Investigating the response of the Botswana High to El Niño Southern Oscillation using a variable resolution global climate model

The Botswana High is an important component of the regional atmospheric circulation during austral spring, summer and autumn. While the high tends to be stronger during El Niño and weaker during La Niña, its direct response to El Niño Southern Oscillation (ENSO) remains unknown. To that end, a variable resolution global climate model (Model Prediction Across Scales version 7, hereafter MPAS) is applied with relatively high resolution (48-km grid spacing) over Southern Africa and a coarser resolution (240-km grid spacing) over the rest of the globe for the study period 1980–2010. The first model experiment uses observed sea surface temperatures (SSTs) everywhere during the study period, while the second experiment uses observed SSTs everywhere except over the Pacific Ocean, where monthly climatological SSTs are imposed. The model results were validated against satellite data (Global Precipitation Climatology Project, GPCP) and reanalysis datasets (Climate Forecast System Reanalysis, CFSR; European Centre for Medium-Range Weather Forecasts version 5, ERA5). The results of the study show that the MPAS model gives a credible simulation of the temporal variability of the Botswana High, the seasonal rainfall and 500-hPa geopotential heights over Southern Africa. In the absence of ENSO forcing, the amplitude of the Botswana High variability reduces but the signal of the variability remains. Hence, this study shows that ENSO enhances the strength of the Botswana High but does not aid in the formation of the high.