Surface currents

The chief surface currents and their relation to prevailing winds are shown in Figure 1.6. In the Equatorial belt between the Tropics of Cancer and Capricorn, the North-East and South-East Trade Winds blow fairly consistently throughout the year, setting in motion the surface water to form the great North and South Equatorial Currents which flow from east to west in the Atlantic, Indian and Pacific Oceans. Across the path of these currents lie continents which deflect the water north or south.

In the Atlantic, the Equatorial Currents are obstructed by the coast of Brazil, and the greater part of the water flows northwards into the Caribbean and Gulf of Mexico. The main surface outflow from the Gulf of Mexico flows strongly northwards past the coast of Florida (the Florida Current), and then out into the North Atlantic as the Gulf Stream. In the South Atlantic, water from the Equatorial Current is deflected southwards as the Brazil Current.

As the water moves away from equatorial regions, its course is influenced by the rotation of the earth. In the Gulf Stream, which at first flows in a north-easterly direction, the Coriolis effect gradually turns the water towards the right until,

Figure 1.6 Prevailing winds and major surface currents, with zones of upwelling and convergence. In the north Indian Ocean the summer circulation is shown. M zones of upwelling; M zones of convergence; ^ prevailing winds; ^ surface currents.

between latitudes 40-45oN, it is flowing eastwards across the Atlantic and becomes known as the North Atlantic Current. By the time it reaches the eastern part of the Atlantic it has been further deflected until it is flowing towards the south as the Madeira and Canaries Currents, eventually merging once again with the North Atlantic Equatorial Current. This vast circulation of surface water in a clockwise gyre surrounds an area of relatively little surface movement, the Sargasso Sea.

Where the North Atlantic Current moves eastwards across the Atlantic it is influenced by winds, the Westerlies, blowing from the south-west. These winds deflect some of the surface water towards the north-east to form the North Atlantic Drift, which flows into the Bay of Biscay and along the west and north of the British Isles, some eventually entering the northern part of the North Sea. North Atlantic Drift water also flows far up into the Arctic along the west and north coasts of Norway (the Norwegian Coastal Current), while some turns westwards south of Iceland (the Irminga Current). Part of this water flows clockwise around Iceland, and part flows on to the west and eventually reaches the west coast of Greenland (the West Greenland Current).

The inflow of water into the Arctic must be balanced by an equivalent outflow. Some of the surface water cools and sinks, and leaves the Arctic as a deep current (see page 16). There is also an outflow of cold surface water from the Arctic which enters the Atlantic as the East Greenland and Labrador Currents, and flows along the coast of Labrador and down the eastern seaboard of the United States, eventually sinking below the warm waters of the Gulf Stream flowing in the opposite direction.

In the South Atlantic, water from the Brazil Current under the influence of the Coriolis effect makes a counter-clockwise rotation, flowing eastwards across the Atlantic between latitudes 30-40OS, and turning in a northerly direction along the west coast of Africa. Here it is known as the Benguela Current which eventually merges with the Equatorial Current. In the southern hemisphere the Westerlies blow from the north-west, and deflect surface water into the Southern Ocean where there are no intervening land masses to interrupt the flow. Here the sea is driven continually in an easterly direction by the prevailing winds and becomes a great mass of moving water, the Antarctic Circumpolar Current, which encircles Antarctica. The surface current is termed the West Wind Drift.

The surface movements of the Pacific Ocean have a broadly similar pattern to those of the Atlantic. The Kuro Siwo Current, flowing in a north-easterly direction past the south island of Japan, is the counterpart of the Gulf Stream in the Atlantic. This water moves eastwards across the North Pacific towards the coast of British Colombia (the North Pacific Current), and then mostly turns south as the California Current. A cold current, the Oyo Siwo, flows down the western side of the Pacific towards the north Japanese island.

The surface circulation of the northern part of the Indian Ocean is complicated by seasonal changes in the direction of the monsoons. During the winter the ocean is warmer than the Asian land mass. Air overlying the sea rises and is replaced by cool air flowing off the land. This prevailing wind blows from the north-east between November and April and is termed the North-East Monsoon. It corresponds with the North-East Trade Winds and sets up a North Equatorial Current flowing from east to west, turning south along the African coast. At this time surface water between Arabia and India moves mainly in a west or south-west direction, and a clockwise gyre develops between India and Burma.

In summer, when the land becomes hot, air rises above the land and is replaced by the inflow of the South-West Monsoon, starting in April and usually blowing strongest in August to September. This carries water-saturated air over the land and causes the Monsoon rains. The reversal in direction of wind changes the direction of flow of surface water, driving water between Arabia and India eastwards and setting up a Monsoon Current from west to east in place of the North Equatorial Current. On reaching Indonesia this current turns south to join the South Equatorial Current. In the South Indian and South Pacific Oceans there is a counter-clockwise surface gyre and a deflection of water into the Southern Ocean, similar to the South Atlantic.

In the Equatorial belt between the latitudes of the North-East and South-East Trade Winds there is a calm zone, the Doldrums, where the effects of wind are minimal. In the Pacific and Indian Oceans, a certain amount of backflow of surface water towards the east occurs in this region, forming the Equatorial Countercurrents. There is relatively little backflow of surface water in the Atlantic, but a short distance below the surface there is an appreciable movement of water from west to east. In the Pacific, there is an even more extensive subsurface current, the Cromwell Current, which transports a large volume of water in an easterly direction between latitudes 2°N and 2°S at depths of 20-200 m.

The distribution of warm or cold surface water has a great influence on climate, and accounts for the climatic differences at equal latitudes on the east and west sides of the oceans. For example, the British Isles lie to the north of Newfoundland but here we have a temperate climate whereas Newfoundland is subarctic. This is because warm water from low latitudes moves across to high latitudes towards the eastern side of the ocean, and cold water moves towards low latitudes along the western side. Consequently, mild conditions extend further north in the east. The Bristol Channel in England lies slightly to the north of the Gulf of St Lawrence in Canada but certainly does not experience the freezing winter conditions of the latter.

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