Dry Spells, Wet Days, and Their Trends Across Southern Africa During the Summer Rainy Season

Dry spell frequency and wet day occurrence during the summer rainy season across southern Africa are examined using daily rainfall for 1982-2019. Evidence shows two sharp gradients in dry spell frequency, extending southeast from southern Angola to the African south coast near the westernmost boundary of preferred cloud band occurrences (diagonal gradient), and west from the Limpopo River Valley along 22-24 degrees S (meridional gradient) (along 14-16 degrees S in early summer) related to regional topography. Trend analysis suggests that the diagonal gradient has weakened during 1982-2019 along with some other large areas in southern Africa whereas this is only true for part of the meridional gradient. Contrastingly, wet days and Normalized Difference Vegetation Index have significantly increased in several important agricultural areas in southern Africa. Evidence suggests these changes are related to those in the midlevel Botswana High, El Nino Southern Oscillation, Southern Annular Mode, and subtropical Indian Ocean dipole.