Wind driven currents are, as the name implies, currents that are created by the force of the wind exerting stress on the sea surface. This stress causes the surface water to move and this movement is transmitted to the underlying water to a depth that is dependent mainly on the strength and persistence of the wind. Most ocean currents are the result of winds that tend to blow in a given direction over considerable amounts of time. Likewise, local currents, those peculiar to an area, will arise when the wind blows in one direction for some time. In many cases the strength of the wind may be used as a rule of thumb for determining the speed of the local current; the speed is figured as 2 percent of the winds force. Therefore, if a wind blows 3 or 4 days in a given direction at about 20 knots, it maybe expected that a local current of nearly 0.4 knot is being experienced.

A wind-driven current does not flow in exactly the same direction as the wind, but is deflected by Earths rotation. The deflecting force (Coriolis force) is greater at high latitudes and more effective in deep water. It is to the right of the wind direction in the Northern Hemisphere and to the left in the Southern Hemisphere. At latitudes between 10N and 10S the current usually sets downwind. In general the angular difference in direction between the wind and the surface current varies from about 10 degrees in shallow coastal areas to as much as 45 degrees in some open ocean areas. Each layer of moving water sets the layer below in motion. And the layer below is then deflected by the Coriolis effect, causing the below layer to move to the right of the overlying layer. Deeper layers move more slowly because energy is lost in each transfer between layers. We can plot movements of each layer using arrows whose length represents the speed of movement and whose direction corresponds to the direction of the layers movements. The idealized pattern for a surface current set in motion by the wind in the Northern Hemisphere is called the EKMAN SPIRAL. Each layer is deflected to the right of the overlying layer, so the direction of water movement shifts with increasing depth. The angle increases with the depth of the current, and at certain depths the current may flow in the opposite direction to that of the surface. Some major wind-driven currents are the West Wind Drift in the Antarctic, the North and South Equatorial Currents that lie in the trade wind belts of the ocean, and the seasonal monsoon currents of the Western Pacific.

Chapters 6 and 7 of Oceanography, Sixth Edition, by M. Grant Gross, contain additional information on the subjects of waves, tides, coasts, and the coastal oceans.

Coastal currents are caused mainly by river discharge, tide, and wind. However, they may in part be produced by the circulation in the open ocean areas. Because of tides or local topography, coastal currents are generally irregular.

Tidal currents, a factor of little importance in general deepwater circulation, are of great influence in coastal waters. The tides furnish energy through tidal currents, which keep coastal waters relatively well stirred. Tidal currents are most pronounced in the entrances to large tidal basins that have restricted openings to the sea. This fact often accounts for steerage problems experienced by vessels.

Attempts at current prediction in the past have only been moderately successful. There has been a tendency to consider ocean currents in much the same manner as wind currents in the atmosphere, when in actuality it appears that ocean currents are affected by an even greater number of factors. It therefore requires different techniques to be used.

In order to predict current information, it must be understood that currents are typically unsteady in direction and speed. This has been well documented in a number of studies. The reasons for this variability have been attributed to the other forces, besides wind and tides, that affect the currents.

Climatological surface charts have been constructed for nearly all the oceans of the world using data from ships drifts. However, this data has been shown to have limitations and should be used as a rough estimate only.

Synoptic Analysis and Forecasting of Surface Currents, NWRF 36-0667-127, provides a composite method of arriving at current forecasts. This method uses portions of other methods that have been used. Forecasters should make themselves aware of the information contained in this publication.

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