9.2: Deep-Ocean Thermohaline Circulation

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Miracosta Oceanography 101
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Deep-Ocean Thermohaline Circulation

Ocean circulation is also influenced by seawater temperature and density. Cold and salty water (concentrated by surface evaporation) sinks. Elsewhere seawater rises where it is displaced by colder and saltier (denser) water (Figure 15.20). Warm water in the tropics flows in currents to polar regions where it cools. In the Arctic region, the formation of sea ice concentrates the salt in seawater, increasing its density so that it sinks. Cold, salty water sinks both in the Arctic and Antarctic regions, feeding deep ocean circulation. The differences in temperature and salinity (or overall density) is the driving force behind deep-ocean thermohaline circulation (Figure 15.21). The coldest and densest (saltiest) water form around Antarctica where massive amount of sea ice forms. When seawater freezes, the sea ice is salt free (expelling the salt). The expelled salt adds to the saltiness (and density) of the coastal waters around Antarctica, causing them to sink in a slow current into the deep ocean basins. Lesser amounts of sea ice form in the northern Arctic region and around Greenland.

Figure 9.3. Thermohaline Circulation: cold and salty ocean water is dense and sinks. Warm water stays at the surface. Evaporation increasing salinity (and increasing density) before it can sink. Formation of sea ice also increases salinity.

Figure 9.4. Thermohaline density stratification and currents of the world's oceans.

The deep ocean basins have slow moving currents (compared with the surface waters exposed to atmospheric winds. As currents move about the globe, evaporation increases salinity. Increased salinity combined with cooling increases seawater density, allowing affected seawater to sink into the deep ocean. The movement of surface waters downward supplies oxygen to the seabed, assisting in the decay of organic matter. The deep, slow-moving water picks up nutrients from the seafloor and from decaying organic particles sinking through the water column. In locations where deep-water upwells to the surface, these nutrients supply the ingredients for phytoplankton blooms, providing food for the food chain.

Animations: global perspectives of ocean currents based on salinity and temperature
Surface Salinity (annual) (NASA)
Perpetual Ocean (2005-2007) [NASA] global ocean circulation time lapse - YouTube video.
22 Years Sea Surface Temperature 1985-2007 [NOAA Polar satellite data] YouTube video
Worldwide Sea Surface Temperature simulation 2008 YouTube video