139736eo.pdf (20MB) - Japan Oceanographic Data Center

CIRCULATION AND PHYSICAL PROCESSES OF THEARABIAN SEA AND THEIR RELATIONSHIPWITH S.W. MONSOON AND PRODUCTIVITYG.S. QURAISHEENational Institute of OceanographyP.E.C.H.S.Karachi 29, PakistanINTRODUCTIONCirculation of the Arabian Sea as well as physical processes and related phenomena havedirect impact on the economic development of the countries in the region. Seasonal changes in theArabian Sea and reversal of monsoons influence various oceanographic and atmospheric processes.The southwest monsoon varies from year to year and causes significant influence on the economy ofPakistan and adjoining countries. The timing of the monsoon and its severity have direct bearing onthe agricultural economy, irrigation, flooding of rivers and even on breeding of marine organisms inthe coastal region. The observations so far obtained and inferences derived from them suggest somepossibility of predicting SW monsoon and other oceanographic parameters.MONSOON AND DYNAMIC HEIGHTSThe atmospheric forcing of the ocean during SW monsoon has been noted from the windstresses during summer months (May - September), when positive stresses with strong gradientsextend to 70'E, covering almost the entire North Arabian Sea (BRUCE, 1981), from the Somali coastto the coasts of Pakistan and India. In view of the pronounced seasonal cycle in the atmospheric andmonsoon oceanic circulation in the Indian Ocean, beginning in April along the Somali coast, there is apossibility of forecasting the SW monsoon intensity if the events are well recorded from the Africancoast to the sub-continent. Such measurements would include dynamic heights, intensity ofupwelling, records of temperature, thickness of the mixed layer, etc.In April, the South Equatorial Current (SEC) along the African coast converges with thesouth- flowing Somali Current to form a loop-like feature which moves northward (MOLINARI,1983). This early indication of the intensity of dynamic heights across the equator appears to havesome relationship with the progressive development of monsoon intensity. One measure of intensityof the SW monsoon is reflected in Indus River discharge, where peak flow develops in August due toheavy rain and from large snowmelt due to persisting warm temperatures. Indus River discharge as aparameter for monsoon intensity has been used to correlate dynamic heights at the equator near Somalicoast (Fig. 1). The variation of surface dynamic heights from 1'N to 1"s off Somali coast have somecorrelation with the intensity of monsoon in Pakistan (QURAISHEE, 1985). Steady and largevariations in the dynamic heights are associated with high river discharges. These two events, i.e.dynamic heights and Indus discharges, have a lag of about 2 months which can be used to predictrnGiiSWfi intensity two mmthhs hence,MONSOON AND SOUTHERN OSCILLATIONOther physical processes, extended over larger oceanic areas, generally known as SouthernOscillations (SO) (PHILANDER, 1983), also appear to have some correlation with the SW monsoonvariability (PANT and PARTHASARATHY, 1981). SO, in fact, represent large scale exchanges ofatmospheric mass between the eastern and western hemispheres, visualised as a see-saw of high229