The mechanism of El

The mean state of the atmosphere and the ocean

Before discussing what processes conspire to produce El Niño events, let us discuss what the mean state of the atmosphere and ocean are, beginning with the atmosphere. The trade winds throughout the equatorial region blow predominantly from higher latitudes toward the equator, and from the east to the west. The low-level convergence at the equator forces the air to rise and then, several kilometers above the surface, move poleward, sinking in the subtropics at about 30° north and south and then moving equatorward at the surface. The meridional cell is known as the Hadley cell, and if there were no continents, the equatorial convergence would be on average the same at all longitudes.

However, there are continents and ocean basins, and their presence is essential in producing El Niño events. The easterly winds over the equatorial Pacific tend to push the surface waters westward, creating a westward current at the surface. The surface waters also diverge away from the equator because of the Coriolis effect, causing upwelling along the equator; that is, cold water from the deep ocean is pulled toward the surface to replace the surface waters moving away. Upwelling in fact occurs at most longitudes in the Pacific (because the equatorial divergence occurs at all longitudes), but it is particularly important in the east because here it must also replenish the surface waters moving westward away from South America. The upwelling water is cold because it comes from the deep ocean, so that the SST of the eastern equatorial Pacific is relatively low, and the surface waters become warmer as they move westward. The thermocline deepens further west, and so the up-welling does not bring as much cold abyssal water to the surface. The upshot of all this is that the SST is high in the western Pacific, up to about 30°C, and low in the eastern Pacific, at about 21°C (figure 6.3, top panel).

As one might expect, such a strong temperature gradient in turn affects the atmosphere. The warm western Pacific region becomes much more unstable with respect to convection than the eastern part, so that rising motion preferentially occurs here. The eastern Pacific is correspondingly cool and thus prone to descent and so, as illustrated in figure 6.5, an east-west overturning circulation is set up in the atmosphere. This is known as the

Walker circulation after the British meteorologist Gilbert Walker, who described it in the 1920s.8 The Walker circulation coexists with and is somewhat analogous to the north-south Hadley circulation, in that both are driven by horizontal temperature gradients at the surface.

Variability and El Niño

Having described the Walker circulation and the corresponding ocean circulation, let's see if we can understand why they vary and give rise to El Niño events. We first note that the ocean and the atmosphere tend to reinforce each other: the atmospheric winds naturally blow from east to west, which sets up an east-west temperature gradient in the ocean. The warm ocean in the west leads to rising motion in the atmosphere, pulling air in at the surface. Thus, the westward atmospheric winds in the Pacific tend to be stronger than they would be if there were no ocean there at all or if the ocean extended all the way around the globe. Such a reinforcement is known as a positive feedback, and it can work both ways. That is to say, positive feedbacks tend to reinforce tendencies, so that if a system starts moving in a different direction, the feedback may again kick in, reinforcing the initial tendency, whatever that tendency may be.

Let's suppose that the Pacific is in a relatively normal state, as illustrated in the top panel of figure 6.5. We now imagine that, for reasons we won't be definitive about right now, the east-west temperature contrast weakens somewhat—perhaps the eastern Pacific Ocean gets a bit

Thermocline During Nina

West Pacific East Pacific

Figure 6.5 Schema of the atmosphere and ocean at the equator in the Pacific, during La Niña/normal conditions (top) and El Niño conditions (bottom). La Niña conditions are similar to normal conditions but with a still steeper-sloping thermocline and maximum SSTs a little further west.

West Pacific East Pacific

Figure 6.5 Schema of the atmosphere and ocean at the equator in the Pacific, during La Niña/normal conditions (top) and El Niño conditions (bottom). La Niña conditions are similar to normal conditions but with a still steeper-sloping thermocline and maximum SSTs a little further west.

climate variability warmer than usual because of some fluctuation in the ocean. The atmosphere can be expected to respond to this by a weakening of the trade winds, to which the ocean in turn responds; with weaker trade winds, the upwelling in the east becomes weaker and the thermocline flattens (as in the lower panel of figure 6.5). The temperature in the east consequently rises a little while temperature in the western Pacific falls, so further reducing the east-west temperature gradient. This reduced temperature contrast results in the main convective regions moving further east and causes the trade winds to further weaken, which in turn further reduces upwelling and causes the temperatures to further increase in the eastern equatorial Pacific; the culmination of this chain of events is an El Niño. Evidently, the sequence of events is crucially dependent on the mutual interaction and feedback between the atmosphere and the ocean, as was first posited by Jacob Bjerknes in 1969. (Jacob Bjerknes (1897-1975) was a Norwegian climate scientist working in the United States.)

The positive feedback we've described must come to an end eventually. The ocean temperature in the east cannot rise indefinitely (and in practice much above 30°C) because eventually the ocean cools via radiation and by giving up heat to the atmosphere. Furthermore, the natural tendency of the trade winds is to blow toward the west, and this effect tries to restore the natural order of things. Once an El Niño is fully developed, it persists for several months before beginning to decay. At that stage, the feedbacks we described above come into play again, but now working in the opposite direction. Indeed, the feedbacks may well cause the system to overshoot its mean state and go into a La Niña state, with enhanced warming in the west, cooler than normal conditions in the east, and stronger than usual trade winds. Then, some time after that, the whole process may begin again and the system may again evolve toward an El Niño state. The seemingly oscillatory nature of the atmospheric cycle historically gave rise to the appellation Southern Oscillation, and the entire phenomenon—atmosphere plus ocean together—is often referred to as the ENSO cycle. Although the basic mechanisms of the oscillation are fairly well accepted, the nature and causes of the transition between the El Niño and La Niña states, and the need or otherwise for some form of kick start to begin the El Niño cycle, remain to be definitely explained.9

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