• Surface ocean currents are driven by winds and involve only 10% of ocean waters.
• Oceanic circulation patterns generate current systems known as gyres.
• Fast-flowing western boundary currents redistribute heat from the relatively warm tropics to cooler high latitudes.
• The Coriolis effect is the name of the apparent deflection of ocean currents or winds to the right of their course in the Northern Hemisphere and to the left of their course in the Southern Hemisphere.
• The global conveyer belt moves heat energy from the tropics to the poles in surface waters and transports cold waters to warmer location by deep ocean circulation.

Ocean Currents
cean surface currents are mainly controlled by climate (temperature, winds) but are also influenced by the distribution of continents and Earth's rotation. Surface currents involve approximately 10% of the world's ocean waters. Sea level is higher at the equator because of thermal expansion of warm waters and diminishes toward the poles. The contrast in the elevation of the ocean surface is about 15 cm (6 inches). In the absence of winds, water would simply flow away from the equator ("downhill") under the influence of gravity. Winds blowing over the ocean exert a frictional drag on surface waters and are the principal force in controlling oceanic circulation. Ocean currents follow wind directions except where wind blows onland. The continents represent barriers to currents, deflecting them to the north or south of their course (Fig. 12).

Figure 12. Distribution of ocean currents. Note circular patterns (gyres) with clockwise pattern north of equator and counterclockwise pattern south of equator.

Global atmospheric circulation patterns generate circular ocean current systems known as gyres that are centered on 30 degrees latitude in each of the major ocean basins (Fig. 12). Circulation of the gyres is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Surface water might take several months to a few years to complete the circuit of a gyre.

Westerly winds cause water to pile up along the western sides of major oceans. These concentrations of surface water generate fast-flowing western boundary currents that redistribute warm tropical waters toward the poles (Fig. 12). These currents (e.g., Gulf Stream, Kuroshio, Brazil) can be thought of as marine rivers, relatively narrow (less than 100 km across) water masses that flow at speeds of 100 to 200 km/day for thousands of kilometers. The Gulf Stream can transport over 50 million cubic meters of water per second, hundreds of times more water than the Amazon, the world's largest river.

In contrast, the eastern boundary currents (e.g., Canary, California, Peru) that complete the eastern leg of each gyre are wider, carry less water, and move more slowly. The Canary current, nearly one thousand kilometers wide, carries just a third of the volume of water in the Gulf Stream and travels at tens of kilometers per day.

Coriolis Effect
Currents are deflected to the right of their course in the Northern Hemisphere and to the left of their course in the Southern Hemisphere: this pattern is termed the Coriolis effect (Fig. 13; for more on the factors that contribute to this phenomenon see The Coriolis effect).

Figure 13. Objects on Earth's equator travel further (and faster) than objects at higher latitudes. A site located along the equator travels at 1675 km/hr, whereas a site at higher latitudes has a lower velocity. It is this contrast in velocity that results in the Coriolis effect. Objects moving north from the equator have a greater component of eastward motion than objects at higher latitudes and thus appear to deflect to the right of their course.

To an observer on Earth, the path of a north- or south-directed wind or ocean current will appear to be deflected. Note that the wind or current doesn’t actually change direction, but the planet beneath it has changed position. An object (rocket, air mass, ocean current, etc.) that travels directly north or south in the Northern Hemisphere appears to be deflected to the right of its course when viewed from a location on the solid Earth's surface. Objects are deflected to the left of their course in the Southern Hemisphere. The net result of these deflections is the circular path of ocean currents.

Global Ocean Conveyer Belt
Surface ocean currents carry warm water away from the equator and toward the poles. Deeper currents are driven by contrasts in water density and are dependent upon temperature and salinity contrasts below 1,000 meters. The pattern of deep currents is termed thermohaline circulation.

Currents in the North Atlantic cool as they approach the northern latitudes. Cold, salty (dense) water sinks in the North Atlantic Ocean south of Greenland and moves southward as the North Atlantic Deep Water (NADW) current at depths of 2 to 4 km (1-2.5 miles; Fig. 14). When the NADW reaches Antarctica it is diverted to the Indian and Pacific Oceans by the Antarctic circumpolar current. The deep water current eventually comes to the surface (upwelling) in the northern Indian and Pacific Oceans before returning to the Atlantic Ocean by a series of surface currents (Fig. 14). A complete loop may take 1,000 years.

Figure 14. Global oceanic circulation. Cold water sinks in northern Atlantic Ocean and travels southward in deep water before upwelling in the Indian and Pacific Oceans. Surface currents return warm water to Atlantic Ocean.

The sinking of this cold, dense water in the North Atlantic is a key step in the global conveyer belt. This system moves energy from the tropics to the poles and back again  and serves to moderate Earth's climate.