Spatio-temporal variability of rainfall and temperature extremes over Southern Africa

Abstract

Climate change can be reflected in terms of  shift in mean climatology as well as shift in the distribution of rainfall and temperature extremes over time. Southern Africa has distinct climate regimes that includes arid and semi-arid climates as well as relatively humid climate. This results in distinct spatio-temporal response of the region to climate change. In this study, major extreme climate indices for the region are derived from daily CHIRPS rainfall, ERA-Interim minimum, maximum and average temperatures to understand the spatio-temporal variability. The long term mean of the coldest annual day-time maximum temperature (90thpercentile) is observed over Lesotho highlands and adjoining areas in South Africa (21.0oC) whereas the warmest day-time temperature (37.4oC) is observed over areas bordering South Africa, southern Botswana and Namibia during the recent four decades. The trend in this indices shows warming (up to 1oC/decade) over southwestern South Africa, along coastal strips of South Africa, much of Mozambique, northwestern Zimbabwe, northern and western Zambia, eastern Angola and cooling over central Botswana. The annual night-time minimum temperature (10th percentile) is increasing northward in contrast to day-time maximum temperature which, in addition, exhibits zonal gradient. In terms of frequency, 10 to 12% of  the days in a year experienced maximum temperature above 90th percentile whereas 9 to 11 % of the days in a year observe night-time minimum temperature below 10th percentile. The annual heat wave duration indices show longest duration (8 days) over Southern Angola, northern Namibia, southern  Zambia, northern Zimbabwe and Botswana and decrease from here northward and southward. Annual number of days with rainfall  above 10 mm is about 10 days over western South Africa, Botswana, southern Zimbabwe, Namibia and southern Angola. In contrast,  it is in the range of 32 to 74 days over northern Angola, Zambia and Mozambique with increasing trend over Botswana, eastern parts of Zambia and Angola.  Similar trend in maximum 5 to 10 day total rainfall is observed over the same areas. Central part of the Southern Africa region exhibits the highest annual continuous dry days (147 to 254 days) whereas the southern and northern parts of the region has the lowest annual continuous dry days ( about 76 days). The long term mean of maximum annual continuous wet days increases northward from 3 days over western parts of South Africa and Namibia to 21 days over  Angola and northern Malawi. Besides secular trend, the rainfall extreme indices have coherent cyclic modes of variability with a period of 3.6 to 3.8 years accounting for 9 to 12 % of the total variance whereas the temperature extreme indices show periodicity of about 2.5 years accounting for 18 to 19% of the total variance. The periodicities are possibly associated with ENSO events that modulate interannual variability.