Extreme rainfall events over the Okavango river basin, southern Africa

Characteristics of extreme rainfall events in the highly biodiverse Okavango River Basin (ORB) in west central southern Africa are not well understood. Hence, this study analyses the frequencies, intensities, spatial distributions, variability and trends of extreme rainfall events for the main rainy season, January-April (JFMA). The events are accumulated over 1-day (DP1) and 3-days (DP3). To identify these events, a method that accounts for both event intensity and spatial extent is used. On average, DP1 (DP3) events contribute -10% (-17%) rainfall totals but in some years their contributions exceed 30%. Tropical-extratropical cloud bands are responsible for most of the events with tropical lows also important. The considerable interannual variability in extreme events appears related to El Nin & SIM;o-Southern Oscillation (ENSO) and the Botswana High. Although ENSO influences the extreme events as well as rainfall totals more generally over southern Africa, by far the wettest season over the iconic Okavango Delta region, in Ngamiland, Botswana, occurred during neutral JFMA 2017. These heavy rains resulted from a deeper Angola Low, weaker mid-level Botswana High which favoured convection in the region together with anomalous westerly moisture fluxes from the tropical South East Atlantic and enhanced low level moisture convergence over most of the region during January-early March. A dry period from mid-March was broken by the second most intense rainfall event on April 22nd, resulting from a cut-off low. Significant increasing trends were found in DP1 frequencies, as well as in rain-days and rain totals over many areas. These trends have important implications for water and agricultural management, and wildlife conservation in the highly biodiverse ORB.