A simple model of the El

El Niño events are good examples of the strong coupling which occurs between the circulations of ocean and atmosphere. The stress exerted by atmospheric circulation - the wind - on the ocean surface is a main driver for the ocean circulation. Also, as we have seen, the heat input to the atmosphere from the ocean, especially that arising from evaporation, has a big influence on the atmospheric circulation.

A simple model of an El Niño event that shows the effect of different kinds of wave motions that can propagate within the ocean is illustrated in Figure 5.10. In this model a wave in the ocean, known as a Rossby wave, propagates westwards from a warm anomaly in ocean surface temperature near the equator. When it reaches the ocean's western boundary it is reflected as a different sort of wave, known as a Kelvin wave, which travels eastward. This Kelvin wave cancels and reverses the sign of the original warm anomaly, so triggering a cold event. The time taken for this half-cycle of the whole El Niño process is determined by the speed with which the waves propagate in the ocean; it takes about two years. It is essentially driven by ocean dynamics, the associated atmospheric changes being determined by the patterns of ocean surface temperature (and in turn reinforcing those patterns) that result from the ocean dynamics. Expressed in terms of this simple model, some of the characteristics of the El Niño process appear to be essentially predictable.

Figure 5.10 Schematic to illustrate El Niño oscillation.

temperature - at higher temperatures more water can be evaporated before the atmosphere is saturated - evaporation from the surface and hence the heat input to the atmosphere is particularly large in the tropics.

The most prominent examples of interactions between atmosphere and ocean circulations are associated with the El Niño Southern Oscillation (ENSO)7 that was first identified in the late nineteenth century as 'seesaw' surface pressure variations between South America and Indonesia (see box). It is during El Niño events associated with ENSO in the east tropical Pacific (see Figure 5.9) that the largest variations are found in ocean temperature.

Anomalies in the circulation and rainfall in all tropical regions and to a lesser extent at mid latitudes are associated with these El Niño events (see Figure 1.4). A good test of the atmospheric models described above is to run them with an El Niño sequence of sea surface temperatures and see whether they are able to simulate these climate anomalies. This has now been done with a number of different atmospheric models; they have shown considerable skill in the simulation of many of the observed anomalies, especially those in the tropics and sub-tropics.

Because of the large heat capacity of the oceans, anomalies of ocean surface temperature tend to persist for some months. The possibility therefore exists, for regions where there is a strong correlation between weather and patterns of ocean surface temperature, of making forecasts of climate (or average weather) some weeks or months in advance. Such seasonal forecasts have been attempted especially for regions with low rainfall; for instance, for northeast Brazil and for the Sahel region of sub-Saharan Africa, a region where human survival is very dependent on the marginal rainfall (see box). To make seasonal forecasts depends on the ability to forecast changes in ocean surface temperature. To do that requires understanding of, and the ability to model, the ocean circulation and the way it is coupled to the atmospheric circulation. Because the largest changes in ocean surface temperature occur in the tropics and because there are reasons to suppose that the ocean may be more predictable in the tropics than elsewhere, most emphasis on the prediction of ocean surface temperature has been placed in tropical regions, in particular on the prediction of the El Niño events themselves.

Later on in this chapter the coupling of atmospheric models and ocean models is described. For the moment it will suffice to say that, using coupled models together with detailed observations of both atmosphere and ocean in the Pacific region, significant skill in the prediction of El Niño events has been achieved for months and up to a year in advance (Figure 5.12) (see also Chapter 7).

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