Variations in extreme wet days during the OND rainy season in Tanzania and their linkages to atmospheric-oceanic anomalies

Extreme rainfall remains one of the leading natural disasters in Tanzania, affecting the majority of citizens and the country’s economy. Understanding the seasonal patterns and associated atmospheric and oceanic anomalies could help mitigate their impacts. The current study investigates changes in seasonal extreme wettest days (EWDs) rainfall in Tanzania and the associated atmospheric and oceanic anomalies during October-December (OND), from 1981 to 2020. EWD is defined as days when daily rainfall meets or exceeds the 99th percentile. Singular value decomposition (SVD), Mann-Kendall, t-test, and Pettitt test methods were applied in this study. The results indicate that approximately 2 EWD events were observed in many areas during OND, with fewer in central regions. Areas such as southwestern Lake Victoria Basin (LVB) and parts of the coastal regions experienced a significant increase in EWDs. The time series showed a non-significant positive trend, with a notable change in 2010. Generally, the country experienced an increase in EWD events in the recent decade. This increase is potentially associated with a shift in tropical SST from a cold phase to a warm phase, the incidence of low-level troughs over the tropical Indian Ocean (TIO), an increase in moisture flux convergence at the TIO, vertical velocity negative anomalies with the ascending limb of Walker-type circulation over the western Indian Ocean, and increased evaporation over the TIO and Congo basin. This is in line with the SVD results, which showed a strong coupling between EWDs in Tanzania and SST over the TIO, Pacific, and Atlantic Oceans, with significant correlation coefficients at the 99% confidence level of R=0.75, R=0.74, and R=0.76, respectively. This study provides valuable findings for farmers, forecasters and water users in Tanzania