Ocean currents move from one place to another in order to equalize temperatures. There are large movements of ocean water usually from a place of warm temperatures to one of colder temperatures or vice versa.
In all the oceans cold currents move from the Polar Regions equator-ward and warm currents from the equatorial regions pole-ward. The effects of ocean currents upon temperatures of the adjacent land areas are variable.
The warm currents raise the temperatures of the nearby coastal regions slightly above the mean values. Similarly, the cold ocean currents tend to lower down the temperatures of the adjacent land areas.
The North Atlantic Drift offers a typical example of the warming effect of the ocean currents on the adjoining land masses. It is a well- known fact that the climate of north-western Europe is modified to a large extent by the warm ocean currents of the North Atlantic.
Along the eastern coast of the United States of America the January isotherms of 30°F (-1.1°C) and 40° F (4.4°C) pass in the vicinity of latitude 40° N. But the January isotherms of 30° F and 40° F pass through Great Britain and the Peninsula of Scandinavia situated in higher latitudes.
This is because of the fact that the North Atlantic Drift, an extension of the warm Gulf Stream, carries with it the warm water to the north-eastern Atlantic, so that the coasts of Great Britain and Norway do not freeze during the winter despite their location in high latitudes.
Because of the prevailing west winds, the moderating effects are carried far inland. Berlin located at latitude 52° N has a mean January temperature equal to that experienced at New York city which is located at latitude 40°N.
The mean temperature for the month of January at London (51°N) is 4.5°C higher than that at New York City. The effects of warm ocean currents are felt most during the colder months of the year.
The effects of cold currents are more pronounced in the tropics or during the summer months in the temperate regions. Cold currents off the western coasts of South Africa or South America lower the temperatures of the adjacent land areas.
The temperature of Walvis Bay located at latitude 23° S is lower than that of Durban which is 6° latitude farther poleward. This is mainly due to the moderating effect of the Benguela cold current flowing near Walvis Bay.
Similarly, the cold California Current keeps the summer temperatures of the subtropical coastal regions of Southern California lower than those of the sea-coasts in the subtropical United States.