FIGURE 12.11. Schematic of the Pacific Ocean-Atmosphere system during (top) cold La Nina and (bottom) warm El Nino conditions.

eastward (through eastward propagation of a wave of depression on the thermo-cline); this deepens the thermocline in the east Pacific some two months later. This in turn raises the SST in the east. The basic postulate—that the ocean responds to the atmosphere—has been confirmed in sophisticated ocean models forced by ''observed'' wind stresses during an El Nino event.

Third, the El Nino-Southern Oscillation phenomenon arises spontaneously as an oscillation of the coupled ocean-atmosphere sys-tem.Bjerknes first suggested that what we now call ENSO is a single phenomenon and a manifestation of ocean-atmosphere coupling. The results noted previously appear to confirm that the phenomenon depends crucially on feedback between ocean and atmosphere. This is demonstrated in coupled ocean-atmosphere models of varying degrees of complexity, in which ENSO-like fluctuations may arise spontaneously. It appears that stochastic forcing of the system by middle latitude weather systems, which can reach down into the tropics to induce ''westerly wind bursts,'' can also play a role in triggering ENSO events.

Once the El Nino event is fully developed, negative feedbacks begin to dominate the Bjerknes positive feedback, lowering the SST and bringing the event to its end after several months. The details of these negative feedbacks involve some very interesting ocean dynamics. In essence, when the easterlies above the central Pacific start weakening at the beginning of the event, it leads to the formation of an off-equatorial shallower-than-normal thermocline signal, which propagates westward, reflects off the western boundary of the Pacific, and then travels eastwards. After a few months delay the thermocline undulation arrives at the eastern boundary, causing the ther-mocline to shoal there, so terminating the warm event.

12.2.4. Other modes of variability

The ENSO phenomenon discussed previously is a direct manifestation of strong coupling between the tropical atmosphere and tropical ocean and it gives rise to coherent variability in the coupled climate. There are other modes of variability that arise internally to the atmosphere (i.e., would be present even in the absence of coupling to the ocean below). Perhaps the most important of these is the annular mode, a meridional wobble of the subtropical jet stream. The cli-matological position of the zonal-average, zonal wind, u, is plotted in Fig. 5.20. But in fact the position and strength of the jet stream maximum varies on all timescales; when it is poleward of its climatological position, u is a few ms-1 stronger than when it is equatorward. These variations in u extend through the depth of the troposphere and indeed right up into the stratosphere. Importantly for the ocean below, the surface winds and air-sea fluxes also vary in synchrony with the annular mode, driving variations in SST and circulation. The manifestation of the annular mode in the northern hemisphere, is known as the North Atlantic Oscillation, or NAO for short; the annular mode in the southern hemisphere is known as SAM, for southern annular mode. Both introduce stochastic noise into the climate system that can be reddened by interaction with the ocean as discussed in Section 12.1.1.

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