Aerographers Mates have a knowledge of the major ocean currents and the meteorological results of the interaction of sea and air. Oceanic circulation (currents) plays a major role in the production and distribution of weather phenomena. Principal surface current information such as direction, speed, and temperature distribution is relatively well known.

Currents in the sea are generally produced by wind, tide, differences in density between water masses, sea level differences, or runoff from the land. They maybe roughly classed as tidal or nontidal currents. Tidal currents are usually significant in shallow water only, where they often become the strong or dominant flow. Nontidal currents include the permanent currents in the general circulatory systems of the oceans; geopotential currents, those associated with density difference in water masses; and temporary currents, such as wind-driven currents that are developed from meteorological conditions. The system of currents in the oceans of the world keeps the water continually circulating. The positions shift only slightly with the seasons except in the Southeast Asia area where monsoonal effects actually reverse the direction of flow from summer to winter. Currents appear on most charts as continuous streams defined by clear boundaries and with gradually changing directions. These presentations usually are smoothed patterns that were derived from averages of many observations.

The speed of a current is known as its drift. Drift is normally measured in knots. The term velocity is often interchanged with the term speed in dealing with currents although there is a difference in actual meaning. Set, the direction that the current acts or proceeds, is measured according to compass points or degrees. Observations of currents are made directly by mechanical devices that record speed and direction, or indirectly by water density computations, drift bottles, or visually using slicks and watercolor differences. Ocean currents are usually strongest near the surface and sometimes attain considerable speed, such as 5 knots or more reached by the Florida Current, In the middle latitudes, however, the strongest surface currents rarely reach speeds above 2 knots.

Eddies, which vary in size from a few miles or more in diameter to 75 miles or more in diameter, branch from the major currents. Large eddies are common on both sides of the Gulf Stream from Cape Hatteras to the Grand Banks. How long such eddies persist and retain their characteristics near the surface is not well known, but large eddies near the Gulf Stream are known to persist longer than a month. The surface speeds of currents within these eddies, when first formed, may reach 2 knots. Smaller eddies have much less momentum and soon die down or lose their surface characteristics through wind stirring.

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