El Nio Southern Oscillation ENSO

El Niño-Southern Oscillation events are a coupled ocean-atmosphere phenomenon. It is a natural feature of the climate system. El Niño involves warming of surface waters of the tropical Pacific in the region from the International Date Line to the west coast of South America, with associated changes in oceanic circulation. It is accompanied by large changes in the tropical atmosphere, lowering pressures in the east and raising them in the west, in what is known as the "Southern Oscillation". tte total phenomenon is generally referred to as ENSO. El Niño is the warm phase of ENSO and La Niña is the cold phase. Historically, El Niño (EN) events occur about every 3-7 years and alternate with the opposite phases of below average temperatures in the equatorial Pacific (La Niña). A convenient way of measur ing ENSO is in terms of the east-west pressure difference, the Southern Oscillation Index, or SOI, which is a scaled form of the difference in mean sea-level pressure between Tahiti and Darwin. A graph of the SOI over the past 30 years is shown in Figure 7.1.

ENSO may be thought in terms of a slopping back and forth of warm surface water across the equatorial Pacific Ocean, tte trade winds, blowing from the east towards the west, normally help to draw up cool water in the east and to keep the warmest water in the western Pacific, ttis encourages low air pressures in the west and high pressures in the east. An El Niño event is when the warm water "spills out" eastwards across the Pacific, the trade winds weaken, pressures rise in the west and fall in the east. Eventually, the warm water retreats to the west again and "normality" is restored, tte movements of water can also swing too far the other way and waters become unusually cool near South America, resulting in what is termed a "La Niña", where the trade winds are unusually strong while pressures are lower than normal over northern Australia. As an El Niño event develops, the compensating shifts of the globe's weather zones and rainfall patterns result in widespread droughts in some regions, heavy flooding in others, and associated regions of warming and cooling, tte regions most affected are the tropical and subtropical regions oflndonesia, Australia, and the Pacific Islands.

Figures 7.2 and 7.3 show relationships with temperature and precipitation throughout the region. Essentially ENSO events cause increased temperature in the tropical South Pacific from just west of the Date Line to the South American coast, whereas temperatures are decreased in ENSO events over Papua-New Guinea south east into the subtropical South Pacific, and New Zealand, tte reverse temperature anomalies occur during La Niña episodes.

Precipitation anomalies can be much more dramatic (Figure 7.3). ENSO events can bring drought and decreased precipitation anomalies over the Phillipines, In-

; Southern Oscillation Index i-1-r

; Southern Oscillation Index

Fig. 7.1. ^e Southern Oscillation Index (SOI) for the last 30 years. Negative excursions indicate El Niño events, and positive excursions indicate La Niña events, ^e irregular nature of ENSO events is evident in the time sequence

Year

Fig. 7.1. ^e Southern Oscillation Index (SOI) for the last 30 years. Negative excursions indicate El Niño events, and positive excursions indicate La Niña events, ^e irregular nature of ENSO events is evident in the time sequence

Fig. 7.3. Correlations between the May - April Southern Oscillation Index and precipitation 1958-2003, after Trenberth and Caron (2000).

Fig. 7.2. Correlations between the May - April Southern Oscillation Index and surface temperature 1958-2004, after Trenberth and Caron (2000)

Fig. 7.3. Correlations between the May - April Southern Oscillation Index and precipitation 1958-2003, after Trenberth and Caron (2000).

Fig. 7.2. Correlations between the May - April Southern Oscillation Index and surface temperature 1958-2004, after Trenberth and Caron (2000)

donesia, northern and eastern Australia, the subtropical Southwest Pacific and the north east of New Zealand. Increased precipitation occurs in the equatorial Pacific from Kiribati (west of the Date Line) through to the Galapagos Islands, tte reverse anomalies occur in La Niña episodes.

Tropical cyclones develop in the South Pacific over the wet season, usually from November through to April. Peak cyclone occurrence is usually during January, February and March based on historical tropical cyclone data analysis, ttose countries with the highest risk include Vanuatu, New Caledonia, Fiji, Tonga and Niue. Taken over the whole of the South Pacific, on average nine tropical cyclones can occur during the November to April season, but this can range from as few as three in 1994/95, to as many as 17 in 1997/98, during the last very strong El Niño, tte mean frequency of tropical cyclones for the 1970 - 2000 period for El Niño episodes is 11.5, and La Niña events 8.6 per season, tte tropical cyclone track densities vary depending on the ENSO state (Figure 7.4). During El Niño episodes a higher frequency of tropical cyclone tracks occur near Vanuatu and Fiji, and their occurrence spreads further east to 160°W to affect the Cook Islands and most of French Polynesia. In contrast, during La Niña events the maximum occurrence is largely confined to the Coral Sea area of the Southwest Pacific centering on 160°E, 20°S and affecting New Caledonia in particular.

Fig. 7.4. Tropical cyclone densities for El Niño (upper) and La Niña (lower) seasons in the Southwest Pacific. Contour interval is 0.25, starting at 1.0

Fig. 7.4. Tropical cyclone densities for El Niño (upper) and La Niña (lower) seasons in the Southwest Pacific. Contour interval is 0.25, starting at 1.0

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