Ocean Currents

Earth is a water planet and the world ocean is always in large-scale motion, slowly rearranging its waters over large extents of latitude and longitude. Whereas tides and waves are vertical motions, ocean currents are horizontal. Currents can be directly forced by surface wind or occur more slowly and deeply because of gradients of temperature, pressure, and salinity. Humans have long been knowledgeable about currents because they are large and readily associated with fishing conditions and navigation. More recently, science has connected changes in currents with global changes in weather and long-term changes in climate. 

There is a known geography to the major currents of the world. Because of the wind emanating outward from the subtropical highs of the global wind and pressure belts, kinetic energy from the fluid that is the atmosphere is imparted to the much denser fluid of the ocean. In that the subtropical highs are semi-permanent features of the atmosphere, this means that air blows from preferred directions over large stretches of oceans for days and weeks at a time. The ocean water starts to move in response though not as fast or in exactly the same direction because of the increased force of friction and the correspondingly lesser Coriolis effect.

The main geographic regularity that can be observed is the presence of oceanic gyres, immense circulations around ocean basins. These gyres have a clockwise flow of water in the Northern Hemisphere and a counterclockwise flow of water in the Southern Hemisphere. This means that there is considerable water being transported from tropical to polar latitudes and vice versa. The main subtropical gyres are joined by Equatorial Countercurrents in the Pacific and Indian oceans.

The strong trade winds on the equatorward side of the gyres cause the westward motion of the water and the piling up of water level in the eastern parts of these ocean basins. The higher water in the east literally runs downhill into the linear area of light and variable Intertropical Convergence Zone winds, and it is this eastward-moving water that is the Equatorial Countercurrent.