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The continental shelf in the northern Gulf of Mexico experiences expansive seasonal hypoxic conditions and eutrophication‐driven acidification in bottom waters. Rising surface ocean temperatures, freshwater and nutrient inputs, and atmospheric CO2will further exacerbate these conditions. Using a high‐resolution, regional circulation‐biogeochemical model, we simulated the spatiotemporal dynamics of oxygen and inorganic carbon in the northern Gulf of Mexico under present and a projected future (2100) climate state. Results indicate a modest expansion of the hypoxic zone, but more severe hypoxia and greater exposure to prolonged hypoxic conditions. The main drivers underlying these changes are a reduction in oxygen solubility (accounting for 60–74% of the change) and increased stratification (accounting for less than 40%). pH is projected to decrease across the shelf with lowest values in hypoxic waters where aragonite saturation will approach the saturation limit. In the model simulations, acidification is primarily driven by atmospheric and offshore CO2levels, while the enhancement in stratification only accounts for 7% or less of the total change in pH. Decreased buffering capacity and increased stratification in the future will enhance respiration‐induced acidification (i.e., a decrease in bottom water pH by respired CO2), which will amplify the climate‐induced acidification. According to the model, the magnitude of future changes varies significantly from year to year. The largest effects are simulated in years with large freshwater discharge and upwelling‐favorable winds. The continental shelf in the northern Gulf of Mexico experiences eutrophication‐driven seasonal low‐oxygen conditions (hypoxia) and acidification (a decrease in bottom water pH by respired CO2). Rising surface ocean temperatures, freshwater and nutrient inputs, and atmospheric CO2will further exacerbate these conditions. We simulated the variations of oxygen and inorganic carbon in the northern Gulf of Mexico at present and under a projected future (2100) climate state. Results indicate more severe and prolonged periods of hypoxia in the future due to reduced oxygen solubility in warmer waters and increased stratification. pH is projected to decrease significantly with lowest values in low‐oxygen waters. Future acidification is primarily driven by rising atmospheric and offshore CO2levels. A decreased buffering capacity of seawater and increased stratification will enhance respiration‐induced acidification, which will further amplify the climate‐induced acidification. The magnitude of projected changes varies significantly from year to year, with the largest effects in years with large freshwater discharge and upwelling‐favorable winds. Hypoxia in the future is simulated to expand and become more severe due to lower O2solubility and enhanced density stratificationRising atmospheric pCO2results in a large drop in pH which will be further amplified by eutrophicationThe projected response to future conditions is larger in years with high freshwater discharge and upwelling‐favorable wind

A large hypoxic zone forms every summer on the Texas‐Louisiana Shelf in the northern Gulf of Mexico due to nutrient and freshwater inputs from the Mississippi/Atchafalaya River System. Efforts are underway to reduce the extent of hypoxic conditions through reductions in river nutrient inputs, but the response of hypoxia to such nutrient load reductions is difficult to predict because biological responses are confounded by variability in physical processes. The objective of this study is to identify the major physical model aspects that matter for hypoxia simulation and prediction. In order to do so, we compare three different circulation models (ROMS, FVCOM, and NCOM) implemented for the northern Gulf of Mexico, all coupled to the same simple oxygen model, with observations and against each other. By using a highly simplified oxygen model, we eliminate the potentially confounding effects of a full biogeochemical model and can isolate the effects of physical features. In a systematic assessment, we found that (1) model‐to‐model differences in bottom water temperatures result in differences in simulated hypoxia because temperature influences the uptake rate of oxygen by the sediments (an important oxygen sink in this system), (2) vertical stratification does not explain model‐to‐model differences in hypoxic conditions in a straightforward way, and (3) the thickness of the bottom boundary layer, which sets the thickness of the hypoxic layer in all three models, is key to determining the likelihood of a model to generate hypoxic conditions. These results imply that hypoxic area, the commonly used metric in the northern Gulf which ignores hypoxic layer thickness, is insufficient for assessing a model's ability to accurately simulate hypoxia, and that hypoxic volume needs to be considered as well. Model intercomparison of three hypoxia models of the northern Gulf of Mexico is presentedBottom water temperature and bottom boundary layer thickness are important for hypoxia simulationOverall stratification strength does not explain model‐to‐model differences in hypoxic conditions

Physical and biogeochemical processes determining the distribution, transport, and fate of nutrients delivered by the Mississippi and Atchafalaya river basin (MARB) to the inner Louisiana continental shelf (LCS) were examined using a three‐dimensional hydrodynamic model and observations of hydrography, nutrients, and organic carbon collected during 12 cruises. Two aspects of nutrient transport and fate on the inner LCS (<50 m depth) were evaluated: (1) along‐shelf and cross‐shelf transports were calculated and (2) nutrient sinks and sources were inferred. On average, 47% of the lower Mississippi River freshwater traveled westward on the LCS, but this percentage was reduced during summer when currents reversed to a predominately upcoast direction. Changes from mainly inorganic to organic nutrients were observed at salinity between 20 and 30, and above 30, organic nutrients were the dominant forms. Westward transport of dissolved inorganic nitrogen (DIN) was about 25% of the combined DIN load from the MARB, whereas westward transport of dissolved organic nitrogen (DON) was 2.8‐fold larger than the MARB DON load. Different from dissolved inorganic nutrients, for which the rivers were the primary source, the dominant source of organic nutrients was advection from offshore. Overall, the inner LCS was estimated to be a net sink for total nitrogen in the amount of −3.14 mmol N m−2d−1and a net sink for total phosphorus in the amount of −0.28 mmol P m−2d−1. These sinks were approximately 33% and 59% of the total N and P sources, respectively, to the inner LCS. Nutrient budgets were developed for the Louisiana shelfTransports were primarily in the form of dissolved organic nutrientsThe inner shelf was a net sink for 33% of the N and 59% of the P inputs

