El Nio And The Southern Oscillation

El Niño episodes of warm coastal currents with accompanying disastrous consequences for marine life and birds recur about every four to seven years and consequently were long known along the west coast of South America.

The related Southern Oscillation (SO) of sea-level pressure between Tahiti (normally high pressure) and Jakarta (or Darwin) (normally low pressure) was identified by Sir Gilbert Walker in 1910 and reinvestigated in the mid-1950s by I. Schell and H. Berlage and in the 1960s by A. J. Troup and J. Bjerknes. A. J. Troup linked the occurrence of El Niño conditions to an oscillation in the atmosphere over the equatorial Pacific in the 1960s. Their wider implications for air-sea interaction and global teleconnections were first proposed by Professor Jacob Bjerknes (of polar front fame) in 1966 who noted the linkages of El Niño or non-El Niño conditions with the SO. The worldwide significance of ENSO events only became fully appreciated in the 1970s to 1980s with the strong El Niño events of 1972 to 1973 and 1982 to 1983. The availability of global analyses showed clear patterns of seasonal anomalies of temperature and precipitation in widely separated regions during and after the onset of warming in the eastern and central equatorial Pacific Ocean. These include droughts in northeast Brazil and in Australasia, and cool, wet winters following El Niño in the southern and southeastern United States.

The occurrence of ENSO events in the past has been studied from historical documents, inferred from tree ring data, and from coral, ice core and high-resolution sediment records. The net effect of major El Niño events on global temperature trends is estimated to be about +0.06°C between 1950 and 1998.


Diaz, H. F. and Markgraf, V. (eds) (1992) El Niño. Historical and Paleoclimatic Aspects of the Southern Oscillation. Cambridge University Press, Cambridge, 476 pp.

Figure 11.50 Schematic cross-sections of the Walker circulation along the equator based on computations of Tourre. (A) Mean December to February regime (non-ENSO); rising air and heavy rains occur over the Amazon basin, central Africa and Indonesia-western Pacific. (B) December to February 1982-3 ENSO pattern; the ascending Pacific branch is shifted east of the date line and suppressed convection occurs elsewhere due to subsidence. (C) Departure of sea-surface temperature from its equatorial zonal mean, corresponding to non-ENSO case (A). (D) Strong trades cause sea-level to rise and the thermocline to deepen in the western Pacific for case (A). E. Winds relax, sea-level rises in the eastern Pacific as watermass moves back eastward and the thermocline deepens off South America during ENSO events.

Source: Based on van Heerden and Taljaard (1998), by permission of the World Meteorological Organization (1985).

dominant. The level of activity of the Southern Oscillation in the Pacific is expressed by the Southern Oscillation index (SOI), which is a complex measure involving sea-surface and air temperatures, pressures at sea-level and aloft, and rainfall at selected locations.

During non-ENSO, high phases (Figure 11.50A) strong easterly trade winds in the eastern tropical Pacific produce upwelling along the west coast of South America, resulting in a north-flowing cold current (the Peru or Humboldt), locally termed La Niña - the girl -on account of its richness in plankton and fish. The low sea temperatures produce a shallow inversion, thereby strengthening further the trade winds (i.e. effecting positive feedback), which skim water off the surface of the Pacific, where warm surface water accumulates (Figure 11.50D). This action also causes the thermocline to lie at shallow depths (about 40 m) in the east, as distinct from 100 to 200 m in the western Pacific. The strengthening of the easterly trades causes cold water upwelling to spread westward, and the cold tongue of surface water extends in that direction sustained by the south equatorial current. This westward-flowing current is wind-driven and is compensated by a deeper surface slope. The westward contraction of warm Pacific water into the central and western tropical Pacific (Figure 11.50C) produces an area of instability and convection

Figure 11.51 El Niño events 1525 to 1987 classified according to very strong, strong and medium. Subsequent strong events occurred in 1991 and 1997.

Source: Quinn and Neal (1992). Copyright © Routledge, London.

