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METEOROLOGT FOR MARINERS ARCTIC OCEAN

The main inflow of water into the Arctic Basin is from the West Svalbard Current. A much smallcr quantity cntcrs through Bering Strait. Frcsh water is added to the Arctic Basin from rivers, notably those of Siberia, and by an excess of prccipitation over evaporation.

The East Greenland Current forms the main outflow of water from the Arctic Ocean. Smali outflows occur duc to the East Svalbard and BJ0RN0YA (Bear Island) Currents which flow south-westwards in the northern part of the Barents Sea and the current which flows eastwards betwecn the islands of the Arctic Archipclago towards Baffin Bay.

Within the castcrn longitudcs of the Arctic Basin thcrc is a weak westerly current, as shown by the drift of the Fram and other ships in the ice. This current emerges to the south-west betwcen Svalbard (Spitsbergen) and Greenland to form the East Greenland Current.

Along the Siberian coast the current generally flows eastwards from the Kara Sea to Bering Strait.

SOUTHERN OCEAN

The cast-going Southern Ocean Current complctcs, on the south side, the counter-clockwise circulation in each of the South Atlantic, South Indian and South Pacific Oceans. The Southern limit of this current is not clearly defined. In generał the easterlies dccrease and become morę north-easterly by about latitude 6o°s. Some easterly component continues southwards to latitudes, varying with longitude and season, betwcen about latitudes 62°s and 67°s. Further south, westerly componcnts incrcase and there is finally a westerly current setting around the coasts of the Antarctic contincnt. These Coastal westerlies are interrupted in the north of Graham Land where the restricted naturę of Drakę Passage produces a north-easterly flow throughout its width.

THE CAUSES AND CHARACTERISTICS OF OCEAN CURRENTS

Introduction

The water of the occans is in a State of continual movement, not only at the surface but at all dcpths. Ocean-current circulation, in its widest sense, takes place in three dimensions but the strongest currcnts occur in an upper layer which is shallow compared with the ocean depth. Apart from the relativcly small-scale vcrtical movcmcnts associatcd with waves, the motion near the sea surface is largely horizontal, but, at depth, it may havc a vertical componcnt.

The navigator is only conccrncd with the currents in that depth of water in which his vesscl is floating, i.e. in a layer extcnding to a depth cqual to the vcsscl’s draught. As, in generał, the current varics with depth, a vessel’s response to the varying currents will represent somc compromise betwcen the response to the current litcrally at the surface and that at the depth of the ship’s draught. Consequcntly currents mcasured by determination of a vesscl’s set and drift are normally regarded as bcing applicable to a depth equal to half the vessel’s draught. Most of our information about so-callcd surface currents comes from a varicty of ships of various draughts. Although such observations may not all be strictly comparable it has been customary to treat them all as representing ‘surface’ currents although in the main they apply to depths varying betwcen about 3 metres and io metres.

The Main Causes of Ocean Currents

The processcs which cause ocean currents are complex and are not yet fully understood. In most cascs thcrc is morę than one factor contributing to the existcncc of a particular current. Frcquently therc is a long chain of rccurrcnt cause and effcct whcrcin it is difłicult to identify a beginning or an cnd.

Two main causes of ocean currents may be distinguished. Thesc are, firstly, wind stress acting on the water surface and, secondly, pressure gradients within the water. The currents resulting from thesc two causes are dcscribcd, rcspcct-ively, as wind-drift currents and gradient currents.

Wind-drift Currents

Wind blowing over a water surface tends to drag the uppermost layer of water in the direction towards which the wind is blowing. As soon as any motion is imparted, howcvcr, the effcct of the carth’s rotation (the Coriolis force) is to dcflcct the movcment towards the right in the northern hemisphcrc and towards the left in the Southern hcmispherc. Although theory suggests that the resulting effcct should be to produce a surface flow (or wind-drift current) in a direction inclined at 45⁰ to the right (left) of the wind direction in the northern (Southern) hemisphcrc, obscrvations show the angle to be less in practice. Various values betwcen 20° and 45⁰ havc been reported. An effcct of the movemcnt of the surface water layer is to impart a lesser movemcnt to the layer immediately bclow, in a direction to the right (left in the Southern

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