Seasonal and interannual variability of surface photosynthetically available radiation in the Arabian Sea

Abstract

Abstract. The regional and monthly intensity of photosynthetically available radiation (PAR) (350-700 nm) just below the sea surface (EdnA}0 for the Arabian Sea is determined from solar irradiance models and 7 years of satellite data (1979)(1980)(1981)(1982)(1983)(1984)(1985). Model results of high spatial resolution (18 km) PAR distribution computed from actual monthly measurements (aerosols, cloud cover, and ozone) displayed small-scale patchiness that is not observed in PAR climatology models. Two elevated PAR periods are observed each year, as opposed to a single elevated period per year 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, interannual changes in intensity and time are observed. Additionally, the PAR cycle is found to vary regionally within the Arabian Sea. The bimodal PAR distribution shows elevated peaks in May and October and minima in December (corresponding to the winter equinox) and July. The second minima occurs at the onset of the southwest monsoon, apparently in response to increased cloud cover and aerosols associated with the monsoon. This summer minima varied latitudinally. It originates in the southern regions (0ø-10 ø latitude) in April and migrates north as the influence of the southwest monsoon moves northward, reaching the northern Oman coast (20 ø latitude) in August. Additionally, the summer minima is less pronounced as the southwest monsoon moves northward. Maximum PAR intensity is observed in early spring (preceding the minima), originating in the southern Arabian Sea and extending 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 approximately the same, despite the interannual variability in the cycle and regional variability. The timing and location of PAR cycles are important since they must be coupled with nutrient availability to understand biological cycles. We determined that for the Arabian Sea, PAR cycles determined by climatology may be inadequate to define the submarine light field and that high-resolution PAR cycles are needed to resolve realistic bio-optical and nutrient cycles.

Basin‐wide seasonal evolution of the Indian Ocean's phytoplankton blooms

“…Note that minima in photosynthetic available radiation were reported during the summer monsoon (both from bio‐optical moored time series [ Dickey et al , 1998, Figure 3] and from satellites [ Arnone et al , 1998]). These minima are due to this season's extreme cloudiness, which is more pervasive near shore.…”

Monsoon-driven biogeochemical processes in the Arabian Sea

“…These minima are associated with cloudiness and boreal winter insolation, respectively (Arnone, Ladner, La Violette, Brock, & Rochford, 1998), in addition to phytoplankton concentrations, and they coincide with the semiannual deepening of the bulk mixed layer. Thus, during the monsoons, nutrients are entrained into a deepening mixed layer while the corresponding light levels are relatively low, though not so low as to prevent positive net photosynthesis in the mixed layer.…”

Estimation of photosynthetically available radiation (PAR) from OCEANSAT-I OCM using a simple atmospheric radiative transfer model

“…However, the relationship between insolation and PAR is highly dependent on different solar zenith angles and different atmospheric conditions (Baker and Frouin, 1987;Pinker et al, 1992;Frouin and Pinker, 1995). PAR models based on climatological input and limited aerosol input do not characterize the regional and temporal variability of the PAR cycle (Arnone et al, 1998). Subsequently, with the advent of satellite data, it was possible to have direct estimation of PAR at synoptic scales.…”