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For typical tropical ocean values h = 100 m, g' = 2 x 10-2 m s-2, the above gives Le ~ 250 km, or about 2.25° of latitude. This is the natural length scale (in the north-south direction) for oceanic motions near the equator.5

This upwelling brings cold water up from depth, and thereby cools the upper layers of the near-equatorial ocean. We have in fact

5Eq. 12-4 is not valid within a distance of order Y ~ Le of the equator, but this does not affect our deductions about upwelling, since the continuity equation can simply be integrated in y across this region to give, at the base of the mixed layer, Y w dy = h [v(Y) - v(-Y)], yielding the same integrated result without needing to know the detailed variations of v(y) between ±Y.

already seen evidence for this upwelling in the thermal structure: the meridional cross-sections of T in Figs. 9.5 and 9.9 of Chapter 9 show clear upward displacement of the temperature contours near the equator. It shows even more clearly in the distribution of tracers, such as dissolved oxygen, shown in Fig. 11.14.

In reality, of course, the tropical Pacific Ocean is bounded to the east (by South America) and west (by the shallow seas in the region of Indonesia), unlike as assumed in the preceding calculation. One effect of this, in addition to the response to the westward wind stress deduced above, is to make the thermocline deeper in the west, and shallower in the east, as depicted in Fig. 12.6. In consequence, the cold deep water upwells close to the surface in the east, thus cooling the sea surface temperature (SST) there. In the west, by contrast, cold water from depth does not reach the surface, which consequently becomes very warm. This distribution is evident in the equatorial Pacific and (to a lesser degree) Atlantic Oceans (Fig. 9.3). A closeup view of Pacific SST distributions during a warm year (1998) and a cold year (1989) are shown in Fig. 12.7. Note the ''cold tongue'' in the cold year 1989 extending along the equator from the South American coast.

Its narrowness in the north-south direction is a reflection of the size of the equatorial deformation radius. Aside from being cold, deep ocean water is also rich in inorganic nutrients; thus, the upwelling in the east enriches the surface waters, sustaining the food chain and a usually productive fishery off the South American coast.

The east-west gradient of SST produced by the wind stress in turn influences the atmosphere. The free atmosphere cannot sustain significant horizontal gradients of temperature, because for a finite vertical wind shear, thermal wind balance, Eq. 7-24, demands that VT ^ 0 as f ^ 0 at the equator. The regions most unstable to convection are those with the warmest surface temperature. So convection and hence rainfall occurs mostly over the warmest water, which is over the ''warm pool'' in the western equatorial Pacific, as depicted in Fig. 12.6. The associated latent heating of the air supports net upwelling over this region, which is closed by westerly flow aloft, descent over the cooler water to the east, and a low-level easterly return flow. This east-west overturning circulation is known as the ''Walker circulation''. While here we have considered this circulation in isolation, in reality it coexists with the north-south Hadley circulation (discussed in Chapter 8). In association

FIGURE 12.6. Schematic E-W cross section of ''normal'' conditions in the atmosphere and ocean of the equatorial Pacific basin. The east-west overturning circulation in the atmosphere is called the Walker Circulation.

Jan-Mar 1998 Jan-Mat 1989

Jan-Mar 1998 Jan-Mat 1989

FIGURE 12.7. Sea surface temperatures (SST) (top) and SST anomalies (departure from long-term Jan-Mar mean; bottom) during an El Nin o (warm event; left) and La Nin a (cold event; right). Green represents warm anomalies, brown cold. Courtesy of NOAA.

FIGURE 12.7. Sea surface temperatures (SST) (top) and SST anomalies (departure from long-term Jan-Mar mean; bottom) during an El Nin o (warm event; left) and La Nin a (cold event; right). Green represents warm anomalies, brown cold. Courtesy of NOAA.

with the circulation, there is an east-west equatorial gradient of surface pressure, with low pressure in the west (where the convection occurs) and higher pressure in the east (see Fig. 7.27).

Note that the low-level easterly flow present in the Walker circulation reinforces the trade winds over the equatorial Pacific. In fact, the causal links that were discussed previously can be summarized as in Fig. 12.8. There is thus the potential for positive feedback in the Pacific Ocean-atmosphere system, and it is this kind of feedback that underlies the variability of the tropical Pacific.

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