BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 20

Chapter Twenty

CHAPTER TWENTY
Surface Currents

INTRODUCTION
The relationship between the surface currents of the ocean and the weather
systems is so strong that no book on maritime meteorology would be
complete without some description of surface currents. Much of our
knowledge of ocean currents is derived from observations made voluntarily
since 1855 by officers in merchant vessels. The navigator needs some
knowledge about these currents because of their effect on the safety and
economical operation of a ship at sea. Ocean currents not only affect
navigation but because some of them are warm and some cold they have an
appreciable influence on climate and weather.
CAUSES OF OCEAN CURRENTS
The surface currents of the ocean can be divided into two main categories:
drift currents caused directly by the wind and gradient currents caused
indirectly by the wind or by density differences in adjacent areas. The wind
is the primary agent which causes the currents; in general the more consistent
the wind the steadier the current.
In the Indian Ocean and China Sea the surface currents are governed by the
Monsoons and therefore they vary seasonally in direction and strength. In the
other oceans the main surface current circulation skirts the perimeter of the
permanent mid-ocean anticyclone in the same sense as the wind; the prime
mover of these currents in each case is the trade winds.
As a constant wind blows over the sea it tends to move the surface water
along with it due to frictional drag. However the Coriolis force, which is due
to the earth's rotation, deflects this flow. Near the surface the water is
deflected about 45 0 from the wind direction and this angle increases with
depth, see fig 20.1. Since the strength of the current decreases with depth the

resultant effect in the whole depth of water affected by the wind is a
deflection at 900 to the wind direction. The actual depth of water affected by
the wind depends upon the strength of the wind and also upon the latitude.
The deflection is to the right in the northern hemisphere and to the left in the
southern hemisphere. Therefore a wind blowing from the north in the
northern hemisphere produces a surface current which sets towards the south
177


Chapter Twenty

west and the resultant deflection in the whole body of water is to the west.
See fig 20.2. It may appear that only the surface water effect is important for
the navigator, however, it should be appreciated that the motion in the whole body
of water is important for the general circulation.
A gradient current. as its name implies, is created by a pressure gradient or slope
in the water level. This gradient may be due to a piling up of the water (e.g., against
a coastline) or due to a density difference between two adjacent bodies of water.
One of the most spectacular examples of a wind induced gradient current is in the
Gulf of Mexico where the west-going North Equatorial Current piles up the water
near the coast and thus not only initiates the Gulf Stream but also enhances the
Caribbean Countercurrent which flows eastward along the coasts of Panama and
Colombia to Barranquilla.

Variations in the density of sea water may be due to temperature or salinity
difference or a combination of both and when surface water of low density lies
alongside water of a higher density a current will be created. In the Western North
Atlantic and Pacific the currents flowing out of the Arctic Ocean are basically
density currents due to the relatively low salinity of the Arctic water. but assisted
by a slight prevalence of northerly winds.

CHARACTERISTICS OF OCEAN CURRENTS
In general, warm surface currents originate, as might be expected, in tropical waters
while cold ones are born in the frigid zones. Probably the best known example of a
warm current is the Gulf Stream, the extension of which, the North Atlantic
Current, brings warm water to Britain's shores and keeps northern Norway ice-free.
A typical cold current example is the Labrador Current which extends down the
East coast of USA to Delaware Bay, inside the Gulf Stream and causing, off
178


Chapter Twenty

the Grand Banks of Newfoundland. a temperature gradient of up to 5.6°C
(10°F).
In some places local cooling of the surface water is caused by a process
called upwelling. When the surface waters tend to be drawn away from a coast
by a persistent wind, cold water from below rises to replace it and this is
known as upwelling and it results in a lowering of the surface water
temperature. upwelling is experienced off the coast of Chile and Peru,
California. North-west Africa and South-west Africa. Upwelling brings
nutrients to the surface and so all of these areas are rich in plankton. The
plankton forms the basis of an extensive food chain.
Apart from seasonal changes. such as occur in the monsoon areas where the
currents reverse in direction. there is much variety in the day to day direction
and strength of almost all individual currents. Drift currents in particular are
susceptible to wind variations and they are liable to temporary local increase
or decrease in strength or eyen to reversal in direction at times. Even the Gulf
Stream, which is the most constant current, may occasionally be affected
locally in this way. The current roses depicted in the Ocean Current Atlases
prepared in the Meteorological Office illustrate this variability Some FAX

