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Effect of Salinity
The ocean does not have a uniform salinity. As ocean water flows
toward the polar region from the Equator, it passes the subtropical
high-pressure zones that have very little precipitation but intense solar
radiation that promotes evaporation. In regions where evaporation is high,
the salt content of the remaining surface water increases, thereby
increasing the density. The Mediterranean Sea receives relatively low
amounts of input from rivers but a large amount of evaporation due to
persistent clear skies and intense solar radiation. As a result, very
dense water is created in the basin which spills out at the basin bottom at
the Strait of Gibraltar, drawing less salty water into the basin at the
upper surface. This effect is graphically revealed in Figure 8.
In regions where precipitation is high, such as the Intertropical Convergence Zone in the central Pacific Ocean shown in the unit on Atmospheric Structure and Circulation (Figure 9), fresh-water rain will ride on top of the saline ocean water. Similarly melting ice in polar regions will be less dense than nearby ocean water of temperature because of its lower temperature and lack of salt.
These concepts of density dependence on temperature and salinity explain the linkage of surface water to the abyssal (deep ocean) circulation. In the North Atlantic ocean, water traveling northward at the surface passes through the subtropical high pressure zone and experiences a density increase due to evaporation. As it continues norward evaporation continues to increase the salinity and hence the density, but cooling due to heat loss to the atmosphere also contributes to increasing the density. Ultimately the density increases to the extent that massive subsidence is created to the north of Iceland. The subsidence region in the Southern Hemisphere is explained for the same reason, since warm water from the tropical Pacific has experienced both evaporation and cooling by the time it reaches the region north of the Weddell Sea (southeast of the southern tip of South America).
The vertical circulation caused by density differences due to differences in ocean temperature and salinity is called the thermohaline circulation. Horizontal global ocean circulation is driven by wind stress at the ocean surface, but vertical mixing is largely due to the thermohaline circulation. A nice aminated depiction is given by the Climate Research Unit of the University of East Anglia.
