<p>The March-May rainfall (long rains hereinafter) in Tanzania exhibits decadal fluctuation with a notable abrupt shift observed around the year 1999. Previous research has suggested possible links with oceanic forcing, but the underlying mechanisms by which sea surface temperature (SST) variability influences the long rains and its changes remain unclear. In this study, we examined the associated decadal variation of the atmospheric circulation and investigated mechanisms driving the decadal variation of the long rains. It is found that the long rains decreased significantly after 1999. The associated wind anomalies show an upper-level convergence over Tanzania and cyclonic circulation around Madagascar. This was associated with anomalous lower-level divergence, subsidence, moisture flux divergence, and decreased long rains in Tanzania. Based on the mechanisms, the SST warming in the South Pacific Ocean (SPO) played a key role in the decadal shifts of rainfall around 1999 through Walker circulation across the Indian Ocean. After 1999, the warm SPO is accompanied by locally enhanced convection. This convection is associated with upper-level outflow in the western tropical Pacific and convergence over Tanzania and is likely an important atmospheric connection between the South Pacific and local atmospheric circulations related to decadal variation. In addition to Walker circulation variability, the long rains are also influenced by the meridional shift of the Inter-Tropical Convergence Zone (ITCZ). Higher (lower) rainfall in Tanzania corresponds to an equatorial (off-equatorial) position of the ITCZ. These findings are helpful for better understanding and prediction of the long rains.</p>