El Niño, climate change, and Southern African climate

Abstract

SUMMARYThe El Nin Äo phenomenon involves a large-scale warming of the equatorial eastern and central Paci®c Ocean. Recent developments in the El Nin Äo±Southern Oscillation (ENSO) phenomenon have raised concerns about climate change. In this review paper, these recent developments are critically assessed and forecasts of possible future changes are reviewed. Since the late-1970s, El Nin Äo episodes have been unusually recurrent, while the frequency of strong La Nin Äa events has been low. Prolonged/recurrent warm event conditions of the ®rst half of the 1990s were the result of the persistence of an anomalously warm pool near the date line, which, in turn, may be part of an abrupt warming trend in tropical sea-surface temperatures that occurred in the late-1970s. The abrupt warming of tropical sea-surface temperatures has been attributed to the enhanced-greenhouse effect, but may be indicative of inter-decadal variability: earlier changes in the frequency of ENSO events and earlier persistent El Nin Äo and La Nin Äa sequences have occurred. Most forecasts of ENSO variability in a doubled-CO 2 climate suggest that the recent changes in the tropical Paci®c are anomalous. Of potential concern, however, is a possible reduction in the predictability of ENSO events given a warmer background climate.El Nin Äo events usually are associated with below-normal rainfall over much of southern Africa. Mechanisms for this in¯uence on southern African climate are discussed, and the implications of possible changes in ENSO variability on the climate of the region are assessed. Recent observed changes in southern African climate and their possible relationships with trends in ENSO variability are investigated. The El Nin Äo in¯uence on rainfall over southern Africa occurs largely because of a weakening of tropical convection over the subcontinent. A warming of the Indian Ocean during El Nin Äo events appears to be important in providing a teleconnection from the equatorial Paci®c Ocean. The abrupt warming of the tropical Paci®c and Indian oceans in the late-1970s is probably partly responsible for increasing air temperatures over southern Africa, and may have contributed to a prolongation of predominantly dry conditions. A return to a wet phase appears to have occurred, despite the persistence of anomalously high sea-surface temperatures associated with the late-1970s warming, and a recordbreaking El Nin Äo in 1997/98.

Variability and trends in the directional wave climate of the Southern Hemisphere

“…Over the past few decades, an increased number of ElNino events have occurred relative to previous decades (Trenberth and Hoar, 1997;Mason, 2001). As El-Nino events are associated with a clockwise rotation of wave direction in the south-western Pacific Ocean (Goodwin, 2005), the increase in El-Nino events is responsible for the clockwise rotation in wave direction observed in this region, over the period 1980-2001 (Figure 10).…”

Rainfall trends in the KwaZulu‐Natal Drakensberg region of South Africa during the twentieth century

“…(e.g. Lindesay et al, 1986;Lindesay, 1988;Van Heerden et al, 1988;Jury et al, 1994;Mason, 2001) and that ENSO warm events [negative values of the Southern Oscillation Index (SOI)] are frequently associated with less than average rainfall and drought over much of southern Africa (e.g. Tyson, 1986;Ropelewski and Halpert, 1987;Janowiak, 1988;Van Heerden et al, 1988;Mason and Jury, 1997).…”

“…Cold events (La Niña) as expressed by positive values of the SOI bring increased rainfall because of the location of the cloud band over southern Africa (Tyson and Preston-Whyte, 2000). For a full review of the mechanisms by which change in the tropical Pacific Ocean affects the atmosphere over southern Africa, see Mason and Jury (1997); Mason and Tyson (2000); Tyson and Preston-Whyte (2000) and Mason (2001). The influence of ENSO events is strongest during the summer rainfall months of December to March when the El Niño/La Niña events have reached maturity (Mason and Jury, 1997).…”

Global analysis of runs of annual precipitation and runoff equal to or below the median: run length

“…• variability of El Niño-southern oscillation (ENSO) teleconnections (Elliot and Angell, 1988;Zhang et al, 1997;Kruger, 1999;McCabe and Dettinger, 1999;Power et al, 1999a;Mason, 2001); • the Pacific decadal oscillation (PDO; Mantua et al, 1997;Barlow et al, 2001; • variability of North Atlantic oscillation (NAO) teleconnections (Hurrell, 1995);…”