Contribution of very heavy rainfall events to MAM and OND rainy seasons in Tanzania and their linkages to IOD and ENSO events

Tanzania experiences extreme rainfall events, occurring in few hours or single day with an exceptionally large amount, followed by dry episodes during the rainy seasons. These events skew seasonal rainfall and lead to uneven rainfall distribution, which adversely affects various sectors in the country. The present study assesses the contribution of very heavy rainfall events (VHREs) on the seasonal distributions of rainfalls over the two main rainy seasons of Tanzania, i.e., March to May (MAM) and October to December (OND), and their relationship with the Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO) events from 1981 to 2020. VHREs, defined as daily rainfall exceeding 100 mm, contribute disproportionately to total seasonal rainfall, altering rainfall distribution and sectoral planning. Results revealed that MAM rainy season features more VHREs (~ 360 events) than OND rainy season (~ 90 events), with non-significant upward trend in MAM and significant positive trend in OND. High VHRE contributions were recorded in Arusha (33.8%), Same (29.5%) and Tabora (24.8%) during MAM, while OND VHREs contributed significantly in Pemba (54.5%), Tanga (44.4%), and Same (39.2%). VHREs occurred most commonly in northeastern, coastal, and southwestern regions during MAM and were confined to the coastal belt in the OND. The anomalous sea surface temperature shows that El Niño events largely enhance VHREs in both seasons, but positive IOD episodes are more dominant during OND. The results underscore the significant contribution of VHREs to seasonal and annual rainfall total and their relevance for seasonal forecasts, agricultural planning, and disaster management in Tanzania.