222 223 (16)

222    METEOROLOGT FOR MARIPfERS

Arabian Sea and thc Bay of Bengal, arc reversed in dircction seasonally by thc monsoons.

The north-cast monsoon current circulation occurs during the hcight of that monsoon from November to January. During the latter part of this monsoon, Fcbruary to Aprii, thc circulation changcs. The south-wcst monsoon circulation prevails from May to September. October is a transitional month. The currents arc thercfore describcd below for the three periods, November to January, February to Aprii and May to September.

Piorlk-east monsoon circulation, Nooember to January. In thc open waters of thc Arabian Sea and Bay of Bengal the current sets in a wcstcrly direction. Thcsc wcsterly sets e.\tend southwards beyond the equator. Near the coasts of the Arabian Sea thcre is a weak circulation in a counter-clockwise sense following the coasts. In the northern part of the Bay of Bengal thcre is a clockwisc circulation. A stronger current sets southwards down thc cast coast of Africa from Ras Hafun to about latitude 2°s. This, the Somali Current, turns eastwards between about latitudes 2°s and 5% to form the beginning of the Equatorial Counter-current. Between Ras Hafun and Ras Asir (C. Guardafui) the current is northerly.

Lata north-cast monsoon period, February to Aprii. The flow in the open waters of thc Arabian Sea and Bay of Bengal remains wcstcrly, though thc currents are morę variable than from Novcmbcr to January. Towards the eąuator, westerlies which are well marked in February become less so in March, and in Aprii the flow changcs to casterly. The Coastal circulation of thc Arabian Sea, howcver, is revcrsed to a eloekwise direction (see ‘Gradient Currents’ in Chapter 16). In February thc Somali Current flows south-westwards along the African coast to about latitude 3°s. During Mareh thc flow becomcs variablc and by Aprii it reverscs to north-eastcrly.

South-west monsoon circulation, May to September. In thc open waters the drift is eastcrly. The Coastal circulation of the Arabian Sea and Bay of Bengal remains eloekwise and is strengthened. The Somali Current continucs to flow northwards, from Cabo Dclgado to Ras Asir (C. Guardafui), and is greatly strengthened. It divides in about latitude 7,°n; part continues along the coast to Ras Asir (C. Guardafui), but the bulk turns eastwards and passes South of Socotra into the generał casterly current. The current south of Socotra in July to September is the strongest known in thc world in the open ocean and rates up to 7 knots have bccn recorded.

Red Sea and Gulf of Aden

The current conforms to the monsoon. During the north-cast monsoon it sets westwards in the Gulf of Aden and passes through the Straits of Bab-al-Mandab to flow up the axis of thc Red Sea. During the south-west monsoon thc current in the Gulf of Aden sets eastwards; in the Red Sea the water flows down the axis of the sea and into the Gulf of Aden.

SOUTH INDIAN OCEAN

Main Circulation

The main circulation of the South Indian Ocean is counter-clockwise. The northern flank of thc circulation is formed by the west-going South

CURRENT CIRCULA TIONS OF THE OCE ANS    223

Equatorial Current which occupies an analogous position to the South Eąuatorial Currents of thc othcr main occans. In this ocean, ho\vcver, the South Equatorial Current lies well south of the equator and in this respcct diflers from the corresponding currents of the Atlantic and Pacific which extcnd to a fcw degrees north of thc cquator. Its northern boundary is usually between latitudcs 6 s and io°s but it varics according to longitude and scason.

The South Equatorial Current, after passing the northern extrcmity of Madagascar, mects thc African coast ncar Cabo Delgado. Herc it dividcs and some of the water flows northwards along thc coast. The remainder flows southwards to form a strong Coastal current which, from Cabo Delgado to Lourcnęo Marqucs, in known as the MoęAMBiQUE Current. Its southward continuation is thc Agulhas Current. This is reinforced by water from the South Equatorial Current setting past thc Southern cxtremity of Madagascar.

Some of thc water of the Agulhas Current recurves to south-eastwards between about longitudes 25°e and 35°e and enters the northern part of thc Southern Ocean Current. The remainder of the Agulhas Current continues along the coastline, and passing ovcr the Agulhas Bank, enters thc South Atlantic Ocean, where it joins the Bengucla Current.

The Southern side of thc main circulation is formed by thc cold water of thc Southern Ocean Current, setting in a generally casterly direction in latitudes south of about 35°s. Thcre is no defined northern boundary to thc Southern Ocean Current; the predominance of casterly sets deereases with decreasing latitude towards the middle of the ocean. Here thc centre of a rather wcak counter-clockwise circulation lies somewhcrc between latitudes 22°s and 35°s and between longitudes 70°e and 95°E.

The east side of the circulation is not well marked. In the northern winter months the Southern Ocean Current turns northwards as it approaches Cape Lecuwin and forms a north-going current parallcl to the west coast of Australia, though there is a narrow belt of south-going current elose inshore. In thc northern summer the Southern Ocean Current ofT south-western Australia sets easterly and turns southerly towards thc coast in latitudcs south of latitude 26 s. Between about latitudcs 20°s and 26°s the current near thc coast runs southwards from about March to August but is northerly in other months. The bulk of thc Southern Ocean Current continues its easterly coursc, south of Australia and Tasmania, into the South Pacific.

Eąuatorial Counter-currcnt

In thc othcr main oceans of the world thc Equatorial Counter-current consists of a relatively narrow belt of casterly currents lying between thc North Equatorial and South Equatorial Currents of thc ocean in question. In the Indian Ocean, also, the Equatorial Counter-currcnt is rcadily distinguished during the north-east monsoon when it forms a belt of easterly currents lying between about latitudes 2^cs and 8°s. In the south-west monsoon, however, the critcrion for determining thc northern boundary of this belt disappears, sińce easterlies are then morę or less continuous north of about latitude 8°s. This easterly flow tends to be lighter and morę variable in thc south and to become morę pronounced further north. In July and August the currents arc light and rather variablc between about latitudes 8°s and 2°n with a morc-dccidcd casterly flow further north, but in June and September thc region of marked casterly flow extcnds southwards as far as latitudes 2°s-4°s.