stations provide information on currents.
The maximum strength of currents. likely to be experienced in different
parts of the main circulation in the open ocean varies quite a lot. In areas
where the current is fairly variable. about 1 knot is the probable maximum and
in areas where it is relatively consistent. rates of 2 to 3 knots may be
encountered. In the Gulf Stream. Kuro Shio and Agulhas Current a 5 knot rate
is sometimes present. The record is between 6 and 7 knots off Socotra, in the
NW Indian Ocean, in August and September.
The rate of currents usually lies between about 2 per cent and 4 per cent of
the wind strength, although the rate of the current depends to a large extent on
the duration of the blow.
In coastal waters currents are often masked due to strong tidal streams.
GENERAL SURFACE CURRENT CIRCULATION
A picture of this is shown on the pull-out chart near the back of this book. The
following is a brief summary:
(a). North Atlantic. The cool south-going currents on the east flank of the
mid-ocean anticyclone merge into the wide band of west-bound current on the
equatorial side and most of tl1is turns northward on reaching the Caribbean to
form the warm Gulf Stream, while a small portion turns east to help form the
179


Chapter Twenty

Caribbean Counter Current. On the northern flank of the anticyclone the Gulf
Stream fans out into the North Atlantic Current, part of 'which eventually turns
SE to complete the circulation while the remainder swings NE as far as the
north coast of Norway. The flow of currents from and around the Arctic is a
little complicated as can be seen from the chart.
(b). North Pacific. In most of this ocean the circulation is very similar to that

of the North Atlantic; a cool south-going current on the east side of the
anticyclone, a wide equatorial current flowing west on the south flank and the
warm north-going Kuro Shio in the west, similar to the Gulf Stream, while
there is an east-going current on the north side. On the extreme west of this
ocean. however. in the area of the China Sea the currents flow NE or SW in
accordance with the monsoons. Like in the North Atlantic there is a fairly
diverse pattern of currents in the Arctic area.
(c). North Indian Ocean. The surface currents here are governed entirely by
the monsoons, so that in the open ocean they flow eastwards during the
summer (SW Monsoon) and westwards during the winter (NE Monsoon). In
the extreme southern part of this ocean, close to the equator, the east-going
Equatorial Counter Current flows eastward throughout the year. Close to the
East African coastline there is a northerly current in summer and a southerly
one in winter (the East African Coast Current, sometimes called the Somali
Current); the summer current here may have a rate as high as 7 knots.
(d). Southern Hemisphere. The surface currents of the South Atlantic, South
Pacific and South Indian Ocean follow, in general, a regular pattern of
anticlockwise circulation round the periphery of the permanent anticyclone in
each ocean.
Each of these oceans has on its eastern side a relatively cold north-going
current and a warmish south-going current on its western side; in each there is
a fairly steady west-going Equatorial Current and Counter Current, but it is
only in the South Indian Ocean that the Equatorial Counter Current is found
south of the Equator. The outstanding feature of the currents in this
hemisphere is in the southern part of each ocean which combine to form what
is called "the Southern Ocean" where there is a very wide band of permanent
east-going current encircling the world.
MEDITERRANEAN
The general circulation of the surface water is counter-clockwise. It enters
from the Atlantic, as a surface current, through the Strait of Gibraltar and

flows thence along the North African coast. Due to the high rate of
evaporation throughout the Mediterranean the salinity of the surface water
gradually increases and it sinks.
180


Chapter Twenty

Hence the outflow of this denser water through the Strait of Gibraltar to the
Atlantic takes place below the surface inflow.
The list of currents in the following table should be studied in conjunction
with the pull-out chart near the back of the book, which portrays the general
pattern of ocean currents.

NOTE: Detailed general and local information about ocean currents is given
in the Admiralty Pilots and in the Ocean Current Atlases prepared in the
Meteorological Office from information supplied by merchant ships. Monthly
Admiralty Routeing Charts show clearly the predominant direction of sea
surface currents for each quarter of the year; also the percentage constancy
and the mean rate of Current in knots in the predominant direction.
181


Chapter Twenty

QUESTIONS
1. Discuss the following: (a) Drift current. (b) Gradient current. (c) Cold
current.
2 Describe the effect of the earth's rotation on ocean surface currents.
3. Name and describe. the currents of the North and South Atlantic Ocean and

give their average daily drifts.
4. Describe the general circulation of the surface currents in the Mediterranean
Sea.
5. (a) Sketch two outline maps of the North Indian Ocean extending eastwards
to include the China Sea. (b) Illustrate the flow of ocean surface currents for
the month of January on one map and for July on the other.
6. Describe how "upwelling" is caused and name some notable localities where
it occurs.
7. A drift bottle thrown into the sea on a voyage from Durban to Cape Town
was eventually picked up on the north coast of Spain. Describe. in your
opinion. the probable track it followed and name the currents which carried it
until arrival at its final destination.
8. Name and describe the currents, including the average daily drifts, you
would expect to encounter on a voyage from Christchurch, New Zealand, to
Rio de Janeiro, via Cape Horn.

182