equatorial countercurrents are major surface flows that carry water eastward in the Atlantic, Indian, and Pacific Oceans. They are located near the equator and are sandwiched between two westward-flowing currents, the North Equatorial Current and the South Equatorial Current. Equatorial counter-currents are unique, in that they flow in the opposite direction of the surface winds. The other major surface currents in the tropics flow in the same direction as the prevailing winds.
The equatorial countercurrents are driven by a distinct surface wind pattern in the tropics. Strong westward trade winds result in westward surface flow in most of the tropical Atlantic and Pacific Oceans. However, several hundred mi. (km.) north of the equator the winds are much weaker, in comparison. The stronger winds to the south pile up water where the winds are weak. As a result, the surface of the ocean can be up to 6 in. (15 cm.) higher and the thermocline (region of strongest decrease of temperature with increasing depth) as much as 328 ft. (100 m.) deeper than it is directly to the north. The excess water flows eastward under the influence of the Earth's rotation, giving rise to the equatorial countercurrents. In the Indian Ocean, the equatorial countercurrent is located several hundred mi. (km.) south of the equator, but is caused by a similar mechanism. In all three oceans, the equatorial countercurrent is concentrated in the upper 656 ft. (200 m.), above the thermocline.
The intensity of the equatorial countercurrent varies from season to season and from month to month. The strongest seasonal changes occur in the Atlantic Ocean. Eastward flow reaches a maximum in the summer and fall, with speeds of up to 12 in. (30 cm.) per second, and disappears in the spring. Seasonal changes are weaker in the Pacific Ocean. Here, the equatorial countercurrent exists year-round, and is strongest in the fall and winter, with speeds slightly
greater than those in the Atlantic. In both the Pacific and Atlantic the equatorial countercurrent is located farthest north in the fall, centered near 8 degrees north. In the Indian Ocean, the countercurrent is present only during the winter.
There are large month-to-month changes in the strengths of the Pacific and Atlantic equatorial coun-tercurrents. The changes are most pronounced during fall in the Pacific, and during summer in the Atlantic, and are associated with clockwise rotating eddies centered between the equatorial countercurrent and the South Equatorial Current to the south. The eddies move westward, driven by the contrast in temperature and flow between the equatorial countercurrent and the South Equatorial Current. Over an entire season, they act to decrease the strength and temperature of the equatorial countercurrent.
The equatorial countercurrent plays an important role in the circulation of mass, heat, and salt in the tropical oceans. It provides one of the pathways through which warm surface water returns eastward after being transported westward in the South Equatorial Current. In the Atlantic Ocean, the equatorial countercurrent also transports significant amounts of fresh water eastward from the mouth of the Amazon River. The Amazon water transported eastward decreases the surface salinity of the western tropical Atlantic Ocean. The strength of the Pacific equatorial countercurrent changes during alternating cycles of El Niño and La Niña. During La Niña, the equatorial countercurrent increases in strength, along with an intensification of the other major equatorial currents in the equatorial Pacific. The equatorial countercur-rent becomes weaker during El Niño.
sEE ALso: Atlantic Ocean; Ekman Layer; Equatorial Undercurrent; Indian Ocean; Pacific Ocean; Wind-Driven Circulation.
BIBLIoGRApHY. George Pickard and William Emery, Descriptive Physical Oceanography (Butterworth-Heinemann, 1990); Stephen Pond and George Pickard, Introduction to Dynamical Oceanography (Butterworth-Heinemann, 1983).
Gregory R. Foltz University of Washington/Joint Institute for the Study of the Atmosphere and Ocean
NOAA/Pacific Marine Environmental Laboratory
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