How much

On average, both the atmosphere and the ocean transport heat poleward, and this transport is illustrated in figure 5.3. The total transport of the atmosphere plus the ocean may be determined fairly directly from satellite measurements. Over the whole planet, there is a balance between the incoming solar radiation and outgoing longwave radiation, and if there were no heat transport, the incoming solar radiation would equal the outgoing infrared radiation at each latitude—a state of pure radiative balance. In fact, at low latitudes there is an excess of incoming solar radiation, whereas at high latitudes there is an excess of outgoing infrared radiation, meaning that at low (high) latitudes Earth is colder (warmer) than it would be if it were in pure radiative balance. The imbalance arises because heat is transported poleward by the motion of the atmosphere and ocean, and if we measure the imbalance at each latitude, then we obtain the total heat transport by the atmosphere and ocean. Perhaps needless to say, this measurement is easier said than done, but the advent of modern satellites that make separate measurements of solar and infrared radiation makes it possible. The most accurate estimates come from the period of the Earth Radiation Budget Experiment, in particular over the period 1985-1989, when intense observations were made, but data continue to be gathered.

The transport in Earth's atmosphere may be calculated directly because we are constantly taking measurements of the air temperature and its flow for weather forecasts. One way to then determine the total heat transport by the atmosphere at a given latitude is to sum up the product of the temperature and meridional velocity over all longitudes and over the entire depth of the atmosphere. Given the heat transport by both the atmosphere and by the atmosphere-ocean system, the heat transport by the ocean follows by simple subtraction, and this transport is shown in the dashed line in figure 5.3a. It is also possible to calculate the ocean transport directly, using in situ ocean measurements; the advantage is that one may be able to elucidate the individual mechanisms of ocean heat transport rather than just the overall effect. Such direct measurements tend to be less accurate than the residual method because of the sparsity of measurements in the ocean, but the two methods are broadly consistent.

Let's first look at the total heat transport. Evidently in mid- and high latitudes the atmospheric transport is two to three times that of the ocean, whereas in low latitudes the two are comparable, with the ocean exceeding that of the atmosphere at very low latitudes. The atmospheric heat transport, which we won't consider in any detail, takes place via two main mechanisms: In low latitudes, the transport occurs via the zonally symmetric Hadley cell, which takes warm air poleward and cooler air equa-torward. In midlatitudes, the heat transport in the atmosphere occurs through the familiar weather systems,

Latitude

Latitude

Figure 5.3. Upper panel: Heat transport in the total atmosphere-ocean system (solid line), in the ocean (dashed line), and in the atmosphere (dotted line). Lower panel: Oceanic heat transport, subdivided into the various basins. Source: Trenberth and Caron, 2004.

Latitude

Figure 5.3. Upper panel: Heat transport in the total atmosphere-ocean system (solid line), in the ocean (dashed line), and in the atmosphere (dotted line). Lower panel: Oceanic heat transport, subdivided into the various basins. Source: Trenberth and Caron, 2004.

which are continually stirring the atmosphere and bringing warm air with low-l atitude origins poleward and cool, high-latitude air equatorward.

As for the ocean overall, and as we might expect, it transports heat poleward in both hemispheres. The transport is associated with a release of heat into the atmosphere at high latitudes, whereas the ocean is being heated by the atmosphere at low latitudes. Measurements show that the release of heat occurs in two primary locations: at the poleward end of western boundary currents, notably in the western Atlantic and western Pacific oceans at about 40° north and south, and at very high latitudes, in particular in the North Atlantic around Greenland. Another notable feature about the ocean transport is that the poleward transport is much larger in the Northern Hemisphere than in the Southern. In fact, if we look at the contributions from the individual basins in the right-hand panel of figure 5.3, we see that the heat transport is northward (that is, toward the equator) in the South Atlantic! Such a transport is quite remarkable, for it implies that the ocean is not being thermally driven by the meridional temperature gradient alone (which would transport heat from hot places to cold places). What could be the driving force? If we recall our discussion of the oceanic meridional circulation in the previous chapter, we won't be too surprised to discover that it is the wind, but not the wind-driven gyres. Let's look at the mechanisms of ocean heat transport in a bit more detail and try to sort this out.

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