South-East Atlantic warm events: composite evolution and consequences for southern African climate

Abstract

Tropical South-Eastern Atlantic sea surface temperature (SST) fluctuations in the period 1951-1991 are investigated in the context of regional climate impacts. Departures of rotated principal component 1 of late summer outgoing longwave radiation (OLR) reflect a dipole between the south-western plateau (Kalahari) and the Angolan coast, which explains 47% of total field variance over southern Africa. The principal component scores are correlated with SST off Angola and with other tropical regions exhibiting global El Niño responses. The spatial loading implies diminished convective rainfall over the Kalahari when tropical SST to the northwest of Angola is > 0,7°C (∼1 standard deviation) above normal, for example in 1984. SST fluctuations in discrete areas of the tropical South-Eastern Atlantic are examined. The annual cycle is dominated by an inverse relationship between SST and pressure, with a maximum (minimum for pressure) in the months February to April. South-easterly trade winds are prevalent throughout the year, but are considerably weaker in the "bight" of Angola. Composite warm events in the late summers of 1963, 1973, 1984 and 1988 are analysed using normalized departures. The results indicate that oceanic warm events are locally generated in the region 0–10°S, 0–15°E, following a reduction in trade winds locally and to the west in the preceding year. Maximum SST is attained typically in March, following a sudden increase after the previous November. The composite precursors suggest that empirical predictability of South-Eastern Atlantic warm events is limited. Relationships between the 1995 event and drought over the Kalahari are discussed. Further research on sea levels, thermocline depth and currents, coupled to numerical modelling, could offer improved insights.