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.

中文翻译：

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海面局部快速升温是阿拉伯海近期脱氧的主要因素

摘要。阿拉伯海（AS）拥有世界上最密集的最低氧气区域（OMZ）之一。观察到O ₂下降近几十年来，在北部的AS中，伴随着亚氧条件的加剧。在同一时期，当地海表温度明显上升，特别是在阿拉伯海湾（也称为波斯湾，以下称海湾）上空，而夏季风则有所增强。在这里，我们重构了1982年至2010年间AS中溶解氧的演变，并探讨了其控制因素，重点是不断变化的大气条件。为此，我们使用了一组基于观测的风，热量和淡水通量的涡旋解析后验模拟。我们发现在AS北部显著脱氧带O ₂超过2％，十年库存20°N滴的北部^-1和7％的十年^-1分别位于顶部200 m和200–1000 m层。这些变化导致亚氧不足量和反硝化率增加了10％^-1和-13％^-1， 分别。使用一组敏感性模拟，我们证明了北部AS的脱氧基本上是由于最近海面特别是海湾地区的快速变暖引起的通风减少而引起的。伴随的夏季风季风增强导致深海和20°N以北的上层海洋的脱氧作用，但增强了其他地方上层海洋的氧化作用。这是因为表面变暖增强了垂直分层，从而限制了中层海洋的通风，而夏季季风的增强则使北大西洋北部的温跃层深度增加，而在其他地区则加深，从而有助于降低O _2。北部AS的上部200 m处的高度，其余AS则增加。我们的发现证实，AS OMZ对上层海洋变暖和印度季风的同时变化非常敏感。最后，我们的结果还表明，局部气候强迫的变化在区域溶解氧变化中起着关键作用，因此需要在全局模型中适当地表示，以减少未来脱氧预测的不确定性。