Fast local warming is the main driver of recent deoxygenation in the northern Arabian Sea

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.

A Regional Review of Marine and Coastal Impacts of Climate Change on the ROPME Sea Area

“…Trends in the I-RSA and M-RSA show an increase in average SST of approximately 1 • C over 34 years [14], with shallow water areas showing a faster warming trend compared to deep water areas [14,15]. A recent study by Lachkar et al [16] demonstrated that, between 1982-2010, SST has risen by 0.5-1 • C in the O-RSA and by up to 1.5 • C in the I-RSA. Data series from the O-RSA indicate a relatively slower increase of 1.2 • C over the past 50 years in the upper 30 m during the summer monsoon [17].…”

“…In the O-RSA, an intensification of suboxic conditions has been linked to greater vertical stratification due to SST warming, which restricts ventilation of the intermediate ocean layer, while strengthening of the summer monsoon wind causes the thermocline depth to rise in the northern Arabian Sea, depleting oxygen in the upper 200 m. Elsewhere in the Arabian Sea, meanwhile, intense summer monsoon winds have the opposite effect of deepening the thermocline and increasing oxygenation [16]. OMZs are expected to expand and intensify during this century, partly in response to climate change [4], and the O-RSA and M-RSA are projected to experience intense deoxygenation, more so than other parts of the Indian Ocean [49,[53][54][55][56].…”

Recent expansion and intensification of hypoxia in the Arabian Gulf and its drivers

“…The circulation model is based on a configuration of the Regional Ocean Modeling System (ROMS) AGRIF version (Shchepetkin and McWilliams, 2005) thoroughly described in Lachkar et al (2021). The model domain covers the entire Indian Ocean at 1/10°eddy-resolving horizontal resolution and uses 32 vertical layers in terrain-following sigma-coordinates, with enhanced vertical resolution near the surface.…”

“…The model is spun-up over a period of 145 years allowing a minimal drift in the upper and intermediate ocean. The details of the spin-up strategy and model drift analysis are provided in Lachkar et al (2021).…”

“…Yet, under the recent rapid growth of the populations and economies of the surrounding countries on the one hand and the changing regional climate on the other hand, these ecosystems are experiencing an ever-increasing pressure from anthropogenic activity (Sheppard et al, 2010;Burt, 2014;Naser, 2014;Lincoln et al, 2021). Examples of anthropogenic stressors altering the local biogeochemistry and potentially threatening the local ecosystems include the rise of eutrophication and a fast rate of warming of up to 0.6 ± 0.3°C per decade, two to three times higher than the global average over the same period (Strong et al, 2011;Al-Yamani and Naqvi, 2019;Burt et al, 2019;Lachkar et al, 2021;Lincoln et al, 2021). The increased nutrient loading (either directly through pollution from terrestrial sources or indirectly due to altered oceanic circulation because of climate change) together with the warming of the Gulf can cause a decline in O 2 and lead to the emergence of hypoxia due to enhanced remineralization, increased stratification and reduced O 2 solubility (Al-Ansari et al, 2015;Al-Yamani and Naqvi, 2019).…”