Fig. 1.38 Meridional section (along 158.5° E; vertical axis depicts depth (m)) a, thermal structure (contours) (°C); and b, stratification (contours) (kg/m3), overlaid with the vertical gradient in the colored map. See color plate section.

structure is intimately associated with the circulation, the theory of the thermocline is also a theory for the circulation in the upper ocean.

Note that both the main thermocline and the main pycnocline are conceptual features defined for the basin scale; thus, for a given station or section, the depth of these layers may not be well defined. At this section (158.5° E), the main thermocline can be readily identified from the left panel of Figure 1.38. It is close to the 20°C isothermal surface in the equatorial ocean, and it gradually shifts to a lower temperature of 11-12°C at mid latitudes. The main thermocline is clearly asymmetric with respect to the equator, which indicates that forcing and boundary conditions for the wind-driven circulation are asymmetric. On the other hand, the main pycnocline is not clearly defined for this section; see the right panel of Figure 1.38.

The depth of the main thermocline varies greatly with geographic location, Figure 1.39. It is rather shallow near the eastern boundaries of the equatorial regimes due to equatorial upwelling driven by the easterlies at low latitudes. It is also shallow along the eastern boundaries of the Southern Hemisphere, owing to the strong coastal upwelling driven by the along-shore component of the trade wind. Thus, the shallowness of the main thermocline in these parts of the ocean is associated with the relatively cold surface temperature induced by local wind-driven upwelling, and these areas are called the "cold tongues" in the oceans.

The main thermocline in the western part of the equatorial oceans is deeper than in the eastern part because warm water is piled up under the equatorial easterly. In the Pacific Ocean this body of warm water is called the Warm Pool. Both the Warm Pool and the Cold Tongue play vitally important roles in the global climate system, especially in the dynamics of the El Nino-Southern Oscillation (ENSO).

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