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Fig. 1.6 Annual mean (NCEP-NCAR) sensible heat flux in the world's oceans (W/m2). See color plate section.

Net air-sea heat flux (W/m2)

Net air-sea heat flux (W/m2)

Fig. 1.7 Annual mean (NCEP-NCAR) net air-sea heat flux in the world's oceans (W/m2). See color plate section.

Fig. 1.7 Annual mean (NCEP-NCAR) net air-sea heat flux in the world's oceans (W/m2). See color plate section.

particular the cold tongues in both the Pacific and Atlantic Oceans. In addition, the western coasts of South America and Africa appear as heat absorption bands, linked to the downward sensible heat flux, which is due to the low sea surface temperature associated with strong coastal upwelling. Both the Kuroshio and the Gulf Stream are major sites of heat loss in the world's oceans. The high-latitude Atlantic Ocean appears as another major site of heat loss in the world's oceans, which is related to the present-day strong meridional overturning in this basin.

Another major feature of this map is that the net heat flux is asymmetric with respect to the equator. Given the strong net heat loss at high latitudes in the Northern Hemisphere, one may expect a similar situation to occur in the Southern Hemisphere. However, a close examination reveals a different pattern. In fact, in the Indian sector and the South Atlantic sector of the Southern Ocean, the net heat flux is downward, i.e., the ocean there gains heat from the atmosphere, instead of losing heat. Comparing Figures 1.6 and 1.7, it is readily seen that these areas of net heat gain in the Southern Ocean are closely related to the downward sensible heat flux associated with the cold water upwelling driven by the strong westerlies in this latitudinal band.

The net air-sea heat flux distribution shown in Figure 1.7 implies that there is a meridional heat transport in the ocean, otherwise the underlying ocean would continuously cool or warm depending on the sign of the heat flux. In order to demonstrate the meridional heat flux, we first calculate the zonally integrated net air-sea heat flux, then integrate the zonal heat flux meridionally, starting from the South Pole, Hf = J0 qad0, where a is the radius of the Earth, 0 is the latitude, 0S is the latitude of the South Pole, and q = q(0) is the meridional distribution of net air-sea heat flux obtained by zonally integrating the flux shown in Figure 1.7. Accordingly, a positive slope of the curve shown in this figure indicates a downward heat flux into the ocean at the latitude of concern, and a negative slope indicates an upward heat flux at this latitude. For example, the strong positive slope over the equatorial band and the latitudinal band of 58° S-42° S indicates strong heat absorption by the ocean (Figure 1.8a).

On the other hand, a positive value of Hf indicates the northward heat flux in the oceans; thus, over the entire Northern Hemisphere, there is a poleward heat flux. As a matter of fact, in the Northern Hemisphere, the poleward heat flux reaches a maximum of nearly 2 PW (1 PW = 1015 W) around 15° N. The corresponding poleward heat flux in the Southern Hemisphere is much smaller and changes its sign several times. In fact, the result obtained from this approach shows a northward heat flux in the latitudinal band of 58° S-20° S; however, values of poleward heat flux obtained from other more comprehensive methods indicate that meridional heat flux in the ocean is mostly southward in the Southern Hemisphere, as discussed in Section 5.3.1. Such a large discrepancy in poleward heat flux is due to the fact that the air-sea heat flux data obtained from observations is not very accurate, especially in the Southern Ocean, where reliable in situ observations are sparse.

Similarly, there is a strong zonal transport of heat in the ocean. In order to demonstrate the zonal heat transport, we integrate the net air-sea heat flux, starting from the longitude of the southern tip of South America. As shown in Figure 1.8b, there is a westward heat flux in the Pacific Basin, with a peak value of 1.4 PW. The zonal heat flux is a manifestation of the zonal asymmetric nature of the thermal forcing of the oceans. This zonal heat flux is intimately linked to the oceanic currents, which are discussed in later chapters.

Surface freshwater flux The oceans exchange freshwater with the atmosphere through evaporation and precipitation, plus river run-off. The river run-off is the result of water vapor from sea surface evaporation and precipitation on land. Freshwater exchange with the atmosphere is one of the most important forcing conditions for both the oceanic general circulation and the climate system. Evaporation is the most crucial vehicle for bringing heat from low-latitude ocean to the atmosphere, where water vapor is carried poleward. Water vapor carries a large amount of latent heat, and this is one of the vital mechanisms of poleward heat transport in the climate system. Water vapor in the atmosphere eventually condenses and releases the latent heat content, returning to the oceans or land as precipitation.

Freshwater flux through the air-sea interface plays a vital role in regulating the hydrolog-ical cycle in the ocean. In particular, freshwater flux is the key ingredient in controlling the salinity distribution in the oceans. Water density is primarily controlled by temperature and salinity, and thus freshwater flux is one of the key players in regulating the thermohaline circulation through its direct connection with the salinity distribution, which is one of the most important topics related to thermohaline circulation and climate. Meridional transport

Northward heat flux

Northward heat flux

Eastward heat flux
Wind Cycle Diagram
Fig. 1.8 a Northwardand b eastward heat transport Hf in the world's oceans, in PW(1PW = 1015W).
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