Global ocean currents help to regulate the world’s climate and have important effects on marine life. They are composed of deep ocean currents and surface currents. Deep ocean currents move both horizontally and vertically between the ocean bottom and 400 metres below the surface. Surface currents flow horizontally in the upper 400 metres. Local shoreline currents are surface currents that are influenced by local geography, weather and tides.
Deep Ocean Currents
Deep ocean currents are created by differences in water density. Cold or salty water is more dense and sinks, whereas warm or less salty water is less dense and rises. In polar regions, much of the ocean water is tied up in ice sheets. The salt remains in the liquid water, so the water in these areas is very salty as well as very cold. This water is consequently very dense and sinks. It is replaced by water from lower latitudes. The deep, dense water then moves slowly across the ocean floor and eventually rises in warmer latitudes. These currents are connected, and are sometimes called the ocean “conveyor belt.” One circuit can take 1000 years to complete.
Deep ocean currents; blue indicates cold, deep currents; red indicates warmer currents closer to surface (used with permission from Argonne National Laboratory)
Surface currents are driven by the wind. The world’s prevailing winds create gyres, or giant circular currents in the major ocean basins. Subtropical high pressure areas (at around 30° latitude) drive these gyres. As wind blows clockwise out of the high pressure are, the water underneath is acted on by the wind. Due to the combination of friction and Coriolis forces, the water doesn’t move exactly parallel to the wind direction. Coriolis force, created by the rotation of the earth, causes water in motion to be pulled to the right in the northern hemisphere (in the southern hemisphere, Coriolis deflects currents to the left).
This tendency of the water to turn to the right causes a slight “hill” of water to form in the middle of the gyre. Therefore, gravity prevents water from piling up very high, and keeps the current moving in a circular pattern.
Major surface ocean currents of the world
What types of currents affect British Columbia?
BC is not directly affected by the deep ocean currents, but these currents are important regulators of the global climate. Cold water from polar regions is circulated to warmer latitudes, and warm water is brought to the poles. This natural heating/cooling system helps to absorb changes in air temperature.
The North Pacific surface current flows with prevailing westerly winds and encounters North America near southern British Columbia. The current splits into two when it encounters the land mass; the Alaska current, which flows north along the BC coast, and the California current, which travels south. The North Pacific current is relatively warm, and brings mild, moist weather to the coast of BC.
Upwelling / Downwelling
Upwelling and downwelling are caused by surface currents and have important effects on marine life. As discussed above, Coriolis force causes water to move to the right in the northern hemisphere. Therefore, a surface current flowing southward along a western coast (or northward along an eastern coast) is drawn to the right and water must rise from below to replace it (see diagram below). This condition is called upwelling, and occurs along the southern coast of B.C. and California. Upwelling brings cold, nutrient-rich water to the surface. Plankton feed on these nutrients and form the basis of a rich marine food web.
Downwelling occurs in the northern hemisphere when a north-flowing current travels along a western coast, such as the Alaska current does along the northern B.C. coast and Alaska panhandle. Coriolis still forces the water to turn to the right, but in this case the land forces the water downward and it is replaced by surface water. The surface water is not as high in nutrients as deeper water.
Upwelling vs. downwelling
Depending on the seasons and on climate fluctuations such as El Niño, the exact position of the surface currents varies. Because Vancouver Island is near the point where the North Pacific Current encounters the coast of North America, it is actually in a “transition zone” where the surface currents can vary considerably. Consequently, the types of marine species that live here also vary. The extent to which shorelines are exposed to currents also determines the types of plant and animal communities that occur there.