Countercurrents

marine currents moving in a direction opposite to that of the prevailing winds in a particular region (for example, the equatorial countercurrents) or counter to the flow of previously known, stable surface currents. Counter-currents occur in the surface, subsurface, and abyssal layers and are a very important element of the overall oceanic circulation. Discoveries made in the 1960’s and 1970’s have shown that the direction of subsurface currents is often opposite to that of the surface flow and that therefore countercurrents exist in virtually all parts of the world ocean.

The equatorial countercurrents in the Atlantic, Pacific, and Indian oceans, which have been known since the 19th century, move eastward in the area of convergence of northeasterly and southeasterly trade winds. Because they are caused by the horizontal unevenness of the zonal component of the wind, their velocity and discharge depend on the strength of the trade winds. Subsurface and abyssal countercurrents, discovered in the mid-twentieth century with the development of methods of measuring subsurface and abyssal currents, are caused by the permanent pressure gradient created by climatic wind conditions over the oceans and other factors that promote the formation of marine currents. Countercurrents are often found in straits connecting seas with different water densities, for example, the Strait of Gibraltar and the Bosporus. In this case the surface current moves toward the basin filled with higher-density water and the bottom countercurrent flows toward the basin with the lower water density.

The best-studied countercurrents are the equatorial subsurface countercurrents: the Cromwell Current in the Pacific Ocean, the Lomonosov Current in the Atlantic Ocean, and the Taxeev Current in the Indian Ocean. The Cromwell and Lomonosov currents, the most stable currents in the high seas, move eastward along the equator as a narrow stream beneath the westerly South Equatorial Current. When the trade winds weaken, the equatorial subsurface countercurrents may “emerge” on the ocean surface. South of the equator, between 4° and 13° S lat. in the Atlantic and Pacific oceans, eastward moving subsurface southern subequatorial countercurrents have been discovered with maximum velocities of 30 cm per sec and a maximum discharge of 20 million cu m per sec.

Among the strongest countercurrents observed in either the subsurface or surface layer is the Peru-Chile Countercurrent, also known as the Gunther Current, which was predicted in 1936 and measured in 1960. It moves southward along the coast of South America from 6° to 23° S lat. between the northward flowing Peru Oceanic and Peru Coastal currents. A similar but weaker countercurrent flows along the coast of Angola and Namibia. The best-studied abyssal countercurrent is the one beneath the Gulf Stream. This countercurrent flows southward at depths of 2–5 km and has maximum velocities of 20 cm per sec. Near the bottom it is replaced by a northward stream of antarctic water. Abyssal countercurrents have also been observed along the edges of the Gulf Stream.