Ocean Currents

Surface Ocean Currents
Coastal currents are affected by local winds. Surface ocean currents, which occur on the open ocean, are driven by a complex global wind system. To understand the effects of winds on ocean currents, one first needs to understand the Coriolis force and the Ekman spiral.

Coriolis EffectIf the Earth did not rotate and remained stationary, the atmosphere would circulate between the poles (high pressure areas) and the equator (a low pressure area) in a simple back-and-forth pattern. But because the Earth rotates, circulating air is deflected. Instead of circulating in a straight pattern, the air deflects toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere, resulting in curved paths. This deflection is called the Coriolis effect. It is named after the French mathematician Gaspard Gustave de Coriolis (1792-1843), who studied the transfer of energy in rotating systems like waterwheels. (Ross, 1995).

                                               

Trade Winds 
 In the Northern Hemisphere, warm air around the equator rises and flows north toward the pole. As the air moves away from the equator, the Coriolis effect deflects it toward the right. It cools and descends near 30 degrees North latitude. The descending air blows from the northeast to the southwest, back toward the equator (Ross, 1995). A similar wind pattern occurs in the Southern Hemisphere; these winds blow from the southeast toward the northwest and descend near 30 degrees South latitude.
 These prevailing winds, known as the trade winds, meet at the Intertropical Convergence Zone (also called the doldrums) between 5 degrees North and 5 degrees South latitude, where the winds are calm. The remaining air (air that does not descend at 30 degrees North or South latitude) continues toward the poles and is known as the westerly winds, or westerlies. The trade winds are so named because ships have historically taken advantage of them to aid their journies between Europe and the Americas.
   

Winds Drive Surface Ocean Currents

Global winds drag on the water’s surface, causing it to move and build up in the direction that the wind is blowing. And just as the Coriolis effect deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, it also results in the deflection of major surface ocean currents to the right in the Northern Hemisphere (in a clockwise spiral) and to the left in the Southern Hemisphere (in a counter-clockwise spiral). These major spirals of ocean-circling currents are called “gyres” and occur north and south of the equator. They do not occur at the equator, where the Coriolis effect is not present .

•     There are five major ocean-wide gyres—the North Atlantic, South Atlantic, North   Pacific,South Pacific, and Indian Ocean gyres. Each is flanked by a strong and narrow“westernboundary current,” and a weak and broad “eastern boundary current” .

One particularly powerful western boundary current is the Gulf Stream. The Gulf Stream, paired with the eastern boundary Canary Current, flanks the North Atlantic gyre. The Gulf Stream, also called the North Atlantic Drift, originates in the Gulf of Mexico, exits through the Strait of Florida. It travels at speeds of 25 to 75 miles per day at about one to three knots (1.15-3.45 miles per hour or 1.85-5.55 kilometers per hour). It influences the climate of the east coast of Florida, keeping temperatures warmer in the winter and cooler than the other southeastern states in the summer. Since it also extends toward Europe, it warms western European countries as well.

Ekman spiral

The Ekman spiral, named after Swedish scientist Vagn Walfrid Ekman (1874-1954) who first theorized it in 1902, is a consequence of the Coriolis effect. When surface water molecules move by the force of the wind, they, in turn, drag deeper layers of water molecules below them. Each layer of water molecules is moved by friction from the shallower layer, and each deeper layer moves more slowly than the layer above it, until the movement ceases at a depth of about 100 meters (330 feet). 

Like the surface water, however, the deeper water is deflected by the Coriolis effect—to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As a result, each successively deeper layer of water moves more slowly to the right or left, creating a spiral effect. Because the deeper layers of water move more slowly than the shallower layers, they tend to “twist around” and flow opposite to the surface current.