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
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
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
COASTAL AND TIDAL CURRENTS
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.
WIND DRIVEN CURRENT PREDICTION
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
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.
COASTAL AND TIDAL CURRENT
Prediction of tidal currents must be based on
specific information for the locality in question. Such
information is contained in various forms in many
Tidal Current Tables, issued annually, list daily
predictions of the times and strengths of flood and ebb
currents and the time of intervening slacks. Due to lack
of observational data, coverage is considerably more
limited than for tides. The Tidal Current Tables do
include supplemental tidal data that can be determined
for many places in addition to those for where daily
predictions are given.