The overturning circulation and the vertical structure

The meridional overturning circulation (MOC) of the ocean is the name commonly given to the circulation in the meridional (i.e., north-south and up-down in water depth) plane. Horizontal variations in this circulation can be important, but let us put them aside for now. This circulation comes about as a consequence of various factors: the temperature gradient between equator and pole, the temperature difference between high northern latitudes and high southern latitudes, the winds, especially over the southern ocean, and turbulent mixing in the ocean interiors; we will discuss all of these more in chapter 4. Suffice it to say now that the dense water at high latitudes sinks and moves equatorward in the deep ocean, filling the abyss in both hemispheres with water that is very cold and dense. The overturning circulation is much stronger in the Atlantic than in the Pacific because the water at high northern latitudes is saltier, and hence denser, in the Atlantic than in the Pacific, but in both basins the deep water is cold and dense. However, the upper ocean at lower latitudes contains warmer water circulating in the gyres, and this water extends down almost a kilometer deep. The upshot is an

Figure 2.4. The zonally averaged density in the Atlantic Ocean. Note the break in the vertical scale at 1,000 m.3

Figure 2.4. The zonally averaged density in the Atlantic Ocean. Note the break in the vertical scale at 1,000 m.3

ocean structure as illustrated in figure 2.4, with warm, light water in the subtropical gyres literally floating on top of the cold, dense abyssal water that has come from high latitudes. A typical vertical profile passing through the subtropical gyre might look something like the schematic shown in figure 2.5. We may identify three distinct features: the mixed layer, the thermocline, and the abyss, so let's talk about these features now.

The mixed layer

The mixed layer is the topmost layer of the ocean, in which the water is (no surprise) well mixed, so that its radiative radiative

layer, typically 50-100 m deep, turbulence and convection act to keep the temperature relatively uniform in the vertical. Below this layer, temperature changes over a depth of a few hundred meters, in the thermo-cline, before becoming almost uniform at depth, in the abyss. Adapted from Marshall and Plumb, 2007.

layer, typically 50-100 m deep, turbulence and convection act to keep the temperature relatively uniform in the vertical. Below this layer, temperature changes over a depth of a few hundred meters, in the thermo-cline, before becoming almost uniform at depth, in the abyss. Adapted from Marshall and Plumb, 2007.

temperature and salinity are almost uniform with depth. The mixing comes about through mechanical stirring from the wind and by convection that occurs when the water becomes statically unstable—that is, when a denser patch of water lies on top of a lighter patch of water. The differences in density normally arise because of the temperature differences (warmer water is light)

but might also arise because of salinity effects (saltier water is denser), for example, if evaporation produces salty water at the surface. Typically, the mixed layer is about 50-100 m thick, but in places it might go much deeper if there is intense convection, which mostly occurs in high latitudes.

The thermocline and the abyss

The thermocline is the layer of water in which temperature varies quite rapidly from the warmth of the mixed layer to the cold of the abyss, and typically it is 500-1,000 m thick. The abyss is a very thick layer of water that stretches from the base of the thermocline to the bottom of the ocean. The water in the abyss has its origins at high latitudes in both Northern and Southern hemispheres and so is cold and dense. The thermocline may thus be regarded as a transition region, or boundary layer, connecting the cold abyss with the warm surface layers. It is a rather complicated transition region because a number of interacting physical processes occur there. First, the thermocline has a seasonal component because the temperature in the mixed layer varies with the seasons, whereas the abyssal temperature is almost constant year round. The temperature difference across the thermocline is thus larger in summer than in winter. In the upper thermocline, the seasonal variations manifest themselves most. This region is called the seasonal thermocline, and it may be several tens of meters thick. The lower, unchanging part of the thermocline is called the permanent thermocline, and this part is up to several hundred meters thick. The thermocline is not just a static transition between cold and warm waters; rather, it contains the circulating waters of the great gyres. Finally, we note that the thermocline is rather different in the subpolar gyres than in the subtropical gyres. Typically, it is weaker in the former because the surface waters are already quite cold there, so there is less of a transition region—that is, a much less distinct thermocline. Indeed in some places in the subpolar gyres, the mixed layer is very deep, with well-mixed convective regions extending well into the abyss. Nevertheless, the upper waters are still circulating and over most of the subpolar gyre there is still a thermocline, albeit a somewhat weak one.

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