Most of energy exchanges of the ocean occurs at the sea surface. They include absorption of solar radiation and terrestrial radiation from the atmosphere, emission of terrestrial radiation by the sea surface, and exchange of energy with the atmosphere (including energy output accompanying evaporation of water). Also, the fact that sea water is pushed and pulled by winds at the surface (wind stress) is, while a small term in the energy balance, important for the motion of sea water.
The ocean can be basically divided into the surface layer and the deep layer. The boundary between them is not well defined, but as a very rough estimate, it is around 500 m depth. The ocean currents we are familiar of are those of the surface layer, and they are basically caused by wind stress, so they are called `wind-driven circulation'.
Let us consider a region of ocean centered in the subtropics. Easterly winds (trade winds) prevail in the lower latitudes, while westerly winds prevail in the higher latitudes. When sea water near the surface is dragged by the winds, rotation of the earth simultaneously affects. As a result, net transport of water is toward higher latitude in the easterly wind zone and toward lower latitude in the westerly wind zone. Thus water accumulates in the subtropics, and pressure becomes high here. In the steady state where the force due to gradient of pressure and Coriolis force balance, clockwise circulation (viewed from above) is formed in the case of northern hemisphere (opposite in the southern hemisphere). This circulation is called the subtropical gyre. When the variation of Coriolis effect with respect to latitude is taken into account, we obtain that the currents toward higher latitudes are concentrated near the western boundary of ocean basins as Kuroshio of the North Pacific and Gulf Stream of the North Atlantic, and that the currents toward lower latitudes distribute widely in the whole ocean basins.
Similarly, in the subpolar zone to the poleward of the maximum of westerly winds, counterclockwise circulation (in the case of northern hemisphere) is formed and called the subpolar gyre. In this case as well, concentrated currents (such as Oyashio in the North Pacific) form near the western boundary of ocean basins. Both types of wind-driven gyres exchange warm low-latitude water and cold high-latitude water nearly horizontally, resulting in transport of energy from lower to higher latitudes. Wind-driven ocean circulation cannot effectively transport energy poleward, however, when there is no coast that blocks ocean currents (as in the case of present southern middle latitudes).
Water masses and fronts:
Similarly to air masses in the atmosphere, we say that water mass is formed when we find water with similar temperature and salinity in a large area. We call such place where temperature and salinity change suddenly (in spatial sense) a front. The boundary between the subtropical gyre and the subpolar gyre is called the subpolar front.
On the other hand, the circulation in the deep layer which comprises most of the mass of ocean is large-scale convection caused by the difference of density of sea water. The density of sea water is determined by temperature and salinity together. So the deep ocean circulation is also called the thermohaline circulation. [The word stems hal in Greek and sal in Latin mean salt.] The density of sea water is larger as temperature becomes lower. There is no inversion of this trend as found in fresh water below 4oC. But the dependence of density on temperature is small near the freezing point (approximately -2oC), and mainly salinity determines density of cold sea water. Salinity is basically determined by water balance between evaporation and precipitation at the surface and supply of fresh water from land. In addition, there is another process when sea ice forms, that the frozen part contains less salt and the part remaining liquid (brine) becomes saltier.
Fig. 4.1-7 shows meridional-vertical cross sections of temperature, salinity, dissolved oxygen, and age of sea water estimated by the radioisotope of carbon (14C), in the present Atlantic Ocean. From many pieces of information like this, it is recognized that most of the deep water in the ocean came from the surface layer in the high latitudes (around 60o -- 70o north) of the North Atlantic. So it is called the North Atlantic Deep Water (NADW). In the deepest parts of the ocean, however, water that sank in the Weddell Sea near Antarctica is found, and called the Antarctic Bottom Water (AABW). Similar cross sections of the Pacific Ocean reveal that the deep water in the Pacific is older in its northern part, and they are NADW that came around the Southern Ocean. As the 14C age tells, the overturning of water in the deep ocean circulation takes one thousand to two thousand years. The time scale is very different from that of wind-driven circulation in the surface layer, which is decades at most.
Though we cannot discern it from the wind-driven circulation with observational data, the thermohaline circulation should include a near-surface current toward the northern North Atlantic that compensates the sinking water. Broecker used a simile of a ``conveyer belt'' (Fig. 4.1-8) to describe the whole cycle of the thermohaline circulation in the global ocean.