Southern Africa Summer Drought and Heat Waves: Observations and Coupled Model Behavior

Abstract

Observations of daily maximum temperature (Tx) and monthly precipitation and their counterpart fields from three coupled models from the Coupled Model Intercomparison Project Phase 3 (CMIP3) archive have been used for exploratory research into the behavior of heat waves, drought, and their joint occurrence across the southern Africa subcontinent. The focus is on seasonal drought and heat waves during austral summer [December?February (DJF)] for land areas south of 15°S. Observational results (Tx available only for South Africa) are compared with those based on CMIP3 twentieth-century climate runs for a common analysis period of 1961?2000 while climate projections for the twenty-first century are also considered using the Special Report on Emissions Scenarios (SRES) A1B forcing scenario. Heat waves were defined when daily Tx values exceeded the 90th percentile for at least 3 consecutive days, while drought was identified via a standardized index of seasonal precipitation. When assessed over the entire study domain the unconditional probability of a heat wave, and its conditional probability given drought conditions, were similar in the models and (for a smaller domain) observations. The models exhibited less ability in reproducing the observed conditional probability of a heat wave given El Niño conditions. This appears to be related to a comparatively weak seasonal precipitation teleconnection pattern into southern Africa in the models during El Niño when drought conditions often develop. The heat wave?drought relationship did not substantially change in climate projections when computing anomalies from future climate means. However, relative to a 1981?2000 base period, the probability of a heat wave increases by over 3.5 times relative to the current climate. Projections across the three models suggest a future drying trend during DJF although this was found to be a model-dependent result, consistent with other studies. However, a decreasing trend in the evaporative fraction was identified across models, indicating that evaluation of future drought conditions needs to take into account both the supply (precipitation) and demand (evaporation) side of the surface water balance.