Context of the suboxic layer in the Arabian Sea
The setting of the Arabian Sea is reviewed in order to examine which of the circumstances causing large oxygen depletion in the ocean are responsible for the suboxic layer (concentrations < 0.1 ml 1−1) in the northern thermocline there. The wind field forces circulations that restrict but do not exclude exchange with the south, and a recent box-model interpretation of trichlorofluoromethane measurements indicates a modest throughflow for the layer of about 5 × 106m3s−1.The associated oxygen-flux divergence is roughly consistent with biochemical determinations of local oxygen-consumption rates, both approaches giving values (3–6 pl 1−1 sτ-1) that are modest in comparison with estimates elsewhere in the world ocean. Despite the high mean-annual surface productivity in the region (nearly 1gCm−2 day−1), it seems plausible that too little of this particulate matter is consumed at thermocline depths to cause an inflated oxygen demand there. Since the layer is neither an isolated pool, nor a sluggish backwater, nor a conspicuous oxygen sink, the suboxic concentrations must be due (as earlier proposed) to the low concentration in the water entering the layer from the south. That depletion in turn seems due to moderate consumption as the water travels the very long trajectories from its zone of sea-surface renewal (Lats. 40–50°S). Although large seasonal variations are expected in both throughflow volume transport and surface productivity (suggesting comparable changes in consumption rate), the volume of the suboxic layer seems big enough to buffer the oxygen levels there against any very noticeable overall variability.
Volume 128 | Issue 8
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