced convergence between the northern and southern hemisphere trade winds and westward-moving disturbances in the low-level equatorial wind field. During the 1997 wet event, it is shown that prior to the heavy rainfall event a ridge of high pressure, on the eastern coast of southern Africa, intensified and propagated eastwards leading to the strengthening of moist easterlies reaching the EAC. The zonal wind component along longitude 40°E showed shears in the flows that were associated with the development of the Mozambique Channel low/trough in the lower troposphere round which southerlies surged northwards. These southerlies converged with the easterlies near the EAC. Thus, the warm and wet air from the east interacted with the relatively cold and mainly continental air from the south generating instability at the EAC.