The waters of the ocean are constantly on the move and it is this movement which has profound influences on the climate and living conditions for plants and animals. These ocean currents, flow in complex patterns and are affected by wind, salinity, heat content, seafloor topography, and even the earth's rotation (see illustration above).

The earth's rotation and seasonal winds push surface water away from some western coasts, so water rises on the western edges of continents to replace it. Upwelling of ocean water brings nutrient-rich cold water from the depths up to the surface. Marine life thrives in these nutrient-rich waters. El Nino coditions are affected by this upwelling and we will look further into these conditions later in the course.

It is known that colder and/or saltier water tends to sink. This occurs when sea water entering polar regions cools or freezes, becoming saltier, thereby forming denser deep water. A global "conveyor belt" is formed and is set into motion when this deep water forms in the North Atlantic, sinks, moves in a southernly direction and circulates around Antarctica. From Antartica, this water moves northward to the Indian, Pacific, and Atlantic basins, and in the process takes a thousand years for water which began in the North Atlantic, to find its way into the North Pacific.

Cold surface ocean currents operate a little differently, they come from polar and temperate latitudes, and tend to flow towards the equator. Warm surface currents flow from the tropics to the higher latitudes. Both are driven by atmospheric winds, and the earth's rotation. This Corriolis effect, causes the water to flow in a circular pattern--clockwise in the Northern Hemisphere, and counterclockwise in the Southern Hemisphere.

On a global scale, large ocean currents are constrained by the continental masses bordering the three oceanic basins. Continental borders cause these currents to develop an almost closed circular pattern called a gyre. Gyres form when the major ocean currents connect.

 One of the better known major oceanic surface currents is the Gulf Stream. It is known as a Western boundary current. These currents flow from tropical to temperate latitudes and are generally narrow, jet like flows that travel at speeds between 40 and 120 kilometers per day. Also, their flow extends deepest, usually 1000 meters below the ocean surface. The Gulf Stream is one of the strongest -- warmest, deepest, and fastest of the western boundary currents. It separates open-ocean water from coastal water. Other western boundary currents are: Kuroshio; Brazil; East Australia; Agulhas.