Figure 11.51 El Niño events 1525 to 1987 classified according to very strong, strong and medium. Subsequent strong events occurred in 1991 and 1997.

Source: Quinn and Neal (1992). Copyright © Routledge, London.

fed by moisture in a convergence zone under the dual influence of both the intertropical convergence zone and the South Pacific convergence zone. The rising air over the western Pacific feeds the return airflow in the upper troposphere (i.e. at 200 mb), closing and strengthening the Walker circulation. However, this airflow also strengthens the Hadley circulation, particularly its meridional component northward in the northern winter and southward in the southern winter.

Each year, usually starting in December, a weak southward flow of warm water replaces the northward-flowing Peru current and its associated cold upwelling southward to about 6°S along the coast of Ecuador. This phenomenon, known as El Niño (the child, after the

Christ child), strengthens at irregular intervals of two to ten years (its average interval is four years) when warm surface water becomes much more extensive and the coastal upwelling ceases entirely. This has catastrophic ecological and economic consequences for fish and bird life, and for the fishing and guano industries of Ecuador, Peru and northern Chile. Figure 11.51 shows the occurrence of El Niño events between 1525 and 1987 classified according to their intensity. These offshore events, however, are part of a Pacific-wide change in sea-surface temperatures. Moreover, the spatial pattern of these changes is not the same for all El Niños. Recently, K. E. Trenberth and colleagues showed that during 1950 to 1977, warming during an El Niño spread westward from Peru, whereas after a major shift in Pacific basin climate took place between 1976 and 1977, the warming spread eastward from the western equatorial Pacific. The atmosphere-ocean coupling during ENSO events clearly varies on multidecadal time scales.

ENSO events result from a radical reorganization of the Walker circulation in two main respects:

1 Pressure declines and the trades weaken over the eastern tropical Pacific (Figure 11.50B), wind-driven upwelling slackens, allowing the ITCZ to extend southward to Peru. This increase of sea-surface temperatures by 1 to 4°C reduces the west-east sea-surface temperature gradient across the Pacific and also tends to decrease pressure over the eastern Pacific. The latter causes a further decrease of trade wind activity, a decrease in upwelling of cold water, an advection of warm water and a further increase in sea-surface temperatures - in other words, the onset of El Niño activates a positive feedback loop in the eastern Pacific atmosphere-ocean system.

2 Over the western tropical Pacific, the area of maximum sea temperatures and convection responds to the above weakening of the Walker circulation by moving eastward into the central Pacific (Figure 11.50B). This is due partly to an increase of pressure in the west but also to a combined movement of the ITCZ southward and the SPCZ northeastward. Under these conditions, bursts of equatorial westerly winds spread a huge tongue of warm water (i.e. warmer than 27.5°C) eastward over the central Pacific as large-scale, internal oceanic (Kelvin) waves. It has been suggested that this eastward flow may sometimes be triggered off or strengthened by the occurrence of cyclone pairs north and south of the equator. This eastward flow of warm water depresses the thermocline off South America (Figure 11.50E), preventing cold water from reaching the surface and terminating the El Niño effect.

Thus, whether La Niña or El Niño develops, bringing westward-flowing cold surface water or eastward-flowing warm surface water, respectively, to the central Pacific, depends on the competing processes of upwelling versus advection. The most intense phase of an El Niño event commonly lasts for about one year, and the change to El Niño usually occurs in about March to April, when the trade winds and the cold tongue are at their weakest. The changes to the Pacific atmosphere-

ocean circulation during El Niño are facilitated by the fact that the time taken for ocean-surface currents to adjust to major wind changes decreases markedly with decreasing latitude. This is demonstrated by the seasonal reversal of the southwest and northeast monsoon drift off the Somali coast in the Indian Ocean. Large-scale atmospheric circulation is subject to a negative-feedback constraint involving a negative correlation between the strengths of the Walker and Hadley circulations. Thus the weakening of the Walker circulation during an ENSO event leads to a relative strengthening of the associated Hadley circulation.

Continue reading here: Teleconnections

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