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Advanced ENSO Theory: The Delayed Oscillator
The Coupled SystemThe propagation of oceanic waves near the equator gives a first order indication of an oscillatory system. An initial westerly wind stress anomaly in the central Pacific leads to the generation of an eastward propagating downwelling Kelvin wave with maximum signal on the equator and an upwelling Rossby wave with maximum signal symmetrically located on either side of the equator. The Kelvin wave travels fairly rapidly to the east where it is able to deepen the thermocline, lessen the upwelling-induced cooling, and thereby warm the SST. Meanwhile, the Rossby wave travels to the western boundary where it reflects back to the east as an upwelling Kelvin wave. This Kelvin wave then propagates to the east where it acts to reverse the earlier warming. This simple scenario is different from ENSO in the sense that it represents oscillations deriving from a single wind impulse, whereas the real ENSO involves a continuous wind-stress forcing throughout the cycle; also the time scale of the ocean oscillations in the present example are much shorter than those of ENSO. These issues are addressed through one other critical element of the theory – the positive feedback, or coupled instability mechanism.The theory asserts that the wind-stress anomaly in the central basin and the equatorial SST pattern are strongly coupled. The physical basis for this is as follows. The near-equatorial atmospheric circulation is driven largely through the heating that results from areas of tropical convection (thunderstorm activity). The areas of convection, in turn, are strongly favored over the warmest tropical waters. Since the typical SST pattern features warm waters in the west, and much cooler waters in the east, convection and its associated heating occurs preferentially in the west, driving a thermally direct circulation cell that features rising motion locally in the west, sinking motion further to the east over the cooler waters, and westward low level trade winds in between. The westward trade winds, however, strongly affect the ocean. First, they induce mean upwelling, as discussed above. But in addition, they act to "pile up" the warm water layer (deepen the equatorial thermocline) toward the west, and shallow it toward the east. In the presence of mean upwelling this contrast is readily transferred to the surface, resulting in SST that is relatively warm in the west, and cool in the east. Herein lies the essential feedback: the westward trade winds tend to reinforce the east-west SST contrast, but at the same time the SST contrast reinforces the winds. Thus the ocean and atmosphere exhibit a positive feedback, in a mutually reinforcing interaction.
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