Movements of the ocean water include the tides, currents, and waves caused by the wind, and the slow creep of ocean water at different temperatures, in which the warmer rises and the colder sinks. These and other movements of ocean water affect man in many ways. They influence the climate, the distribution of plants and animals, especially those that are found on remote islands, the sea routes followed by man in his explorations and migrations, and the erosion and deposition of material along shorelines.
The relative temperatures of masses of ocean water affect the temperatures of the lower air, which in turn markedly affect atmospheric pressure. Cold surface water tends to be surmounted by high atmospheric pressure, and warm-water areas by lower atmospheric pressure. In turn the weather of continents, especially on their windward side, often depends on the great offshore areas of high and low air pressure that affect the temperature, precipitation, windiness, and cloudiness. Another weather factor is the temperature of the currents along windward coasts that have important influences upon land temperatures. Likewise the food supply for fish is related to the temperature and movements of ocean currents. Abundant food supplies for fish are found in cold currents, especially those in which water is welling up from the depths.
The surface circulation of ocean water depends mainly on prevailing winds, rotation of the earth, and location of the continents. Subsurface movements are slow and might best be referred to as creeping of the water. They result chiefly from heating of the water in the tropics and chilling in the polar regions. The chilled water creeps slowly toward the tropical regions where surface water is heated and moves on top of the ocean toward higher latitudes. Small differences in salinity result locally from large surface evaporation, heavy precipitation, and inflow of big rivers of fresh water.
The trade winds and prevailing westerlies start drifts of the ocean water that the rotation of the earth and north-south running coasts of the continents help to divert into gigantic eddies. The drift in the north Pacific is divided at about the longitude of Hawaii into two circulating drifts that have a clockwise motion, and of which the western is much the larger. The similar drift in the south Pacific moves counterclockwise. The water of the main North Pacific Drift comes chiefly from the westward-flowing North Equatorial Current. This current was set in motion by the northeast trade winds, which in low latitudes blow nearly from an easterly direction. The water is warm because of being heated during its passage for 10,000 miles through the tropics; therefore, the western and northern parts of the North Pacific Drift have temperatures above normal for the latitudes. Where this warm water moves by Japan it is called Kuroshio or the Japan Current. Part of this water may reach the coast of North America. The North Pacific Drift carries enormous quantities of warm water from the tropics into the temperate zone. The influence of the South Pacific Drift is less obvious than its counterpart in the northern hemisphere partly because there are no large land masses to be affected by it in the colder latitudes. Most of Australia is closer to the equator than the United States or Japan. The current is also scattered by the many islands in the western Pacific through which it flows.
The trade winds cause an Equatorial Current to flow westward both north and south of the doldrums. When these two currents reach the islands north and northwest of New Guinea the water piles up. Part is deflected north to feed the North Pacific Drift, and part returns eastward through the calm doldrums to form the Equatorial Countercurrent. Both the Equatorial Current and the Equatorial Countercurrent are warm and steady in their flow.
In the great southern ocean that extends around the world without interference from continents a current develops that flows towards the east and is called the West Wind Drift. Along the west coast of South America is the cold Peru (Humboldt) Current, which flows north almost to the equator. This in part represents a portion of the West Wind Drift that has been deflected north, and in part a welling up of cold water from the depths when the southeast trades blow away the warm surface water. The south-flowing Kamchatka Current (Oyashio) off northeast Asia comes from the Bering Sea and hugs the east coast because of the deflective effect of the earth's rotation. Offshore winds and resulting welling up of cold water help account for its being a cold current. The southward-flowing California Current off the Pacific coast of the United States is another cold current whose temperature, low for the latitudes, results from offshore winds and the welling up of cold water after the warmer surface water is blown away. North of the North Pacific Drift is an eastward-flowing cold current called the Aleutian whose water is doubtless a mixture from the Kamchatka Current and the North Pacific Drift.
The location of the cold currents is of importance in that they contain abundant food for fish so that the best fishing grounds coincide with cold waters. The great fishing grounds of northeast Asia and northwest North America are examples of such association with regions of cold currents.
On desert islands in the Peru Current off the coast of Peru there live millions of sea birds that consume fish for food. The guano formed from bird waste is an excellent fertilizer. Each bird deposits several dollars' worth of guano per year, making the birds among the most valuable in the world. Occasionally it happens that the Peru Current is replaced by a southward-flowing warm current called El Niño (literally, The Child), because this warm current is likely to flow farthest south about Christmas time. When this happens the supply of fish food decreases and the fish migrate or die in vast numbers, and flood-producing rains may fall on the coastal desert because warm moist winds replace the usual cool, dry winds.