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234 METEOROLOG!' FOR MARINERS

Figurc 16.2. Mcan rcsultant winds ovcr the Atlantic Ocean, August

thc prevailing winds produces a corresponding seasonal variation in thc pre-dominant current.

The differing degrees of variability in various regions arc illustrated by the threc current roses in Figurę 16.3. Here, Figurę i6.3(a) shows thc frequcncy distribution typical of a region of strong and fairly constant current, Figurc i6-3(b) shows the sort of distribution which is characteristic of a trade-wind arca where the degrcc of prcdominance is less marked, and Figurę 16.3(0) shows that characteristic of a region of variable currcnts where there is no dcclded p redom i nancc.

Figurę 16.3. Three characteristic current distributions

Strength of Current

Over large arcas of the open ocean in the middlc and higher latitudes the most frequently reported current speed is around \ kn, and most of thc individual reports varv between nil and ij kn. In certain other regions the generał lcvel of speed is higher and the extremcs corrcspondingly so. For examplc, in some equatorial regions (notably in the mid Pacific and western Atlantic) the mcan speed is in the rangę 1-1J kn and speeds in cxccss of 3 kn have been reported. Rathcr similar valucs occur in thc Gulf Stream but speeds ^UP to 51 kn have been recorded in Florida Strait. Still higher speeds are reported in thc Somali Current during thc south-west monsoon. Herc from about June through Septcmbcr averagc speeds are abovc 2 kn and on one occasion a speed of 7 kn was reported.

Warm and Cold Currents

In generał, currents which set continuously eastwards or westwards acquirc temperatures appropriatc to the latitudc concerncd. Currents which set northwards or southwards ovcr long distanccs, however, transport water from higher to lower latitudes, or vice versa, and so advect lowcr or higher temperatures from thc region of origin. The Gulf Stream, for example, transports water from the Gulf of Mcxico to the central parts of thc North Atlantic where it gives risc to temperatures wcll abovc thc latitudinal avcrage. Between the Gulf Stream and the American coast the water is much coldcr sińce it dcrivcs from Arctic regions by way of thc Labrador Current. The transition from this cold water to the much warmer water of the Gulf Stream is marked by a vcry strong gradient of sca surfacc temperatures. Both here and elsewhere strong temperaturę gradients indicatcd by sea temperatures isotherms can be uscd to dctcct thc boundarics between currents.

Among the principal warm currents may be listed:

Gulf Stream    Moęambique Current

Kuro Shio    Agulhas Current

Brazil Current    East Australian Coast Current

The principal cold currents are:

Labrador Current    Oya Shio

East Greenland Current    Falkland Current

California Current    Peru Current

Kamchatka Current    Bcnguela Current