Drought metrics and temperature extremes over the Okavango River basin, southern Africa, and links with the Botswana high

The Okavango River Basin (ORB), including the World Heritage site Okavango Delta, is a region of high biodiversity projected to suffer increased early summer drying under climate change. Little work has been done on drought over this sensitive region. Here, various drought metrics are analysed over the ORB. These include a cumulative drought intensity index, based on the product of maximum temperature anomaly and maximum duration of a dry spell, and the Standardized Precipitation-Evapotranspiration Index. Strong gradients in dry spell and hot day frequencies shift south over the ORB from August to November as the tropical rain-belt moves increasingly south of the equator, the Congo Air Boundary declines and the Botswana High strengthens and moves southwestwards. By December, the tropical gradient in dry spell frequencies has vanished while that across the Limpopo River and southern ORB region, where the Botswana High is centred, is prominent. sub-seasonal analyses highlight October-November 2013-2021 as particularly dry and hot over the Okavango Delta region. This dry and hot epoch appears related to a stronger and southward shifted Botswana High and reduced low-level moisture convergence. On interannual scales, strong relationships were found with the Botswana High and El Nino-Southern Oscillation (ENSO). The early summer shows a strong drying-warming trend, related to a significant strengthening of the Botswana High. These trends, together with the Coupled Model Intercomparison Project Phase 6 projected early summer drying over southern Africa, may impact severely on the sensitive ecosystems of the ORB and on agriculture, with important implications for the management of agricultural activities, water resources and biodiversity.