Changes in March to May rainfall over Tanzania during 1978-2017 Changes in March to May rainfall over Tanzania

This study assessed the change in March to May (MAM) rainfall over Tanzania during 1978-2017. It is revealed that the MAM rainfall experienced more years of negative (positive) anomalies after (before) the late 1990s. The atmospheric and oceanic background underlying such a change in MAM rainfall was addressed. The results show that the recent decrease in MAM rainfall is associated with the reduction of water vapour transport from the Indian Ocean to Tanzania, the enhancement of tropospheric moisture divergence, and the decrease of convective activities over Tanzania. Changes of sea surface temperature (SST) in the 1990s from a cold phase to a warm phase over the southwestern Indian Ocean and western tropical Pacific Ocean and that from a warm phase to a cold phase over the southern Atlantic Ocean also have a significant contribution. Associated with the SST warming over the southwestern Indian Ocean, an anomalous cyclonic circulation prevails over the tropical Indian Ocean in the upper level, favourable for the increase of subsidence in Tanzania. Concurrently, the Mascarene High shifts eastward, which may weaken the water vapour transport from the southern Indian Ocean toward Tanzania. The warming of SST over the western tropical Pacific Ocean is linked to a Walker-like circulation with the descending limb over the western Indian Ocean and Tanzania, conducive to the subsidence over Tanzania. Related to the SST cooling over the southern Atlantic Ocean, anomalous atmospheric convergence in the upper level is dominant over Tanzania, concurrent with the easterly outflow toward the Congo basin and the westerly and northerly outflow toward the western Indian Ocean from Tanzania in the low level, favouring the decrease of MAM rainfall.