The structure and variability of inter-annual circulation anomalies and precursors over northern Tanzania are investigated. Northern Tanzania experiences bimodal rains centered on: March to May ('long' rains) and October to December ('short' rains), our focus here. Areal rainfall departures in the period 1960 to 1990 are used to identify wet and dry years. Composite fields of sea surface temperature (SST), outgoing longwave radiation (OLR) and winds in the lower and upper troposphere are analysed at lags 0, -2, -4 and -6 mo. The historical mean is subtracted to produce anomalies. The patterns indicate an east-west gradient in SST anomalies across the Indian Ocean which corresponds with a zonal circulation and enhanced convection over the western basin, and increased rainfall over northern Tanzania. A multiple regression model is developed using 3 tropical predictors in the July to September period. The model accounts for about half of short-rain rainfall variance in the 1970s and 1980s.

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 tropicalsubtropical interactions.KEY WORDS: Intra-seasonal oscillations · Rainfall · Tanzania Resale or republication not permitted without written consent of the publisherClim Res 15: 239-248, 2000239-248, & Wang (1993, who found a prominent centre of action in the central Indian Ocean for 30 to 60 d convective variability. Rui & Wang (1990) documented the development and dynamic structure of tropical intraseasonal convection anomalies using pentad outgoing longwave radiation (OLR) and ECMWF-derived 200 and 850 hPa wind divergence. Murakami (1988), Anyamba (1992) and Matarira & Jury (1992) have discussed intra-seasonal variability in the African sector; however, the evolution of synoptic-scale forcing and its relationship to local convection have not been described.External precursors and regional controls of wet spells over tropical east Africa are documented here for the November onset event. Rainfall during this period contributes up to 40% of the annual total in northern Tanzania (Nyenzi 1988). Using principal component analysis, Ogallo (1989) found that the first principal component explained 70% of the total rainfall variance over tropical east Africa, with a bimodal distribution and widespread spatial loading. The short rains are associated with a shift of the ITCZ to south of the equator over the western Indian Ocean following dissipation over India (Asnani 1993). During the 'ITCZ onset event', which usually occurs in November, convection is concentrated over the plateau of northern Tanzania, and water resources, depleted during the preceding dry winter season, are replenished.Tanzanian wet spells are hypothesised to be related to moisture advection from the western Indian Ocean, enhanced upper westerly shear and in situ convection due to local instability and heating. To diagnose the mechanisms underlying intra-seasonal variability, ECMWF gridded meteorological field data for the period 1986 to 1992 were used to provide insight into the synoptic-scale process...