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Inter-annual variability of mesoscale eddy occurrence in the western Arabian Sea

Sergey A. Piontkovski1*, Muna H.Al-Tarshi2 , Sabrina M.Al-Ismaili3 , Sultana S.H.Al-Jardani3 and Yasmin H.A.Al-Alawi3

1Sultan Qaboos University, P.O.Box 34, Muscat 123, Sultanate of Oman email:This email address is being protected from spambots. You need JavaScript enabled to view it.
2Ministry of Environment and Climate Affairs, P.O.Box 323, Muscat 100, Sultanate of Oman e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
3 Ministry of Agriculture & Fisheries Wealth, P.O.Box 4271, Muscat 119, Sultanate of Oman e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
*Corresponding author

The western Arabian Sea is known for its intensive upwelling developed seasonally along the Omani coast as well as for numerous mesoscale eddies originating in and passing through waters with seasonally high or low primary productivity. The frequency of cyclonic and anticyclonic eddy occurrence for the 13 year period (from 2002 to 2015) 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, in order to elucidate the interannual variability and statistical linkages between parameters. The linear increase of eddy occurrence over years was observed during intermonsoon seasons, against the background of decreasing wind speed and the kinetic energy of the main geostrophic flow. Along with that, a positive correlation between the number of eddies 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 more heterogeneous spatial distribution of threshold concentrations- by lifting them to the upper layers and creating an unfavorable environment for populations of large pelagic species.

Mesoscale eddies, Arabian Sea, fish landings, remote sensing, dissolved oxygen concentration

Mesoscale Eddies and Yellowfin Tuna Catches in the Western Arabian Sea
College of Agricultural and Marine Sciences, Sultan Qaboos University, P.O. Box 34, Al-Khod 123, Sultanate of
E-mail ID: 1 This email address is being protected from spambots. You need JavaScript enabled to view it.; 2 This email address is being protected from spambots. You need JavaScript enabled to view it.

Daily maps of sea surface heights retrieved with 4-km spatial resolution from the MODIS-Aqua scanner and obtained over 13 years of remote sensing (from 2002 to 2015) were used to quantify monthly variations of 1108 cyclonic and 1090 anti-cyclonic eddies.
The seasonal cycle of eddy occurrence has demonstrated two peaks, both corresponding to the spring and fall Inter-monsoon seasons. The amount of eddies observed during Inter-monsoon seasons was by ~ 30% higher than those of the South-west (winter) and North-east (summer) monsoons. Seasonal cycles of eddy occurrence were compared to Yellowfin Tuna landings. Two peaks of landings corresponding to April and October (which are cores of the spring and fall Inter-monsoon seasons) were observed. A positive linear correlation between the total number of cyclonic plus anti-cyclonic eddies and Yellowfin Tuna landings was statistically significant. Also, it holds for the cyclonic and anticyclonic eddies analyzed separately. Results of the Principal Component Analysis incorporating 12 environmental parameters (including wind speed, sea surface temperature, dissolved oxygen, nutrients, chlorophyll-a, sardine catches, and key indices of atmospheric anomalies) demonstrated that mesoscale eddies might be treated as ecologically important regional phenomena affecting seasonal variations of Yellowfin Tuna landings over the Omani shelf.
Key Words: Arabian Sea; Mesoscale Eddies; Yellowfin Tuna

Physical Controls on Oxygen Distribution and DenitrificationPotential in the North West Arabian Sea
Bastien Y. Queste1, Clément Vic2, Karen J. Heywood1, and Sergey A. Piontkovski3
1Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, UK
2Department of Ocean and Earth Sciences, University of Southampton, Southampton, UK
3College of Agricultural Marine Sciences, Sultan Qaboos University, Muscat, Oman

At suboxic oxygen concentrations, key biogeochemical cycles change and denitrification becomes the dominant remineralization pathway. Earth system models predict oxygen loss across most ocean basins in the next century; oxygen minimum zones near suboxia may become suboxic and therefore denitrifying. Using an ocean glider survey and historical data, we show oxygen loss in the Gulf of Oman (from 6–12 to <2 μmol/kg1) not represented in climatologies. Because of the nonlinearity between denitrification and oxygen concentration, resolutions of current Earth system models are too coarse to accurately estimate denitrification. We develop a novel physical proxy for oxygen from the glider data and use a high-resolution physical model to show eddy stirring of oxygen across the Gulf of Oman. We use the model to investigate spatial and seasonal differences in the ratio of oxic and suboxic water across the Gulf of
Oman and waters exported to the wider Arabian Sea.

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