Summary
This study investigates the circulation anomalies associated with the intraseasonal evolution of wet and dry years over western Tanzania (29–37° E, 11.5–4.75° S) and how the onset and withdrawal of the rainy season as well as its wet spell characteristics are modified. It is found that for wet years, the rains begin earlier and end later, with strong wet spells occurring during the season, and there tend to be a greater number of moderate wet spells (although not necessarily more intense wet spells) than in dry years. In dry years, late onset and early cessation of the rainy season occur, often with an extended dry spell soon after the onset, and there tend to be a greater number of dry spells within the season. Large negative outgoing long wave radiation (OLR) anomaly values tend to be located between 20° and 40° E with anomalous westerly flow at 850 hPa occurring across the continent from 10° E to the tropical western Indian Ocean during wet spells in the anomalously wet seasons. Anomalously dry seasons are characterised by large positive OLR anomalies over 30–50° E as well as easterly anomalies at 850 hPa and westerly anomalies at 200 hPa. Eastward propagating intraseasonal anomalies are slower during the wet years implying that the convection remains over Tanzania longer. On the intraseasonal scale, Hovmoeller analyses of OLR and 850 and 200 hPa zonal wind indicate that convection over western Tanzania may be associated with a flux of moisture from the tropical southeast Atlantic and Congo basin followed by weak easterlies from the tropical western Indian Ocean.
On interannual scales, wet (dry) years are characterized over the Indian Ocean by weaker (stronger) equatorial westerlies and weaker (stronger) trades that lead to less (more) export of equatorial moisture away from East Africa and increased (decreased) low-level moisture flux convergence over southern Tanzania, respectively. These anomalies arise from an anticyclonic (cyclonic) anomaly over the tropical western Indian Ocean during wet (dry) austral summers that may be related to cool (warm) SST anomalies there. Large scale modulation of the Indian Ocean Walker cell is also evident in both cases, but particularly for the dry years.
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Current affiliation: Tanzania Meteorological Agency, P.O. Box 3056, Dar es Salaam, Tanzania
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Mapande, A., Reason, C. Links between rainfall variability on intraseasonal and interannual scales over western Tanzania and regional circulation and SST patterns. Meteorol. Atmos. Phys. 89, 215–234 (2005). https://doi.org/10.1007/s00703-005-0130-2
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DOI: https://doi.org/10.1007/s00703-005-0130-2