Synoptic evolution of composite wet spells over northern Tanzania

Abstract

The synoptic-scale structure and evolution of wet spells over northern Tanzania are described. Pentad (5 d) ECMWF data for major convective events during November in the period 1986 to 1992 were averaged to form a sequential composite, and a seasonal reference mean is subtracted to produce anomalies. The composite results show that a NW-SE line of convergent moisture shifts southward from the NW Indian Ocean 2 pentads before the wet spell, while uplift occurs over the highlands to the north. A high-pressure cell intensifies in the SW Indian Ocean 1 pentad before the wet spell. During this time a low-level vorticity dipole is maintained in the west Indian Ocean, either side of the equator. The main flux of moisture is from the southern Indian Ocean. A southern subtropical, upper westerly trough amplifies during the onset phase. The results offer statistical guidance in medium-range weather forecasts that may assist agricultural management. Dynamical insights are also gained to improve knowledge of climate-weather teleconnections and tropical-subtropical interactions.