Fig. l.l7 Sea surface density (kg/m3), based on WGA-Gl.
is primarily controlled by temperature, except for the high latitudes, where surface density is primarily controlled by sea surface salinity (Fig. 1.17). The high sea surface temperature, in combination with low salinity, gives rise to the lowest sea surface density in the Bay of Bengal and the Warm Pool. There is a zone of low-density water extending from the low-salinity tongues in the eastern equatorial boundary of the Pacific Ocean (Fig. 1.15). In the subtropical basins, density is relatively low in the western basins, as compared with the eastern basins, and this feature is apparently related to the surface temperature pattern shown in Figure 1.14.
One of the salient features in the surface density, temperature and salinity distribution in the world's oceans is the asymmetry in the zonally averaged meridional distribution. In fact, surface water in the Southern Hemisphere is colder and saltier than that in the Northern Hemisphere. As a result, surface water in the Southern Hemisphere is heavier than that in the Northern Hemisphere (Fig. 1.18). The asymmetric nature of meridional distribution of water properties at the sea surface reflects many physical aspects of the climate system. First, the Southern Hemisphere is mostly a water hemisphere, while the Northern Hemisphere is mostly a land hemisphere. More important, the combination of the Antarctic continent and the adjacent circumpolar water channel creates the coldest and densest surface water in the world's oceans, which sinks to the bottom of the world's oceans and dominates the abyssal circulation.
To examine the distribution of surface temperature and salinity more closely, we subtract the zonal mean value and plot the deviation from the zonal mean for both the surface temperature and salinity. The most interesting features that can be identified from the deviation from the zonal mean surface temperature and salinity distribution are as follows.
First, there is a strong east-west asymmetry of temperature distribution in both the Pacific and Atlantic Basins (Fig. 1.19). This feature is closely related to the circulation driven by wind stress. The most direct connection is the coastal upwelling driven by the along-shore wind. For example, the equatorward wind at mid latitudes in both hemispheres drives coastal upwelling along the eastern boundaries, and coastal downwelling near the western boundaries. The strong upwelling driven by trade winds near the coast of Peru brings cold water rich in nutrients from deeper in the ocean to the upper surface layer and thus creates one of the largest fishing grounds in the world's oceans. There are other similar cold-water upwelling zones near the eastern coasts of North and South Atlantic Ocean. As a result, there is an outstanding east-west temperature contrast at mid latitudes in the Pacific and
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