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Thermohaline Gradients
Ocean water is not uniform in its composition. Within a given water column (i.e., an area of ocean from the surface down to the sea floor) there are differences in both temperature and salt concentration. In general, warmer, fresher water is found at the surface because it is less dense and cooler, and saltier water is found at the bottom. When a mass of water becomes cooler or saltier it becomes denser and sinks. Similarly, the gradual warming of deep water can cause it to rise in an upwelling. Thus, thermohaline gradients are a major generator of ocean current.
Surface and Deep Currents
Thermohaline effects are the predominant driving forces behind deep ocean currents below the first 400 meters of the ocean. Although they don't greatly influence the surface currents, they are dependent upon the heating, cooling and freshwater flux (the addition or evaporation of fresh water) in these upper layers of the water. The two main mechanisms that ultimately affect the deep currents are turbulent mixing, where tides and surface currents mix with and warm the underlying layers of ocean, and thermohaline forcing.
Thermohaline forcing is when fresh water or heat is added to deep water, forcing it to rise. When a large mass of deep water rises to the surface, it is called upwelling. Deep water is rich in nutrients but low in oxygen--two factors that influence what forms of sea life can live in this water.
The North Atlantic Current
The North Atlantic current is a good example of thermohaline effects on ocean currents. A large current of relatively warm, fresh water moves up through the Atlantic Ocean near the surface. When this water reaches the northern latitudes along the coast of Greenland and Eastern Canada, salinity increases (through freezing and wind evaporation) while the northern climate cools the water. This causes large quantities of dense water to sink and form the North Atlantic Deep Water mass.
This sinking mass of water crawls along the ocean basin. The process is a major driving force for the global "conveyor belt" of ocean currents. These currents have profound effects on global climate. For instance, without this current, warm water wouldn't move up the East Atlantic and the climate of Western Europe would become somewhat cooler. However, some researchers, such as Columbia University's Richard Seager, posit that the effect of this current on Europe's climate is somewhat overstated.
Pacific Currents
The Pacific Ocean is relatively devoid of deep currents because it lacks a powerful thermohaline force, such as the one that drives Atlantic currents. While the Pacific has powerful wind-driven surface currents, especially around the equator, it is almost completely devoid of currents below 2,000 meters.
Thermohaline effects still contribute to the relatively shallow currents of the North Pacific. For instance, saline water from the Sea of Japan adds density to the surface water, resulting in vertical ventilation within this area of ocean. Below this circulating layer, the water is essentially static. This makes the bottom of the North Pacific the oldest deep water body in the ocean.