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The intra-seasonal variability of rainfall over the northeastern Highlands (NEH) of Tanzania during the March, April, May (MAM) rainfall season and associated circulation anomalies have been studied for selected wet and dry years. Station rainfall data were used to extract the onset, peak, and cessation dates of the rainfall season and National Center for Atmospheric Research/National Center for Environmental Prediction (NCEP/NCAR) data were used to study the atmospheric circulation features associated with the intra-seasonal variability. It was found that the anomalously wet MAM seasons over the NEH of Tanzania tend to result from an earlier onset of the rains and a late cessation and thus, a longer-than-average rainfall season, whereas, dry years were characterized by a shorter-than-average duration of the rains. The moisture flux plots indicate that the composite wet MAM seasons were associated with advection of moisture from both the Indian Ocean and the Congo Basin. For the dry MAM seasons, a cyclonic anomaly off the Somali coast leads to southerly moisture flux anomalies over Kenya and westerly anomalies over the Indian Ocean which, together with easterly moisture flux anomalies over the Congo Basin, act to divert and transport low-level moisture away from Tanzania. Lag correlations between time series of moisture over the Congo (0-8°S; 20-29°E) and MAM rainfall over the NEH indicated a phase locking of the two variables that occurred during the years with larger rainfall anomalies. A significant correlation was observed two months before the onset of the rainfall season in January that indicates that some predictability of the MAM seasonal rainfall total based on the Congo Basin zonal wind may exist. However, given the constraints of the available data and the regional atmospheric circulation, it is possible that additional predictors may be required when developing forecast models of the MAM seasonal rainfall. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Extreme weather and climate events have major negative effects on Tanzania's economy. Recently, the prolonged drought from 1998 to 2005 caused devastating crop failures, livestock losses and reductions in the country's water reservoir levels, which, in turn, created food shortages and rationing of hydroelectric power and water. The present study analyses the atmospheric circulations associated with the 1998 to 2005 drought that affected the northeastern highlands of Tanzania. Analysis of large-scale circulation indicates that the drought during the October to December (OND) short rainy seasons, from 1998 to 2005, was related to westerly low-level moisture flux divergence and subsidence over northern Tanzania associated with an eastward shift of the Walker cell. For the March to May (MAM) long rainy seasons, the prolonged drought was related to strong moisture flux divergence over the region and subsidence associated with the occurrence of the descending branch of a Walker-type circulation over the region. Rainfall onset was undefined for most years in both the OND and the MAM seasons, and, in all cases, unevenly distributed rains with prolonged dry spells were experienced in a shorter than average rainy season. However, an out-of-season rainfall peak occurred in January, after the end of the OND rains. This delayed peak has important implications for agricultural and water resource activities. The circulation patterns observed in the Tanzanian region from 1998 to 2005 were similar to those experienced during a previous prolonged drought (1973 to 1976), suggesting that some predictability of drought may exist. [ABSTRACT FROM AUTHOR]