Regional climate is strongly influenced by thermal properties of the Earth's surface, in particular, by neighboring ocean that moderates extreme conditions (Chap. 15). An analogous influence is exerted on global-mean climate. Owing to its large heat capacity and capacity to hold substances in solution, the ocean serves as a reservoir of energy and carbon. It thereby provides thermal inertia to the climate system, figuring importantly in exchanges with the atmosphere of heat and carbon dioxide.

The Earth has only one ocean. It is compartmentalized in major basins, like the Atlantic and Pacific. Those bodies of water are interconnected by currents, circulation systems that exchange mass between ocean basins. Ocean circulations are driven by atmospheric wind stress, which transfers momentum to the ocean. They are also driven by transfers of heat and moisture, which, through density, influence the buoyancy of seawater.

COMPOSITION AND STRUCTURE

Seawater is a solution of pure water and a variety of salts, mostly sodium chloride, which accounts for more than 80% of its ionic composition. Salinity S is the relative concentration of dissolved material, measured in grams per kilogram or parts per thousand (‰). Each is approximately equal to Practical Salinity Units (PSU). Displayed in Fig. 17.1 is surface salinity. It varies between 30 and 40 g/kg. However, values are clustered about 35 g/kg. High salinity is found in the subtropical Atlantic and the Mediterranean Sea. There, as over the subtropical Pacific, salinity is increased by evaporation.