The climatology and characteristics of drought in the eastern Cape of South Africa

Abstract Along the south‐eastern seaboard of Africa, a drought‐prone area is found between the humid subtropics and a deser. Convective mid‐summer precipitation in the north‐east is contrasted with stratiform winter rains in the west. Mean annual precipitation is <500 mm while potential evaporation is >2000 mm. According to time series analysis of rainfall records, dry spells recur at intervals of 3·5 and 18·2 years, most notably in the period 1940–1970. By contrast, excessive rainfall characterized the mid‐1970s. Extreme dry and wet years are analysed for circulation, convection and sea‐temperature differences. Below‐normal sea temperatures to the south‐west and east of the study area are associated with drought in the eastern Cape. Circulation differences indicate that low‐level westerly winds sweep across the region, in association with a stationary subtropical westerly wave of wavelength 3000 im and amplitude 1500 km. A positive outgoing longwave radiation anomaly indicative of reduced cloudiness and water vapour is noted in a north‐west—southeast band extending from Namibia across the eastern Cape watershed. Anticyclonic vorticity anomalies of the order of 10 −5 s −1 across the subtropics of southern Africa are common in dry years. Lower divergence to upper convergence anomalies of the order of 2 × 10 −6 s −1 are the driving forces of subsidence over the eastern Cape. The analysis of climatic patterns associated with dry spells will assist in forcasting seasonal rainfall, planning agricultural production, and optimizing water resources.