3.6.2 El Niño-Southern Oscillation and Tropical/Extratropical Interactions

3.6.2.1 El Niño-Southern Oscillation

El Niño-Southern Oscillation events are a coupled ocean-atmosphere phenomenon. El Niño involves warming of tropical Pacific surface waters from near the International Date Line to the west coast of South America, weakening the usually strong SST gradient across the equatorial Pacific, with associated changes in ocean circulation. Its closely linked atmospheric counterpart, the Southern Oscillation (SO), involves changes in trade winds, tropical circulation and precipitation. Historically, El Niño events occur about every 3 to 7 years and alternate with the opposite phases of below-average temperatures in the eastern tropical Pacific (La Niña). Changes in the trade winds, atmospheric circulation, precipitation and associated atmospheric heating set up extratropical responses. Wavelike extratropical teleconnections are accompanied by changes in the jet streams and storm tracks in mid-latitudes (Chang and Fu, 2002).

The El Niño-Southern Oscillation has global impacts, manifested most strongly in the northern winter months (November–March). Anomalies in MSLP are much greater in the extratropics while the tropics feature large precipitation variations. Associated patterns of surface temperature and precipitation anomalies around the globe are given in Figure 3.27 (Trenberth and Caron, 2000), and the evolution of these patterns and links to global mean temperature perturbations are given by Trenberth et al. (2002b).

Figure 3.27. Correlations with the SOI, based on normalised Tahiti minus Darwin sea level pressures, for annual (May to April) means for sea level pressure (top left) and surface temperature (top right) for 1958 to 2004, and GPCP precipitation for 1979 to 2003 (bottom left), updated from Trenberth and Caron (2000). The Darwin-based SOI, in normalized units of standard deviation, from 1866 to 2005 (Können et al., 1998; lower right) features monthly values with an 11-point low-pass filter, which effectively removes fluctuations with periods of less than eight months (Trenberth, 1984). The smooth black curve shows decadal variations (see Appendix 3.A). Red values indicate positive sea level pressure anomalies at Darwin and thus El Niño conditions.

The nature of ENSO has varied considerably over time. Strong ENSO events occurred from the late 19th century through the first 25 years of the 20th century and again after about 1950, but there were few events of note from 1925 to 1950 with the exception of the major 1939–1941 event (Figure 3.27). The 1976–1977 climate shift (Trenberth, 1990; see Figure 3.27 and Section 3.6.3, Figure 3.28) was associated with marked changes in El Niño evolution (Trenberth and Stepaniak, 2001), a shift to generally above-normal SSTs in the eastern and central equatorial Pacific and a tendency towards more prolonged and stronger El Niños. Since the TAR, there has been considerable work on decadal and longer-term variability of ENSO and Pacific climate. Such decadal atmospheric and oceanic variations (Section 3.6.3) are more pronounced in the North Pacific and across North America than in the tropics but are also present in the South Pacific, with evidence suggesting they are at least in part forced from the tropics (Deser et al., 2004).

Figure 3.28. Pacific Decadal Oscillation: (top) SST based on the leading EOF SST pattern for the Pacific basin north of 20°N for 1901 to 2004 (updated; see Mantua et al., 1997; Power et al., 1999b) and projected for the global ocean (units are nondimensional); and (bottom) annual time series (updated from Mantua et al., 1997). The smooth black curve shows decadal variations (see Appendix 3.A).

El Niño-Southern Oscillation events involve large exchanges of heat between the ocean and atmosphere and affect global mean temperatures. The 1997–1998 event was the largest on record in terms of SST anomalies and the global mean temperature in 1998 was the highest on record (at least until 2005). Trenberth et al. (2002b) estimated that global mean surface air temperatures were 0.17°C higher for the year centred on March 1998 owing to the El Niño. Extremes of the hydrological cycle such as floods and droughts are common with ENSO and are apt to be enhanced with global warming (Trenberth et al., 2003). For example, the modest 2002–2003 El Niño was associated with a drought in Australia, made much worse by record-breaking heat (Nicholls, 2004; and see Section 3.8.4, Box 3.6). Thus, whether observed changes in ENSO behaviour are physically linked to global climate change is a research question of great importance.