SEASONAL AND INTERANNUAL VARIABILITY OF SURFACE PHOTOSYNTHETICALLY AVAILABLE RADIATION IN THE ARABIAN SEA

The regional and monthly intensity of photosynthetically available rad iation (PAR) (350-700 nm) just below the sea surface (Ed(PAR)(-)) for the Arabian Sea is determined from solar irradiance models and 7 years of satellite data (1979-1985), Model results of high spatial resoluti on (18 km) PAK distribution computed from actual monthly measurements (aerosols, cloud cover, and ozone) displayed small-scale patchiness th at is not observed in PAR climatology models. Two elevated PAR periods are observed each year, as opposed to a single elevated period per ye ar observed in the North Atlantic during the summer. When the biannual cycle for each of the 7 years is compared with the 7-year average, in terannual changes in intensity and time are observed. Additionally, th e PAR cycle is found to vary regionally within the Arabian Sea, The bi modal PAR distribution shows elevated peaks in May and October and min ima in December (corresponding to the winter equinox) and July. The se cond minima occurs at the onset of the southwest monsoon, apparently i n response to increased cloud cover and aerosols associated with the m onsoon. This summer minima varied latitudinally. It originates in the southern regions (0 degrees-10 degrees latitude) in April and migrates north as the influence of the southwest monsoon moves northward, reac hing the northern Oman coast (20 degrees latitude) in August. Addition ally, the summer minima is Less pronounced as the southwest monsoon mo ves northward. Maximum PAR intensity is observed in early spring (prec eding the minima), originating in the southern Arabian Sea and extendi ng northward into the central Arabian Sea. The timing of the northward movement of this spring maximum is slightly different each year. The net yearly PAR intensity for each of the 7 years appears to remain app roximately the same, despite the interannual variability in the cycle and regional variability. The timing and location of PAR cycles are im portant since they must be coupled with nutrient availability to under stand biological cycles. We determined that for the Arabian Sea, PAR c ycles determined by climatology may be inadequate to define the submar ine light field and that high-resolution PAR cycles are needed to reso lve realistic bio-optical and nutrient cycles.