Projected changes in extreme precipitation in Africa under global warming. Part 1: Southern Africa

This study investigates likely changes in mean and extreme precipitation in southern Africa in response to changes in radiative forcing using an ensemble of global climate models prepared for the IPCC Fourth Assessment Report (AR4). Extreme seasonal precipitation is defined in terms of 10-year return levels obtained by inverting a generalised Pareto distribution fitted to excesses above a pre-defined high threshold. Both present (control) and future climate precipitation extremes are estimated. The future to control climate ratio of 10-year return levels is then used as an indicator for the likely changes in extreme seasonal precipitation.

A Bayesian approach to multi-model ensembling is adopted. The relative weights assigned to each of the model simulations is determined from bias, convergence and correlation. Using this method, the probable limits of the changes in mean and extreme precipitation are estimated from their posterior distribution.

Over the western parts of southern Africa, an increase in the severity of dry extremes parallels a statistically significant decrease in mean precipitation during austral summer months. A notable delay in the onset of the rainy season is found in almost the entire region. An early cessation is found in many parts. This implies a statistically significant shortening of the rainy season.

A substantial reduction in moisture influx from southwestern Indian Ocean during austral spring is projected. This and the pre-austral spring moisture deficits are possible mechanisms delaying the rainfall onset in southern Africa. A possible offshore (northeasterly) shift of the tropical-temperate cloud band is consistent with more severe droughts in the southwest of southern Africa and enhanced precipitation further north in Zambia, Malawi and northern Mozambique.

This study shows that changes in the mean vary on relatively small spatial scales in southern Africa, and differ between seasons. Changes in extremes often, but not always, parallel changes in the mean precipitation.