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Circulation Systems
Ocean water is constantly moving around Earth as a result of two circulation systems, the surface system (the upper 400 meters of an ocean) and the deep-water system (deeper than 400 meters). The surface system is primarily wind driven and involves only 10% of total ocean volume. The deep-water system is mostly density driven and involves 90% of total ocean volume.

Circulation Forces
Four forces influence ocean water movement within these circulation systems:

  1. Solar heating causes water to expand, actually raising sea surface height in areas around the equator.
  2. Winds push water up in a “mound” in the direction the wind is blowing.
  3. Gravity pulls water down the “mound” of water against the pressure gradient.
  4. Coriolis effect: The rotation of the Earth causes a clockwise shift in the direction of wind and ocean current flow in the Northern Hemisphere and a counterclockwise shift in the Southern Hemisphere known as the “Coriolis effect”. Similarly, the direction of ocean current flow rotates in a clockwise or counterclockwise direction with increasing depth from the ocean surface, depending on the location in the Northern or Southern Hemisphere. Over large distances, these surface currents form ocean “gyres” that rotate around large areas of relatively stationary water.

Because of the Coriolis effect, winds blowing from the north along the west coast of Northern Hemisphere continents tend to drive surface ocean currents to the right of the wind direction, namely offshore toward the west. Colder, nutrient-rich deeper water is then drawn up from beneath to replace the water that was pushed offshore, resulting in an “upwelling of cold” zone. Upwelling zones occur on the west coasts of Southern Hemisphere continents where there are strong northerly winds, and also occur in some equatorial zones of where winds and ocean currents diverge. Because of the rich supply of nutrients upwelling zones support abundant phytoplankton and zooplankton on which larger animals feed.

Thermohaline Circulation
Density-driven circulation is called thermohaline circulation. Cold, dense water from the polar regions sinks and flows along the ocean floor toward the equator where it slowly rises to the surface, circulating in a great convection system. Circulation in deep ocean water takes a long time, because there is less energy there than in surface systems. Denser water masses sink below lighter ones and spread laterally when they meet other water masses at what are called convergence zones.

As these water masses mix, their salinity, temperature, and oxygenation gradually change. In fact, large water masses are identified by these physical chemical characteristics. A deep-water mass called the Mediterranean Outflow Water is a result of the high evaporation rate in the Mediterranean, which increases the salinity of the water mass. As this water leaves the Mediterranean basin, it spreads out and sinks into the Atlantic Ocean until it meets the Atlantic Deep Water mass, which is much cooler and denser.

El Niño
Models of the continuing cycle of ocean circulation depict trends in currents and circulation that can be used to forecast climate, sea levels, and oceanic-atmospheric interactions. One of these types of atmospheric interactions is El Niño, a disruption of the ocean-atmosphere system in the tropical Pacific that has widespread consequences for weather around the planet.

Under normal weather conditions, trade winds blow west across the tropical Pacific and pile up warm surface water so that the sea surface is about one-half meter higher at Indonesia than at Ecuador. The normal upwelling of cold water from deeper sea levels keeps the temperatures off the west coast of South America cool. During El Niño, however, the trade winds are weaker in the central and western Pacific, and ocean waters in the eastern Pacific become cooler. Upwelling of cold ocean water off the coast of South America becomes less efficient, and the surface temperature of the ocean there rises. Rainfall follows the warmer water eastward, so that rainfall increases, sometimes catastrophically, across the southern United States and Peru, while droughts occur in the West Pacific. The ability to predict these trends will enable us to better prepare for changes in our climate and weather.

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