230 231 (14)

230

hcmisphcrc) of that of the surfacc laycr. Thus, with increasing depth, thc spccd of the wind-induced currcnt bccomcs progressively less while thc angle between thc dircction of wind and current gradually inereases.

Many investigators have cndcavoured to dcterminc thc ratio bctwccn the spccd of thc surface current and thc spccd of the wind responsible. This is a complex problem and many diflferent answers havc bccn put forward. An avcragc cmpirical value for this ratio is about 1140 (or 0*025). Some invcstigators claim a variation of the factor with latitudc but thc dcgrcc of any such variation is in disputc. In the main the variation with latitude is comparatively smali and, in vicw of the other uncertainties in determining the ratio, can probably be disregarded for most purposes. The production of a currcnt in response to thc wind is not an instantancous rcaction. Initially the response is slow and it takes time for a steady State to become established. This time varies according to latitudc but is something like 24 hours.

It would seem reasonable to expcct that hurricane-force winds might give risc to currcnts in excess of 2 knots, but it is rarc for such winds to persist for morę than a few hours without a change in direction. With such high winds obscrvations of wind-drift currcnts are not usually availablc. Strong currents have indeed bccn reported in conncction with hurricancs but thcsc havc been complicated by sccondary cflects, such as the piling-up of water against the coast, which leads to thc production of a gradient currcnt.

Gradient Currents

These are causcd by pressure gradients in thc water. They occur whenever the water surface develops a slope, whether under the action of thc wind, or through a juxtaposition of waters of differing temperaturo and or salinity. The initial water movcmcnt is downslope but the cfTcct of thc carth’s rotation is to deflcct thc movcmcnt through 90° to the right (Icft in thc Southern hcmisphcrc) of this dircction.

An interesting cxamplc of a gradient current occurs in thc Bay of Bcngal in February. In this month thc currcnt circulation is eloekwise around thc coasts of the Bay, thc flow being north-casterly along much of thc cast coast of India. With thc north-cast monsoon still blowing, thc currcnt herc sets against the wind. The cxplanation of this phenomenon is that thc cold wind blowing off the land cools thc water at thc head of thc Bay. A temperaturę gradient thus arises bctwccn thc cold water in thc north and warm water in the south. Bccause of thc density difTcrcncc thus crcatcd a slope dcvclops, downhill towards thc north. The resulting northward flow is deflcctcd to thc right, i.c. eastward, and so sets up thc generał eloekwise circulation.

Complex Currents

Whilc it is convenient to distinguish between thc two foregoing types of currcnt in terms of their basie causcs, thc currcnts cncountcred in practice arc frcquently complcx in charactcr. We havc scen how thc cfTcct of thc wind (in the northern hemisphcrc) is to produce a surfacc flow in a direction up to 45° to thc right of thc wind direction, also that succcssivc lowcr laycrs of water arc deflcctcd progrcssivcly further to thc right. 'I his angle of dcficction inereases whilc thc spccd of motion dccrcascs until thc latter bccomes ncgligible by the time thc angle of motion has become diamctrically opposed to the original wind direction. This iinplics a bulk transport of thc water within thc aflccted

CAUSES AND CliARACTERISTICS OF CURRENTS 231

laycr roughly at right angles to the direction of thc wind. This transport leads to an accumulalion of water to the right of the direction towards which the wind is blowing. This in tum tcnds to producc a gradient currcnt flowing downslope from the region of accumulation. The cflect of thc carth’s rotation diverts this through 90° to thc right (northern hemisphere) and thus produces a flow in the direction of the original wind. In this way the original wind-drift currcnt is strcngthcncd by a gradient currcnt causcd indirectly by thc wind.

In many cases a wind-drift current is superimposcd upon a gradient currcnt which may bc duc to temperaturo and/or salinity diflerenccs. The resulting current is then a combination of thc two efleets.

Near the ocean boundaries further complications arise. For example, ovcr a widc belt in thc cquatorial regions of the Atlantic, thc tradc winds produce a west-going currcnt setting towards thc Caribbean Sea and Gulf of Mexico. Bccause of the configuration of thc latter thcrc is an accumulation of water in the Gulf so that water levels are higher on the western coast of Florida than on the castcrn coast. The resulting castward flow through thc Florida Strait constitutcs the Florida Currcnt which, after turning northwards past the Bahamas becomes the Gulf Stream.

EfFect of Evaporation

Evaporation may in some cases contribute to current formation. For examplc, in a relatively shallow sea like the Mediterranean, thc ratc of cvaporation is high, and thc inflow of water from rivcrs is not suflicient to maintain thc lcvcl of the sea. Water thcrcforc flows in from thc Atlantic, through the Strait of Gibraltar, to make good the deficiency. The cflcct of thc carth’s rotation is to tend to divcrt thc cast-going flow through the Strait to the south. Thcrc can be no actual dcflcction within thc Strait bccause of its narrowness, but further cast thc inflow is thus dellected against thc African coast and so produces a counter-clockwisc circulation. By cvaporation, thc Mediterranean surfacc water bccomcs salinę and dcnsc. It thcrcforc sinks and the cxcess of this denser bottom water cmcrgcs ovcr the sili forming thc shallow Strait of Gibraltar and bclow the incoming water.

EfTect of Wind Blowing over a Coastline

This may bc illustratcd by rcfcrcncc to the Benguela Currcnt oflf SW Africa. Thcrc the coast is orientated nnw-sse and thc prcvailing wind is sf.’1v. The dircct cflcct of thc wind is to producc a surface current setting roughly towards thc west (say 30° to the lcft of thc direction towards which thc wind is blowing). Howevcr, thc indircct cflcct, taking account of the sub-surfacc layer, is to cause a transport of water towards thc sw and this by establishing a gradient from sw to ne leads to a gradient currcnt setting Nw’ly (90° to the left of ne). The rcsult-ant of this and thc direct current is a roughly WNW*ly currcnt which rcmovcs water from thc Coastal regions. This is madc good by thc upwellir.g of water from lowcr lcvcls. The water thus brought to the surface is cold in comparison with the sea surfacc temperatures appropriatc to thcsc latitudcs. The arca of upwelling is rcvcalcd on maps of sea surfacc temperaturę as an area of Iow temperaturę surroundcd by higher temperatures.

Relation of General Current Circulation to that of Wind

Although thcrc arc many local cxccptions, tliere is in generał a close relation between thc largc-scalc patterns of prevailing currcnts in the oceans, and thc

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