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 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 knot is the probable maximum and in areas where it is relatively consistent rates of to knots may be encountered In the Gulf Stream Kuro Shio and Agulhas Current a knot rate is sometimes present The record is between and knots off Socotra, in the NW Indian Ocean, in August and September The rate of currents usually lies between about per cent and 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 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 Discuss the following: (a) Drift current (b) Gradient current (c) Cold current Describe the effect of the earth's rotation on ocean surface currents Name and describe the currents of the North and South Atlantic Ocean and give their average daily drifts Describe the general circulation of the surface currents in the Mediterranean Sea (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 Describe how "upwelling" is caused and name some notable localities where it occurs 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 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