Rainfall variability in Northern Tanzania in the March-May season (long rains) and its links to large-scale climate forcing

Mean rainfall in Northern Tanzania is characterized by 2 annual maxima: in March-May, the long rains, and in October-December, the short rains. The interannual variability of rainfall in the March-May season and its relationships to large-scale fields from the NCEP/NCAR Reanalysis in the African-Indian Ocean sector were analysed. It was found that the March-May precipitation has characteristics different to that in October-December, the former being much less persistent within the season and being associated with intraseasonally changing large-scale patterns of surface temperature, sea-level pressure and winds. March and April rainfall anomalies are linked to zonal thermal contrast between the Indian Ocean and the Eastern African land mass, to zonal surface winds anomalies and to vertical velocity anomalies. On the other hand, May rainfall anomalies are associated with a meridional surface temperature contrast between the Indian Ocean and the Asian continent and meridional surface winds anomalies, indicating a relationship to the Indian Monsoon. Local convergence seems to play a smaller role than for March anomalies, Indeed, Northern Tanzanian rainfall anomalies in May are correlated with the area-averaged Indian rainfall in the following months of June, July and August. The consequences for possible statistical seasonal prediction schemes are briefly discussed.