Mesoscale Eddies of the Western Arabian Sea

The western Arabian Sea is known for a vigorous field of mesoscale eddies. These eddies induce problems in Omani fishery, in the form of sudden changes of artisanal catches, fish kill incidents and oxygen depletions over the Omani shelf. The seasonality of eddy occurrence is poorly understood, but highly desirable in light of the announced extension of the Omani shelf economic zone off shore; the fishery area affected by oceanic eddies will increase.

The goal of the ongoing SQU-funded project is to elucidate seasonal and inter-annual changes of eddy occurrence and their potential impact on fisheries.

The frequency of cyclonic and anticyclonic eddy occurrence for the 13 year period retrieved with 4-km spatial resolution from MODIS-Aqua sea surface height anomalies, and data on wind speed, atmospheric anomalies, geostrophic currents and landings of large pelagic species (Yellowfin tuna, Kingfish and others) were analyzed. The linear increase of eddy occurrence over years was observed during inter-monsoon seasons, against the background of decreasing wind speed and the kinetic energy of the main geostrophic flow (the Oman Coastal Current). Along with that, a positive correlation between the number of eddies over years and the variation of fish catches was elucidated. Presumably, the increasing number of cyclonic eddies could mediate the variance of fish catches, making them less predictable over years. The mechanism of this link could be based on the impact of cyclonic eddies on the oxycline depth. Concentrations of dissolved oxygen less than 1.5-2.5 ml L-1 reportedly act as the hypoxic threshold for the Yellowfin tuna. An enhanced number of cyclonic eddies could generate highly heterogeneous spatial distribution of threshold concentrations- by lifting them to the upper layers and creating an unfavorable environment for populations of large pelagic species.

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