E 60e 90e 120e 150e 180 150w 120w 90w 60w 30w

40 80 120 160 200 240

Fig. 1.3 Annual mean (NCEP-NCAR) net short-wave radiation (W/m2). See color plate section.

There are several places where the latent heat flux is maximal (Fig. 1.4). First, the global maxima of latent heat loss exist in western boundary outflow regimes, such as the Kuroshio and the Gulf Stream, where the warm water brought by these strong western boundary currents meets cold and dry air from the continents, and strong latent heat loss is induced. These places are closely linked to a high rate of evaporation, as will be discussed shortly. Second, centers of strong latent heat loss exist at extratropics/subtropics in both hemispheres. There are areas of very low rate of latent heat flux associated with the cold tongues of surface waters in the eastern equatorial Pacific and Atlantic Oceans. Latent heat loss is generally small at high latitudes, where low sea surface temperature cannot sustain much evaporation.

The backward radiation from the ocean to the atmosphere and outer space is made up of two components: short-wave radiation reflected from the sea surface and long-wave radiation. The long-wave radiation is due to the fact that the equivalent radiation temperature of the Earth is rather low; this outgoing radiation is directly controlled by sea surface temperature and the local atmospheric conditions. The bulk formula for long-wave radiation is

i.e., the heat flux associated with long-wave radiation is the outgoing long-wave radiation from the ocean to the atmosphere minus the backward long-wave radiation from the atmosphere to the ocean. Both these terms are proportional to the fourth power of temperature at

Latent heat flux (W/m2)

Latent heat flux (W/m2)

Fig. 1.4 Annual mean (NCEP-NCAR) latent heat flux due to evaporation (W/m2). See color plate section.

Fig. 1.4 Annual mean (NCEP-NCAR) latent heat flux due to evaporation (W/m2). See color plate section.

the sea surface and of the atmosphere. The annual mean net heat flux of long-wave radiation is shown in Figure 1.5.

Owing to the competition of these two processes, the pattern of outgoing long-wave radiation is more complicated than other fluxes. In general, it is high near the western boundary outflow regimes in the subtropical basins of both hemispheres, especially in the Pacific Ocean. In comparison, it is much lower in the equatorial band and at high latitudes.

Sensible heat loss to the atmosphere is intimately related to the difference between the sea surface temperature and the atmospheric temperature. The most important sites of large sensible heat flux from the ocean to the atmosphere are over the Gulf Stream in the North Atlantic Ocean and the Kuroshio in the North Pacific Ocean. These high sensible heat flux regimes are clearly related to the warm water flowing in both the Gulf Stream and the Kuroshio. Note that, at high latitudes, the annual mean flux of sensible heat is actually from the atmosphere to the ocean. In particular, sensible heat flux in the Indian and Atlantic sectors of the Southern Ocean is from the atmosphere to the ocean, indicating that sea surface temperature is lower than the atmospheric temperature (Fig. 1.6). Such a low sea surface temperature is closely related to the cold deep water brought up by the strong Ekman upwelling driven by the Southern Westerlies under the current land-sea distribution.

The net air-sea heat flux, which is the sum of the four previous terms in the heat balance, is shown in Figure 1.7. As expected, there is a strong heat gain along the equatorial band, in

1.1 Surface forcing for the world's oceans Net long-wave radiation (W/m2)

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