Mid-21st century changes in extreme events over Northern and Tropical Africa

Many climate model studies project global increases in extreme events, including droughts and intense rainfall, in association with increasing atmospheric greenhouse gases. The purpose of this study is to project changes in extreme temperature and precipitation over northern and tropical Africa for the mid-21st century due to increased CO2 forcing/global warming using a regional climate model, and to evaluate confidence in those predictions.

Output from a regional climate model is used to evaluate six main indicators for climate extremes over Africa. They are extreme temperature range, daily diurnal temperature range, heat wave days, number of dry days, number of extreme wet days, and extreme wet day rainfall intensity. Confidence in projections is evaluated by validating against observational and reanalysis data to assess how realistically the model can reproduce each of these indicators, assessing the degree of agreement among individual ensemble members for each indicator, and comparison with other modeling studies of the 21st c. when available.

The projections indicate a widespread decrease in the annual extreme temperature range including over West Africa (i.e., 0.5 – 1 K), the Sahel (i.e., 2 – 3 K), and Congo basin (i.e., up to 4 K). The daily diurnal temperature range is predicted to decrease over West Africa and Central Africa during the boreal spring and fall by 0.3 – 1.2 K, over the Sahel during the West African monsoon season by 0.5 – 1.5 K, and across the Congo basin during the boreal winter and spring by 0.5 – 2.0 K. The daily temperature range increases over the Horn of Africa during the boreal winter by 0.3 – 1.2 K, and over Kenya/Tanzania during the boreal summer by 0.2 – 1.5 K associated with greater warming of the daily maximum temperature due to a decrease in the atmospheric relative humidity. A 20 to 120 day increase in the number of heat wave days is predicted over most of the Sahel and Saharan Africa.

The number of dry days is predicted to decrease by 15 - 25% over the Congo basin and by 3 – 7% over Central Africa and the Sahel east of 0°E, while across the coastal plains of Tanzania, Kenya, and Somalia the number of dry days is projected to increase by 5 – 10% associated with a weakening of the boreal spring long rains. The number of extreme wet rainfall days is projected to increase over West Africa by 40 – 60%, the southern Sahel by 50-90%, and the Ethiopian Highlands by 50 - 90%. Over the Congo basin the number of extreme wet days decreases by 30 – 60%, primarily associated with a reduction during the short rains of the boreal fall. Conversely, the number of extreme heavy rainfall days over Kenya and Tanzania is predicted to increase, associated with changes in the short rains of the boreal fall/early winter.