In the Arabian Sea (AS), spatiotemporal nutrient limitation patterns of primary production and the possible role of nutrient inputs from the atmosphere are still not well understood. Using a biogeochemical model forced by modeled aerosol deposition, we show that without high atmospheric iron inputs through dust deposition during the summer monsoon, primary production over the AS would be reduced by half. Atmospheric iron deposition also supports most of the nitrogen fixation over the AS. However, our ocean biogeochemistry modeling results suggest that dinitrogen fixation constitutes a negligible fraction of the primary production. Finally, we show that atmospheric inputs of nitrogen, mostly from anthropogenic activities in India, have a negligible impact on primary production.

The combination of high primary productivity and weak ventilation in the Arabian Sea (AS) and Bay of Bengal (BoB) generates vast areas of depleted oxygen, known as oxygen minimum zones (OMZs). The AS OMZ is the world's thickest and hosts up to 40% of global denitrification. In contrast, the OMZ in the BoB is weaker and denitrification free. Using a series of model simulations, we show that the deeper remineralization depth (RD) in the BoB, potentially associated with organic matter aggregation with riverine mineral particles, contributes to weaken its OMZ. When the RD is set uniformly across both seas, the model fails to reproduce the observed contrast between the two OMZs, irrespective of the chosen RD. In contrast, when the RD is allowed to vary spatially, the contrasting distributions of oxygen and nitrate are correctly reproduced, and water column denitrification is simulated exclusively in the AS, in agreement with observations.

The biogeochemical cycling in oxygen-minimum zones (OMZs) is dominated by the interactions of microbial nitrogen transformations and, as recently observed in the Chilean upwelling system, also through the energetically less favorable remineralization of sulfate reduction. The latter process is masked, however, by rapid sulfide oxidation, most likely through nitrate reduction. Thus, the cryptic sulfur cycle links with the nitrogen cycle in OMZ settings. Here, we model the physical-chemical water column structure and the observed process rates as driven by formation and sinking of organic detritus, to quantify the nitrogen and sulfur cycles in the Chilean OMZ. A new biogeochemical submodule was developed and coupled to the Regional Ocean Model System (ROMS). The model results generally agree with the observed distribution of reactive species and the measured process rates. Modeled heterotrophic nitrate reduction and sulfate reduction are responsible for 47% and 36%, respectively, of organic remineralization in a 150 m deep zone below mixed layer. Anammox contributes to 61% of the fixed nitrogen lost to N2 gas, while the rest of the loss is through canonical denitrification as a combination of organic matter oxidation by nitrite reduction and sulfide-driven denitrification. Mineralization coupled to heterotrophic nitrate reduction supplies ∼48% of the ammonium required by anammox. Due to active sulfate reduction, model results suggest that sulfide-driven denitrification contributes to 36% of the nitrogen loss as N2 gas. Our model results highlight the importance of considering the coupled nitrogen and sulfur cycle in examining open-ocean anoxic processes under present, past, and future conditions.

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.

Hindcast simulations of the Arabian Gulf and the Sea of Oman using the Regional Ocean Modeling System (ROMS) are quantitatively evaluated with basin‐wide hydrographic data and time series measurements. The model shows comparable skill in reproducing moored observations of current velocities structure in upper and bottom depths. The skill in simulating observed temperature is higher of 0.93 (scale 0–1) in upper depths compared to 0.52 in bottom depths. Model results are sensitive to parameterization of water clarity. A lower sensitivity was noticed to KPP, GLS, and MY2.5 turbulence closures. When coastal turbid water parameterization is used, accuracy of the model in reproducing seasonal and spatial variations of temperature and salinity increased by 25% compared to the clear water case whereas only 10% increase was noticed when applying KPP turbulent closure. The model reproduces well anticlockwise circulation in the Gulf. A stronger surface inflow of fresher water to the Arabian Gulf through the Strait of Hormuz is simulated in summer compared to winter conditions, mainly due to upper layer horizontal gradient of density between the Arabian Gulf and the Sea of Oman. Less seasonal variability of outflow between 0.15 and 0.20 m s−1 at 50 m to bottom depth around the Strait of Hormuz was noticed in the model results. Modeled surface layer stratification is stronger in summer than winter and varies spatially in the Arabian Gulf with highest stratification near the Strait of Hormuz. Overall, the stratification in shallow water area of the Arabian Gulf remains low throughout the year.

Two oil spill events were investigated using multisensor satellite images in the Al Khafji and Al Fujairah regions. Oil slicks were characterized with Red-Green-Blue (RGB) and floating algae index (FAI) images. Oil slicks near Al Khafji were detected on April 19, 2014 by Landsat 8 and covered around 29.04 km 2 . Sequential VIIRS and MODIS/Aqua images collected on the same day observed the same slicks, which indicated different appearances in the RGB images. Another event was recorded near Al Fujairah on May 11, 2014 by both Landsat 8 and Aqua. The total area coverage of oil slicks was 114.6 km 2 . The estimated slick trajectories from GNOME driven by ocean circulation data from the Regional Ocean Model System (ROMS) and meteorological data were assessed with the movement patterns of satellite-detected oil slicks. The average absolute percentage error of velocity of slick movement between satellite observation and GNOME simulation was 33% for both events. The directions of slick movement showed an agreement between satellite observation and model simulation in 5 out of 7 trajectories' cases. This implies that the integration of multisensor satellite measurements and spill trajectory modeling is very helpful to forecast and predict the fate and transport of oil spills. Résumé. Deux déversements d'hydrocarbures ontétéétudiés en utilisant des images satellitaires multicapteurs dans les régions d'Al Khafji et d'Al Fujairah. Les nappes de pétrole ontété caractérisées avec des images rouge-vert-bleu (RVB) « red-green-blue (RGB) » et avec l'indice d'algues flottantes « floating algae index » (FAI). Les nappes de pétrole près d'Al Khafji ontété détectées le 19 avril 2014 par Landsat 8 et elles couvraient environ 29,04 km 2 . Les images séquentielles VIIRS et MODIS/Aqua acquises le même jour ont observé les mêmes nappes, indiquées par différents aspects dans les images RVB. Un autreévènement aété détecté près d'AlFujairah le 11 mai 2014à la fois par Landsat 8 et Aqua. La superficie totale de ces nappesétait de 114,6 km 2 . Les trajectoires des déversements d'hydrocarbure estiméesà partir de GNOME, en utilisant les données de la circulation océanique du Regional Ocean Model System (ROMS) et les données météorologiques, ontétéévaluées en utilisant les déplacements des nappes de pétrole détectées par satellite. Le pourcentage d'erreur absolue moyenne de la vitesse de mouvement de la nappe entre l'observation par satellite et la simulation de GNOMEétait de 33% pour les 2évènements. Les directions de mouvements des nappes ont montré un accord entre l'observation par satellite et la simulation du modèle dans 5 des 7 cas de trajectoires. Ceci implique que l'intégration de mesures satellitaires multicapteurs et la modélisation des trajectoires des déversements sont très utiles pour prévoir et prédire le devenir et le transport des déversements de pétrole.

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.

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