Unit 3: Oceans // Section 3: Ocean Currents
Mixing is a key dynamic in the oceans, creating currents and exchanges between cold, deep waters and warmer surface waters. These processes redistribute heat from low to high latitudes, carry nutrients from deep waters to the surface, and shape the climates of coastal regions.
Several types of forces cause ocean mixing. Waves and surface currents are caused mainly by winds. When winds "pile up" water in the upper ocean, they create an area of high pressure and water flows from high to low pressure zones. Ocean currents tend to follow Earth's major wind patterns, but with a difference: the Coriolis force deflects surface currents at an angle of about 45 degrees to the wind—to the right in the Northern Hemisphere, left in the Southern Hemisphere. (For more about the Coriolis force, see Unit 2, "Atmosphere.") This pattern is called Ekman transport, after Swedish oceanographer Vagn Ekman. Each layer of the ocean transfers momentum to the water beneath it, which moves further to the right (left), producing a spiral effect (Fig. 4).
Figure 4. Ekman spiral
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Source: © 2004. Oceanworld, Texas A&M University.
At deeper levels, ocean mixing is caused by differences in density between colder, saltier water and warmer, fresher water. Because the density of water increases as it becomes colder and saltier, it sinks at high latitudes and is replaced by warm water flowing northward from the tropics. (This pattern, called the Thermohaline Circulation, is a key mechanism that helps to regulate Earth's climate and is discussed further below and in Section 4.) Cold water typically flows below warmer water, but when winds blowing along coastlines deflect warm surface currents away from shore through Ekman transport, they allow cold, nutrient-rich water to rise to the surface. This coastal upwelling process occurs against western coastlines in the Atlantic, Pacific, and Indian oceans.
The combined effects of these forces create circular currents called gyres in the world's largest oceans, centered at about 25° to 30° north and south latitudes. Gyres rotate clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere, driven by easterly winds at low latitudes and westerly winds at high latitudes. Due to a combination of friction and planetary rotation, currents on the western boundaries of ocean gyres are narrower and flow faster than eastern boundary currents. Warm surface currents flow out of ocean gyres from the tropics to higher latitudes, and cold surface currents flow from colder latitudes toward the equator (Fig. 5).
Figure 5. Ocean currents
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Source: © 2004. Arctic Climate Impact Assessment, Graphics Set 1, p. 21.
How does ocean circulation affect Earth's climate? The oceans redistribute heat from high to low latitudes by moving warm water from the equator toward the poles. They also cause a net transfer of heat from the Southern to the Northern Hemisphere. As currents flow, they warm or cool the overlying atmosphere. The most famous example is the Gulf Stream, the fast-moving western boundary current that flows north through the Atlantic Ocean and makes northern Europe much warmer than Canadian provinces lying at the same latitudes.
In areas where coastal upwelling brings cold water up from the depths, cold currents have the opposite effect. As one illustration, San Diego, California, and Columbia, South Carolina, lie at the same latitude, but coastal upwelling in the eastern Pacific brings cold water into the California Current system, which runs south along the California coast. This process helps to keep peak summer temperatures in San Diego at about 78°F, compared to 95°F in Columbia.
Ocean waters are warmest in the tropics and coldest at the poles because the sun heats the equator more strongly than high latitudes. Surface water temperatures can be 30°C warmer at the equator than in polar regions (Fig. 6). Short-term variations in ocean surface temperatures, from day to night and summer to winter, are mainly influenced by the sun's energy. However, as we will see below in Section 5, some longer-term temperature changes are not driven directly by the sun but rather by complex atmosphere/ocean interactions that occur on seasonal, annual, or multi-year cycles.
Figure 6. Global sea surface temperatures, July 1–4, 2005
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Source: © NOAA Satellite and Information Service. National Environment Satellite, Data, and Information Services.
The oceans respond to temperature changes more slowly than the atmosphere because water has a higher specific heat capacity (SHC) than air, which means that it takes more energy input to increase its temperature (about four times more for liquid water). Because water warms and cools more slowly than land, oceans tend to moderate climates in many coastal areas: seaside regions are typically warmer in winter and cooler in summer than inland locations. On a larger scale, by absorbing heat the oceans are delaying the full impact of rising temperatures due to global climate change by decades to centuries. (For more details, see Unit 13, "Looking Forward: Our Global Experiment.")