Understanding Possible Physical Mechanisms Associated With Variability of March to May Seasonal Extreme Wettest Days Rainfall in Tanzania

Abstract

Extreme rainfall remains the most impactful natural disaster affecting the environment and ecological system in Tanzania. Understanding possible physical mechanisms behind these events is crucial for mitigating associated risks. Therefore, the interannual variability of extreme wettest days (EWDs) during March to May from 1981 to 2020 was examined using daily ground observations and gridded data from the Climate Hazards Group InfraRed Precipitation with Station data. EWDs were determined by using 99th percentile-based method. The variability of EWDs was assessed using empirical orthogonal function (EOF) and wavelet methods. To understand their connectivity with physical mechanisms, methods such as regression and correlation were applied in the analysis. Results show a significant increase in EWDs under 95% confidence level, especially in recent years, with a notable peak in 2020, explaining 19.3% of the variance in the leading EOF1, which is positively loaded across most of Tanzania. EOF1's principal component exhibits interannual variability with predominantly positive values, indicating a close relationship between high rainfall regions and EWDs. Wavelet analysis reveals significant oscillations of EWDs at 2 to 5-year intervals, linked to climate phenomena like the Indian Ocean Dipole and El Niño-Southern Oscillation. Climatologically, southwest-oriented vertical integrated moisture flux (VIMF) vectors are predominant, moving westward over Tanzania due to an anticyclonic system in the southwestern Indian Ocean. The study concludes that EWD variability is influenced by the convergence of southerly and westerly VIMF vectors along Tanzania's coastal zone and the western Indian Ocean. Warming sea surface temperature anomalies in various oceans (i.e., northwestern Atlantic Ocean, tropical Indian Ocean and northern Pacific Ocean) are positively correlated with EWDs in Tanzania. These anomalies enhance or suppress EWDs by creating low (upper) level convergence (divergence) winds over the tropical Indian Ocean, linking to the ascending (sinking) limb of Walker-type circulation over the Indian (Pacific and Atlantic) Ocean.