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Decadal variability in the Botswana High and its relationships with rainfall, temperature and circulation over the southern African region are examined. This High exists in the mid-troposphere over subtropical southern Africa from early spring through to early autumn. Although its seasonal and interannual variability have been previously studied, lower frequency variability in the Botswana High has not. An EOF analysis of 500hPa geopotential height from the NCEP Twentieth Century reanalyses reveals two modes both of which display substantial decadal to multidecadal variability. These modes, which explain almost 90% of the variance, reflect shifts in the summer core position of the High over central Namibia / western Botswana and in its strength. This decadal variability in the Botswana High is associated with sizeable changes in regional rainfall and surface temperature as well as changes in the onshore flow of moist marine air and uplift over many parts of southern Africa. Such changes are large enough to have important impacts on regional society and may also obscure climate change signals over the region.