Abstract
Abstract. The Arabian Sea (AS) hosts one of the most intense oxygen minimum zones (OMZs) in the world. Observations suggest a decline in 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 may have intensified. Here, we simulate 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 winds and heat and freshwater fluxes from an atmospheric reanalysis. We find a significant deoxygenation in the northern AS, with O2 inventories north of 20∘ N dropping by over 6 % per decade between 100 and 1000 m. These changes cause an expansion of the OMZ volume north of 20∘ N at a rate of 0.6 % per decade as well as an increase in the volume of suboxia and the rate of denitrification by 14 and 15 % per decade, respectively. We also show that strong interannual and decadal variability modulate dissolved oxygen in the northern AS, with most of the O2 decline taking place in the 1980s and 1990s. Using a set of sensitivity simulations we demonstrate that deoxygenation in the northern AS is essentially caused by reduced ventilation induced by the recent fast warming of the sea surface, including in the Gulf, with a contribution from concomitant summer monsoon wind intensification. This is because, on the one hand, surface warming enhances vertical stratification and increases Gulf water buoyancy, thus inhibiting vertical mixing and ventilation of the thermocline. On the other hand, summer monsoon wind intensification causes a rise in the thermocline depth in the northern AS that lowers O2 levels in the upper ocean. 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.
Highlights
Rising ocean temperatures decrease O2 solubility in seawater, increase respiration-driven oxygen consumption and enhance vertical stratification, reducing interior ocean ventilation (Oschlies et al, 2018)
A vertical transect at 65◦ E indicates that most of the O2 decline is concentrated north of 20◦ N between 100 and 300 m (Fig. 4). This deoxygenation results in a significant intensification of the oxygen minimum zones (OMZs) over the 3-decade study period, with the volume of suboxic (O2 < 4 mmol m−3) water increasing by nearly 14 % per decade north of 20◦ N and by around 10 % per decade when considering the entire Arabian Sea domain (Figs. 5 and S18)
We reconstruct the evolution of dissolved oxygen in the Arabian Sea (AS) from 1982 through 2010 using a series of hindcast simulations performed with an eddy-resolving ocean biogeochemical model forced with ERA-Interim atmospheric reanalysis
Summary
Rising ocean temperatures decrease O2 solubility in seawater, increase respiration-driven oxygen consumption and enhance vertical stratification, reducing interior ocean ventilation (Oschlies et al, 2018). These changes collectively cause the ocean to lose oxygen as it warms up, a process termed ocean deoxygenation. Observational and modeling evidence suggest that the majority of the observed oxygen decline is caused by changes in ocean ventilation and biogeochemistry (Bindoff et al, 2019). In the upper 1000 m, a growing consensus points towards a loss of O2 of 0.5 %–3.3 % between 1970–2010 (Bindoff et al, 2019). The analysis of local time series suggests much stronger trends at particular sites
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