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The fundamental reason for the world’s surface ocean currents is the sun. The heating of the earth by the sun has produced semi-permanent pressure centers near the surface. When wind blows over the ocean around these pressure centers, surface waves are generated by transferring some of the wind’s energy, in the form of momentum, from the air to the water. This constant push on the surface of the ocean is the force that forms the surface currents.
Around the world, there are some similarities in the currents. For example, along the west coasts of the continents, the currents flow toward the equator in both hemispheres. These are called cold currents as they bring cool water from the Polar Regions into the topical regions. The cold current off the west coast of the United States is called the California Current.
Likewise, the opposite is true as well. Along the east coasts of the continents, the currents flow from the equator toward the poles. These are called warm current as they bring the warm tropical water north. The Gulf Stream, off the southeast United States coast, is one of the strongest currents known anywhere in the world, with water speeds up to 3 mph.
These currents have a huge impact on the long-term weather a location experiences. The overall climate of Norway and the British Isle is about 18°F warmer in the winter than other cites located at the same latitude due to the Gulf Stream.
While ocean currents are shallow level circulations, there is global circulation that extends to the depths of the sea called the Great Ocean Conveyor. Also called the thermohaline circulation, it is driven by differences in the density of the seawater that is controlled by temperature (thermal) and salinity (haline).
It is estimated that once the water sinks in the North Atlantic Ocean that it takes 1,000-1,200 years before that deep, salty bottom water rises to the upper levels of the ocean.