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https://doi.org/10.5194/bg-2020-325
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-325
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 Sep 2020

14 Sep 2020

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This preprint is currently under review for the journal BG.

Fast local warming of sea-surface is the main factor of recent deoxygenation in the Arabian Sea

Zouhair Lachkar1, Michael Mehari1, Muchamad Al Azhar1,4, Marina Lévy2, and Shafer Smith1,3 Zouhair Lachkar et al.
  • 1Center for Prototype Climate Modeling, New York University Abu Dhabi, Abu Dhabi, UAE
  • 2Sorbonne Université (CNRS/IRD/MNHN), LOCEAN-IPSL, Paris, France
  • 3Courant Institute of Mathematical Sciences, New York University, New York, USA
  • 4Plymouth Marine Laboratory, Plymouth, UK

Abstract. The Arabian Sea (AS) hosts one of the most intense oxygen minimum zones (OMZs) in the world. Observations show a decline of O2 in the northern AS over the recent decades accompanied by an intensification of the suboxic conditions there. Over the same period, the local sea-surface temperature has risen significantly, particularly over the Arabian Gulf (also known as Persian Gulf, hereafter the Gulf), while summer monsoon winds have intensified. Here, we reconstruct the evolution of dissolved oxygen in the AS from 1982 through 2010 and explore its controlling factors, with a focus on changing atmospheric conditions. To this end, we use a set of eddy-resolving hindcast simulations forced with observation-based winds and heat and freshwater fluxes. We find a significant deoxygenation in the northern AS with O2 inventories north of 20° N dropping by over 2 % decade-1 and 7 % decade-1 in the top 200 m and the 200–1000 m layer, respectively. These changes cause an increase in the volume of suboxia and the rate of denitrification by 10 % decade-1 and 13 % decade-1, respectively. Using a set of sensitivity simulations we demonstrate that deoxygenation in the northern AS is essentially caused by a reduced ventilation induced by the recent fast warming of the sea surface, in particular in the Gulf. Concomitant summer monsoon wind intensification contributes to deoxygenation at depth and in the upper ocean north of 20° N but enhances oxygenation of the upper ocean elsewhere. This is because surface warming enhances vertical stratification, thus limiting ventilation of the intermediate ocean, while summer monsoon wind intensification causes the thermocline depth to rise in the northern AS and deepen elsewhere, thus contributing to lowering O2 levels in the upper 200 m in the northern AS and increasing it in the rest of the AS. Our findings confirm that the AS OMZ is strongly sensitive to upper-ocean warming and concurrent changes in the Indian monsoon winds. Finally, our results also demonstrate that changes in the local climatic forcing play a key role in regional dissolved oxygen changes and hence need to be properly represented in global models to reduce uncertainties in future projections of deoxygenation.

Zouhair Lachkar et al.

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Short summary
This study documents and quantifies a significant recent oxygen decline in the upper layers of the Arabian Sea and explores its drivers. Using a modeling approach we show that the fast local warming of sea surface is the main factor causing this oxygen drop. Concomitant summer monsoon intensification contributes to this trend, although to a lesser extent. These changes exacerbate oxygen depletion in the subsurface, threatening marine habitats and altering the local biogeochemistry.
This study documents and quantifies a significant recent oxygen decline in the upper layers of...
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