Inter-Annual Variation and Its Large-Scale Circulation Drivers of October-December Rainfall Onset Date in Tanzania

This study investigates the inter-annual variation of the October-December (OND) rainfall onset date in Tanzania, including spatial-temporal patterns, atmospheric anomalies, and climatic drivers such as the Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO), utilizing daily rainfall data, reanalysis datasets, and climate indices (1993-2023). The onset dates were determined using a threshold-based method, while spatial and temporal variability was analyzed through Empirical Orthogonal Function analysis. Results show that: i) the onset dates were determined using a threshold-based method, while spatial and temporal variability was analyzed through Empirical Orthogonal Function analysis. In Tanzania, there is a unimodal rainfall regime, such as the Southern, Southern Coast, and Southwestern Highlands, rainfall onset typically occurs later, around November or December. ii) Anomalous easterly upper-level winds with convergence and westerly lower-level winds with divergence at low-level, suppress moisture transport with descending over the region will delay rainfall onset during late Onset date (LOD) years. In contrast, the opposite situation will be during Early Onset Date (EOD) years. iii) Both ENSO and IOD have close relationship with the rainfall onset day in Tanzania. Warm SSTA in eastern-central Pacific and western Indian Ocean during El Niño years/positive IOD years, can induce LOD associated circulation patterns and then delay rainfall onset day over most part of the region, especially over east part of the region. However, the warm SSTA in western Pacific to eastern Indian Ocean during La Niña/negative IOD years will induce EOD-like circulation patterns, which will make rainfall onset earlier over most part of the country and the Indian Ocean plays more critical role on the rainfall onset day in Tanzania.