Open Arabian Sea Research Articles

Modelling the seasonality of subsurface light and primary production in the Arabian Sea

Seasonal changes in mixed-layer depth and phytoplankton biomass in the Arabian Sea are assessed with climatologies of ship-based hydrographic measurements and ocean-color observations from satellite. At the close of the intermonsoons in November and especially May, the open Arabian Sea resembles the stereotypic, unperturbed tropical ocean, with a thin oligotrophic mixed layer and a pronounced subsurface chlorophyll maximum. Both the northeast and southwest monsoons disrupt this typical tropical hydrography through mixed-layer deepening and eutrophication in the central and northern Arabian Sea. Computations using a spectral model of light penetration suggest that seasonal changes in mixed-layer thickness and phytoplankton concentration result in pronounced fluctuations through the annual cycle in the radiant flux reaching the base of the mixed layer. At the close of the fall and spnng intermonsoons the base of the model euphotic zone is in the thermocline across all of the open Arabian Sea. The euphotic zone appears to rise Into the mixed layer of the northern Arabian Sea during both the winter and summer monsoons. Strong seasonality in total primary production and its partitioning between the mixed layer and thermocline is predicted by a photosynthesis-irradiance model for a site in the western Arabian Sea (14.36 N, 57.38' E). Modeled mixedlayer primary production depicts an intense peak for the southwest monsoon and a secondary northeast monsoon peak separated by intermonsoon periods of low production. During the fall and spring intermonsoons, in the presence of a subsurface chlorophyll maximum, the model estimate of primary production in the thermocline exceeds that in the mixed layer. Our model calculations suggest that the subsurface chlorophyll maximum present in the Arabian Sea during the spring intermonsoon is a precursor of the regional, summer, phytoplankton bloom.

Bacterioplankton distributions and production in the northwestern Indian Ocean and Gulf of Oman, September 1986

Abstract Bacterial abundance and 3H-thymidine incorporation were measured throughout the water column during September–October 1986, along transects in the north western Arabian Sea and Gulf of Oman. Bacterial abundances and production estimated from incorporation rates were high (> 1 × 109 cells 1−1 and 30–92% of primary production, respectively) along the oceanic portions of the transects. These elevated levels may indicate a response to the decline of summer phytoplankton blooms stimulated by monsoonal deepening of the mixed layer. Bacterial production and abundance profiles had complex vertical structure with multiple subsurface maxima related to chlorophyll and oxygen distributions. Production and abundance both declined exponentially with depth below 100–200 m. Rates of dissolved organic carbon (DOC) release from the sinking particle flux may have been adequate to support bacterial production in the Gulf of Oman, but in the open Arabian Sea this source appeared to be insufficient to meet the bacterial demand. The bacterial production estimates reported herein are very conservative because very low conversion factors were used. In general these results suggest that the carbon sources usually assumed to support bacterial production (e.g., phytoplankton exudation, particle breakdown) supply only a fraction of the bacterial demand in the northwest Indian Ocean.

A contribution to the dynamic climatology of Arabia

Maps and transects are presented for January, April, July, and October. The uppertropospheric wind regime is dominated by Westerlies during winter, and the Tropical Easterly Jet in summer; seasonal reversals being related to the meridional gradient in the tropospheric thickness pattern. The relation between the kinematics of flow and the surface climate is most readily apparent during the extreme seasons. In winter, winds from northerly directions prevail over the Arabian Peninsula. Highest speeds are found in the Northeast monsoon flow over the open Arabian Sea. A confluence zone between winds from westerly and northerly directions extends from Ethiopia to the Southern part of the Red Sea, broadly coincident with a band of convergence. Cloudiness and sea surface temperature decrease from the equatorial seas northward. In summer a prominent core of maximum speed is found within the Southwest monsoon over the Western Arabian Sea. Divergence prevails upstream and convergence downstream from this speed maximum. Marked cyclonic vorticity is characteristic for the area to the left of the high speed core, that is along the Southeast coast of Arabia. The concomitant positive curl of wind stress is conducive to oceanic upwelling and cold coastal waters. Concurrently, the increase of surface friction from the open sea to the continent, that is towards the left of the flow core, favors atmospheric subsidence along the Southeast coast of Arabia. The gradient of cloudiness across the coast is also particularly steep in summer. The relative abundance of rainfall along the coasts of the Southern Red Sea seems at least in part to be related to the presence of a confluence zone during much of the year.