Density depends on temperature and salinity of the water.
Thermohaline circulation of the oceans
Cold and salty water is dense and will sink. Warm and less salty water will float. Although tides are generally a dominant driver of water motion in shallow coastal waters, their relative importance in the oceans is less.
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It should be noted, however, that tides are mainly generated in the oceans by the gravitational forces of moon and sun and are amplified when propagating onto the continental shelf see the article Ocean and shelf tides. This wind field pattern results from the low atmospheric pressure in the tropics warm ascending air and high atmospheric pressure in the subtropics cooled descending air.
The near-surface air flow - toward the equator at low latitudes and toward the poles at high latitudes so-called Hadley cells - is deflected by earth rotation , hence giving rise to the Westerlies and the Trade winds.
The oceanic thermohaline circulation : an introduction / Hendrik M. van Aken - Details - Trove
The thickness of the surface layer entrained by wind is of the order of meters about the thickness of the thermocline at low- and mid-latitudes , up to a maximum of m. Due to earth rotation the main ocean current system consists of large anticyclonic gyres clockwise rotating in the northern hemisphere and anticlockwise in the southern hemisphere . The Antarctic Circumpolar Current is situated in the Southern Ocean and constantly circles around Antarctica because there are no land masses to interrupt the currents. It is an eastward-flowing current driven by the dominant western winds at this latitude.
The most famous ocean current, the Gulf Stream , is a vast moving mass of water, transporting an enormous amount of heat from the Caribbean across the ocean to Europe. It passes by the US east coast as a narrow jet, due to the northward increase of the Coriolis effect  and then spreads out as a meandering current over the ocean while generating a series of meso-scale eddies and whirls. The North Atlantic Gyre is completed by the Canary Current in the Eastern Atlantic that transports relatively cold water south and west. The Kuroshio is a warm boundary current in the north-western Pacific, similar to the Gulf Stream.
In regions where Ekman transport deflects the boundary current from the coast, water from the deep ocean rises to the ocean surface, see figure 2. This phenomenon is called 'upwelling' and is very important for enrichment of surface waters with organic matter and nutrients. Upwelling zones are characterized by a very rich marine life with abundant resources for fishery.
taylor.evolt.org/voluh-maluenda-dating-apps.php Upwelling also occurs at the equator at the Pacific Ocean Equatorial upwelling. The North Equatorial Current is deflected to the north and the South Equatorial current to the south as a consequence of the Coriolis effect. This produces upwelling of nutrient rich water and cooling of the surface water near the equator of the Pacific, see figure 3. Downwelling zones exist north and south of the equator. Instability of the coupled ocean-atmosphere dynamics produces large fluctuations in the climate of the Pacific region, which are felt at the global scale.
Weakening of the easterly trade winds allows warm water from the Western Pacific to flow back with the Equatorial Counter Current to the eastern South American boundary, where upwelling currents of cold deep ocean water are shut off. This results in relative warming of the Eastern Pacific lowering the sea surface atmospheric pressure and relative cooling of the Western Pacific increasing the sea surface atmospheric pressure and hence induces a further weakening of the easterly trade winds.
This feedback strengthens the so-called El Nino phase of the oscillation  . The shut-off of the food-rich upwelling currents has major consequences for marine life and fisheries. After a number of years three on average, but variable the system sweeps back to the opposite phase, called La Nina. The onset and offset of the oscillation are still not fully understood.
Deep ocean circulation is primarily driven by density differences. It is called thermohaline circulation , because density differences are due to temperature and salinity. However, the water masses moving around by thermohaline circulation are huge. Density gradients alone are not sufficient for sustaining the deep ocean circulation.
Upwelling and mixing processes, to bring deep ocean water back to the surface, are required too .
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The density of surface water increases when frigid air blows during winter across the ocean at high latitudes. The water density increases further by evaporation and by salt expulsion when sea ice is formed. From these regions, a cold deep water layer spreads over the entire ocean basins. The thermohaline circulation moves water masses around between the different ocean basins  . The conveyor belt is fed in the northern North Atlantic with high-salinity water due to evaporation supplied by the Gulf Stream , which sinks to great depth after cooling down in the Arctic region, forming the North Atlantic Deep Water NADW.
The replacement of this dense sinking water generates a continuous surface flow feeding the conveyor belt.
Combination of wind-driven and thermohaline circulation. Hydrological cycle in the ocean. Mixing and Circulation in SCS.
Lozier, Susan, Duke Uni. Double diffusion in the ocean. AAIW decadal change and eddy activity. Compositional transport during sea-ice solidification. Zhong, Jin-Qiang, Tongji Uni. Zheng, Quanan, Uni. Maryland, USA. Lin, Yihua, IAP. Tian, Jiwei, OUC. Water mass formation. Wang, Wei, OUC. Mixing in ACC.