Seasonal and intra-seasonal dynamics and precursors of rainfall over northern Tanzania

Kabanda, Tibangayuka Abbas

Seasonal and intra-seasonal dynamics and precursors of rainfall over northern Tanzania

Master Thesis

1995

Publisher

University of Cape Town

Department

Department of Oceanography

Faculty

Faculty of Science

Abstract

The structure and variability of inter-annual and intra-seasonal circulation anomalies and their precursors are investigated over northern Tanzania. Area rainfall departures are computed in the period 1960-1990 and wet and dry years are identified. Northern Tanzania experiences bimodal rains centred on the seasons: March to May (Masika) and October to December (Vuli). Station inter-correlations in Masika are of order +0.4, whereas in Vuli correlations are of order+0.7. Using composite sequences of gridded sea surface temperatures (SST),outgoing longwave radiation (OLR) and winds in the lower- and upper troposphere, the- precursors of flood and drought are assessed. Composites are averaged for the specific season and for periods 2, 4 and 6 months prior; and the historical mean is subtracted to produce anomalies. In this way, evolution of extreme rainfall scenarios is tracked for predictive purposes. The patterns analysed show that when positive SST anomalies persist in the Indian Ocean, sympathetic negative OLR values and local rainfall enhancement are observed. Using pentad (5 day mean) rainfall time series for the area, wet spells are analysed for structure and evolution. ECMWF data for the peak in Vuli rainfall each year from 1986 to 1991 are averaged to form a single composite. The historical mean is subtracted from the composite to produce anomalies for the wet spell and the preceeding pentads.

Keywords

Oceanography

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