Intra-seasonal variability over the northeastern highlands of Tanzania

摘要

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.