Analogously to the air, in the waters of the earth there are certain currents. The forces that create them are varied. There are drift currents depending on the wind, currents produced by the tide, and currents resulting from the difference of the level of density.
Drift currents are surface currents, produced mainly by the wind. They are not affected by its seasonal behaviour, its speed or direction. The constant currents on the western side of the continents are produced by the passat, or trade winds, which blow in one direction year round. Monsoon winds produce alternating currents in the northern part of the Indian Ocean
As;a consequence of the earth's rotation, the oceans are impacted by a force, which produces a deviation, the so-called Coriolis force, or Coriolis effect. In the northern hemisphere, it produces a deviation, or bending, to the right, and in the southern hemisphere to the left. This force produces spiral bending of drift currents away from the wind direction. The water flows exactly at right angle away from the wind. This means, that in the region of westerly winds, water in the northern hemisphere runs in southerly direction, while in the region of the trade winds it runs in the northerly direction.
Tide currents are produced by the lunar gravity. The gravitational force of the moon produces the tide. In the ocean, these currents are almost negligible, but are very noticeable in the coastal areas, where the tide occurs four times a day (high tide twice a day, low tide twice a day). In contrast with the surface currents, tide currents encompass the entire water mass all the way to the ocean floor.
Deep-ocean circulation dominates in the abyss. It is produced by uneven thermal distribution and salt content (thermohaline circulation). These horizontal density variations occur when, for example, the ocean in polar regions transfers heat into the atmosphere. As it cools, the water gets heavier and sinks. This sinking interrupts the contact of the atmosphere with the surface water.
When during this exchange the water reaches a certain depth, where the densities are again even, it stops sinking and flows from the polar region towards the equator. The evaporation of water in the hot tropics allows the entrance of fresh water from the polar regions. This is, for example, the origin of the deep-ocean current that runs from the Strait of Gibraltar to the Atlantic Ocean.
Ocean currents moving from the equator to higher geographical latitudes, that is to say, towards the earth's poles, are warm currents, whereas currents flowing from higher geographical latitudes towards the lower latitudes are cold ocean currents.
One of the warmer ocean currents is the Gulf Stream, which runs through the northern Atlantic. The Gulf Stream moderates winter temperatures in western Europe, which are higher than the average winter temperatures in these geographical latitudes. Thanks to Gulf Stream, even high up north the Atlantic coast doesn't freeze.
In view of its volume and regularity, the Gulf Stream runs alongside the American coast like an independent ocean river. East of the Mid-Atlantic Ridge, this stream joins in with the surrounding waters and its speed decreases from 10 km/h to 1 km/h.
Alongside the tropical west coasts of the continents run the old ocean currents. However, they do not have such a great impact on the local climate as the Gulf Stream does, since the marine air there is not carried by the winds into the interior. Local trade winds blow parallel to the coast. Due to the deviation to the left (Coriolis force), surface waters are pushed away from the land. Cold water rises to the surface and cools only the immediate coastal belt.
The rising cold ocean currents and cool water frequently produce fog and also cause the formation of deserts. We see this in the northern hemisphere, for example, the desert in lower Carolinas, and in the southern hemisphere, a good example is the desert in Namibia, in southwest Africa.
People always tried to take advantage of these marine currents while seafaring. In the past, this helped in the discovery of new continents. Today, ocean currents serve to shorten the travel time.