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Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 15, 159-186, 2018
https://doi.org/10.5194/bg-15-159-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
10 Jan 2018
Intensification and deepening of the Arabian Sea oxygen minimum zone in response to increase in Indian monsoon wind intensity
Zouhair Lachkar1, Marina Lévy2, and Shafer Smith1,3 1The Center for Prototype Climate Modeling, New York University in Abu Dhabi, Abu Dhabi, UAE
2Sorbonne Université (UPMC, Paris 6/CNRS/IRD/MNHN), LOCEAN-IPSL, Paris, France
3Courant Institute of Mathematical Sciences, New York University, New York, NY, USA
Abstract. The decline in oxygen supply to the ocean associated with global warming is expected to expand oxygen minimum zones (OMZs). This global trend can be attenuated or amplified by regional processes. In the Arabian Sea, the world's thickest OMZ is highly vulnerable to changes in the Indian monsoon wind. Evidence from paleo-records and future climate projections indicates strong variations of the Indian monsoon wind intensity over climatic timescales. Yet, the response of the OMZ to these wind changes remains poorly understood and its amplitude and timescale unexplored. Here, we investigate the impacts of perturbations in Indian monsoon wind intensity (from −50 to +50 %) on the size and intensity of the Arabian Sea OMZ, and examine the biogeochemical and ecological implications of these changes. To this end, we conducted a series of eddy-resolving simulations of the Arabian Sea using the Regional Ocean Modeling System (ROMS) coupled to a nitrogen-based nutrient–phytoplankton–zooplankton–detritus (NPZD) ecosystem model that includes a representation of the O2 cycle. We show that the Arabian Sea productivity increases and its OMZ expands and deepens in response to monsoon wind intensification. These responses are dominated by the perturbation of the summer monsoon wind, whereas the changes in the winter monsoon wind play a secondary role. While the productivity responds quickly and nearly linearly to wind increase (i.e., on a timescale of years), the OMZ response is much slower (i.e., a timescale of decades). Our analysis reveals that the OMZ expansion at depth is driven by increased oxygen biological consumption, whereas its surface weakening is induced by increased ventilation. The enhanced ventilation favors episodic intrusions of oxic waters in the lower epipelagic zone (100–200 m) of the western and central Arabian Sea, leading to intermittent expansions of marine habitats and a more frequent alternation of hypoxic and oxic conditions there. The increased productivity and deepening of the OMZ also lead to a strong intensification of denitrification at depth, resulting in a substantial amplification of fixed nitrogen depletion in the Arabian Sea. We conclude that changes in the Indian monsoon can affect, on longer timescales, the large-scale biogeochemical cycles of nitrogen and carbon, with a positive feedback on climate change in the case of stronger winds. Additional potential changes in large-scale ocean ventilation and stratification may affect the sensitivity of the Arabian Sea OMZ to monsoon intensification.

Citation: Lachkar, Z., Lévy, M., and Smith, S.: Intensification and deepening of the Arabian Sea oxygen minimum zone in response to increase in Indian monsoon wind intensity, Biogeosciences, 15, 159-186, https://doi.org/10.5194/bg-15-159-2018, 2018.
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This study provides a new contribution to our understanding of the coupling between the oxygen minimum zones (OMZs) and climate. It explores how idealized changes in summer and winter Indian monsoon winds affect the productivity of the Arabian Sea and the size and intensity of its OMZ. We find that intensification of Indian monsoon winds can amplify climate warming on decadal to centennial timescales.
This study provides a new contribution to our understanding of the coupling between the oxygen...
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