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

pressure in the South Pacific and low pressure in the Indian Ocean. These exchanges exhibir a cycle of2 to 6 years. El-Niiio has been found to be related with the SO. A long data base of 112 years(1866-1977) of monsoon intensity shows that wet monsoons (strong) and dry monsoons (weak) havecorrelation with El-Niiio. The investigation on physical causes and relationship between SO andmonsoon intensity can provide a good forecasting tool.MONSOON AND THERMAL STRUCTUREMonsoon intensity and thermal structure of the Arabian Sea also show reasonable correlation(SHUKLA, 1983). SST and temperatures at depth are affected by upwelling along the Somali andArabian coasts and circulation of upwelled water (SWALLOW, 1985) outflow and mixing of Red Seawater at about 750 m depth and Persian Gulf water at about 250 m depth (MEERA PATHMARAJAH,1982).Cooling of the eastern Arabian Sea (SWALLOW, 1983) may be due to evaporation andmixing with deeper cool water, resulting in deepening of mixed layer. On the basis of the fewavailable data, the depth of 20°C isotherm in the centre of North Arabian Sea in May appears to havesome correlation with the August discharges of Indus River (QURAISHEE, 1985). The depth of20°C isotherm (Table l), which is considered to be almost in the middle of permanent thermocline,sinks to greater depths when large mixed layers are formed. The high floods of the River Indus in1967, 1973, 1976, 1978 and 1983 are reasonably correlated to deeper depths of 20°C isotherm, i.e.180 - 200 m. Thus there is a reasonable correlation between temperature structure, particularly thedepth of thermocline, and the intensity of monsoon rainfall.MONSOON AND EDDY CIRCULATIONSatellite imageries, using high resolution infrared radiometer, show that circulation of theArabian Sea contains warm and cold core eddies (CAGLE and WHRITNER, 1981). The eddycirculation appears to become intensified and locally persist (QURAISHEE, 1984) during the SWmonsoon (May - September). These months are dominated by upwelling along the Arabian coast, andthe cold water plume and wedge extend eastward. Comparatively weak upwelling also appears alongthe Pakistan coast west of Karachi. In the middle of these two upwelled cold water areas ananticyclonic eddy is found with a warm core. Records generally show that this eddy circulation isrepeated in the SW monsoon.The warm core eddies appear to transfer energy from the ocean to the atmosphere.Consequently the rainfall on the subcontinent is affected. The thermal field in the warm core eddy ofJune 1983 has been investigated (QURAISHEE, 1984). The normal surface temperature in June is29'C, based on WYRTKI's (1971) bimonthly charts, when the mixed layer depth is about 20-40 m.Eddy circulation, however, disturbs these normal conditions: cool water (26.0"C) prevails near thecoast, and warm water (29.0"C) is trapped in the centre of the eddy. The trapping of warm water isreflected by the configuration of isotherms at 50 m and 100 m, and less prominently at 200 m and 500m (Fig. 2). The 20°C isotherm lowers to a depth of about 230 m (Fig. 3) which is as much as 50 mdeeper than normal (see above). Thus information about the thermal structure in an eddy field also canprovide a useful parameter for prediction of monsoon intensity.EDDY CIRCULATION AND PRODUCTIVITYIn 1983 high values of phosphate and nitrate were observed at the core of a warm-core eddy.Mixing of the nutrients from the adjoining cool upwelled water, particularly from deeper depths (50 -300 m), is apparent. High phosphate concentrations (2.23 pg-at P -1) have been observed at thesurface.230