South America

120" E

El Niño events are perturbations of the ocean-atmosphere system. It is not known whether the perturbations originate in the atmosphere or the ocean, but for convenience we will start by considering what happens in the atmosphere during an El Niño event. The prevailing winds over the equatorial Pacific are the South-East Trades. Their strength depends on the difference in surface atmospheric pressure between the subtropical high pressure region in the eastern South Pacific - where cool, dry air converges and subsides - and the low pressure region over Indonesia - where warm, moist air rises, producing cumulonimbus clouds and heavy rainfall (Figure 5.21). During an El Niño event, the Indonesian Low has anomalously high pressure (i.e. is a weaker low than usual) and moves eastwards into the central Pacific, while the South Pacific High becomes anomalously low. The South-East Trades weaken, and there are bursts of westerlies in the western Pacific.

B\ reference to F igure 5.4. can you suggest what effect a relaxation ol the

South-East Traites will have on the upper waters of the equatorial Pacific?

The sea-surface slope will "collapse', so that both it and the thermocline become near-horizontal, enabling a considerable volume of warm mixed-layer water to move eastwards across the ocean. In the western Pacific, the collapse in the Trade Winds occurs abruptly, and so the resulting change in the upper ocean - a depression in the thermocline accompanied by a slight rise in sea-level (cf. Figure 5.18(a)) - propagates eastwards along the Equator as a pulse, or series of pulses, of Kelvin waves. At the eastern boundary, the equatorial Kelvin waves split into northward- and southward-travelling coastal Kelvin waves (cf. Figure 5.18(b)), as well as being partially reflected as Rossby waves (cf. Figure 5.20). The speed of these Kelvin waves has been calculated to be about 2.5 m s-1. but the bulge travels slightly faster than this because it is carried forward by flow in the Equatorial Undercurrent.

Figures 5.22 and 5.23 (overleaf) summarize the main differences between normal conditions in the Pacific basin, and conditions during an El Niño event. As shown in part (a) of these Figures, the highest sea-surface temperatures of 28-29 °C are normally found in the western ocean; during an El Niño event, this area of exceptionally warm water moves into the central ocean, along with the vigorous convection of moist air usually associated with the Indonesian Low. and the Intertropical Convergence Zone shifts southwards and eastwards (Figure 5.23(b)).

Why could this eastward shift of the region of \ igorous convection, and in the position of the ITCZ. be the result of an eastward movement of exceptionally warm surface water, rather than the indirect cause of it?

A warm sea-surface leads to increased upward convection of moist air and. as discussed in Section 2.3.1. the increase in convection is particularly marked when sea-surface temperatures exceed -28 °C. The eastward movement across the Pacific of the Indonesian Low. the shift in the position of the ITCZ. and the exceptionally warm surface water are therefore all intimately linked together. Thus, although the general eastward movement of warm mixed-layer water may be explained in terms of the slackening of the Trade Winds in response to a change in the strength and position of the Indonesian Low. this is clearly only part of the story.

Figure 5.22 Cross-section along the Equator in the Pacific (a) in a normal year, and (b) during an El Niño event. The pink area represents the pool of very warm water usually located in the western Pacific. The curly arrows represent upwelling. Note that although upwelling may not be entirely suppressed during an El Niño event, the upwelled water now comes from within the warm mixed layer and is not nutrient-rich.



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