FORECASTING
SURFACE CURRENTS
LEARNING OBJECTIVES:
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Distinguish
between tidal and nontidal currents.
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Define
the terms associated with currents.
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Classify
currents as wind driven, coastal, or tidal.
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Identify
publications available to obtain tidal and current information.
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Although
the forecasting of surface currents has been performed by aerographers for a
number of years, the prominence of such forecasting became more evident when a
number of incidents involving large sea-going oil tankers occurred.
Collisions and grounding involving tankers
caused great amounts of pollutants, mainly oil, to be spilled on the water
surface. The movement, both direction and speed, of such contaminants is
directly controlled by the surface currents in the affected area. More concerned
emphasis has now been placed on the ability of forecasters to predict the
movement of such contaminated areas. In the past, NAVMETOC units have provided
forecasts to assist in the location of personnel or boats adrift in the open sea
as well as forecasts used in estimating ice flow.
With the growing concern about pollution
and contamination of ocean waters, it is anticipated that more requests for
current and drift forecasts will be directed to NAVMETOC units.
In this section, we will discuss the
general characteristics of currents, how they form, and the different types of
currents.
There are presently no hard and fast rules
or techniques that are universally followed. Most units involved in surface
current forecasts have their own innovations and methods.
Figure 6-10.-Sample surf
worksheet.
Figure 6-11.-Example of final forecast
form.
Aerographer’s 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.
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 wind’s 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 Earth’s 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 layer’s 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 ship’s
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.
Prediction of tidal currents must be based
on specific information for the locality in question. Such information is
contained in various forms in many navigational publications.
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.
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