Spatio-temporal Dynamics of the Onset and Cessation of OND Rainy Season across Bimodal Regions of Tanzania

Abstract

This study investigates the temporal variability of the onset, cessation, and duration of the October-November-December (OND) rainy season in the bimodal rainfall region of Tanzania. Utilizing daily precipitation data from the Multi-Source Weighted-Ensemble Precipitation dataset for the period 2014–2023, the study employs the Accumulated Daily Index method to determine the onset and cessation of rains, with the seasonal duration computed as the difference between these dates. Variability in onset, cessation, and season length was quantified using the Coefficient of Variation. The findings highlight significant spatial variability in onset, with early rains in northwestern areas (e.g., Bukoba) and coastal zones, commencing as early as late September. Most regions experience onset between early October and late November, while cessation of rains is typically observed between mid-January and early February, with coastal and Lake Victoria Basin regions showing greater variability. The study identifies significant variations of the OND rainy season, particularly in the western and coastal areas, with shifts toward a November-December-January (NDJ) rainfall season in some regions. This suggesting a transition from OND to NDJ regime in some regions. These results underline the necessity for enhanced forecasting models to address the implications of altered rainfall regimes on agriculture, water management, disaster mitigation, and other socio-economic activities. This study provides critical insights into rainfall variability, aiding in the formulation of adaptive strategies to optimize socio-economic benefits while mitigating risks associated with climate variability.