The multidecadal expansion of northern Gulf of Mexico continental shelf hypoxia is a striking example of the adverse effects of anthropogenic nutrient enrichment on coastal oceans. Increased nutrient inputs and widespread shelf hypoxia have resulted in numerous dissolved oxygen (DO) water quality problems in nearshore coastal waters of Louisiana. A large hydrographic dataset compiled from research programs spanning 30 years and the three-dimensional hydrodynamic-biogeochemical model CGEM (Coastal Generalized Ecosystem Model) were integrated to explore the interconnections of low DO waters across the continental shelf to nearshore coastal waters of Louisiana. Cross-shelf vertical profiles showed contiguous low DO bottom waters extending from the shelf to coastal waters nearly every year in the 30+ year time series, which were concurrent with strong cross-shelf pycnoclines. A threshold Brunt–Väisälä frequency of 40 cycles h–1was critical to maintaining the cross-shelf subpycnocline layers and facilitating the formation of a contiguous low DO water mass. Field observations and model simulations identified periods of wind-driven bottom water upwelling lasting between several days to several weeks, resulting in both physical advection of oxygen-depleted offshore waters to the nearshore and enhanced nearshore stratification. Both the upwelling of low DO bottom waters and in situ respiration were of sufficient temporal and spatial extent to drive DO below Louisiana’s DO water quality criteria. Basin-wide nutrient management strategies aimed at reducing nutrient inputs and shelf hypoxia remain essential to improving the nearshore coastal water quality across the northern Gulf of Mexico.

The processes underlying the strong Kuroshio encountering a cape at the southernmost tip of Taiwan are examined with satellite‐derived chlorophyll and temperature maps, a drifter trajectory, and realistic model simulations. The interaction spurs the formation of submesoscale cyclonic eddies that trap cold and high‐chlorophyll water and the formation of frontal waves between the free stream and the wake flow. An observed train of eddies, which have relative vorticity about one to four times the planetary vorticity (f), is shed from the recirculation that occurs in the immediate lee of the cape as a result of flow separation. These propagate downstream at a speed of 0.5–0.6 m s−1. Farther downstream, the corotation and merging of two or three adjacent eddies are common owing to the topography‐induced slowdown of eddy propagation farther downstream. It is found that the relative vorticity of a corotating system (1.2f) is 70% weaker than that of a single eddy due to the increase of eddy diameter from ~16 to ~33 km, in agreement with Kelvin's circulation theorem. The shedding period of the submesoscale eddies is strongly modulated by either diurnal or semidiurnal tidal flows, which typically reach 0.2–0.5 m s−1, whereas its intrinsic shedding period is insignificant. The frontal waves predominate in the horizontal free shear layer emitted from the cape, as well as a density front. Energetics analysis suggests that the wavy features result primarily from the growth of barotropic instability in the free shear layer, which may play a secondary process in the headland wake. Recirculation flow is induced, shedding submesoscale cyclonic eddiesHeadland eddy generation is influenced by the Kuroshio and tidal flowsEddies corotate and merge further downstream

The hypoxic zone on the Louisiana Continental Shelf (LCS) forms each summer due to nutrient‐enhanced primary production and seasonal stratification associated with freshwater discharges from the Mississippi/Atchafalaya River Basin (MARB). Recent field studies have identified highly productive shallow nearshore waters as an important component of shelf‐wide carbon production contributing to hypoxia formation. This study applied a three‐dimensional hydrodynamic‐biogeochemical model named CGEM (Coastal Generalized Ecosystem Model) to quantify the spatial and temporal patterns of hypoxia, carbon production, respiration, and transport between nearshore and middle shelf regions where hypoxia is most prevalent. We first demonstrate that our simulations reproduced spatial and temporal patterns of carbon production, respiration, and bottom‐water oxygen gradients compared to field observations. We used multiyear simulations to quantify transport of particulate organic carbon (POC) from nearshore areas where riverine organic matter and phytoplankton carbon production are greatest. The spatial displacement of carbon production and respiration in our simulations was created by westward and offshore POC flux via phytoplankton carbon flux in the surface layer and POC flux in the bottom layer, supporting heterotrophic respiration on the middle shelf where hypoxia is frequently observed. These results support existing studies suggesting the importance of offshore carbon flux to hypoxia formation, particularly on the west shelf where hypoxic conditions are most variable. Formation of hypoxia, or low dissolved oxygen, is a seasonal occurrence on the Louisiana Continental Shelf associated with stratification of the water column and excess nutrient loads delivered via the Mississippi and Atchafalaya River systems. Frequently referred to as “dead zones,” bottom‐water hypoxia results in stress or death of aquatic organisms, especially those that cannot move to areas with more oxygen. To study the sources and distribution of organic matter that supports oxygen consumption, we applied a three‐dimensional hydrodynamic‐biogeochemical model named CGEM (Coastal Generalized Ecosystem Model). CGEM simulations between 2003 and 2007 successfully simulated spatial and temporal patterns of hypoxia and important biological processes that control its formation. Our simulations revealed that highly productive nearshore waters serve as a source of organic matter that supports oxygen consumption offshore. We identified seasonal bottom‐layer currents that transport organic matter offshore and interannual variations in river discharge that influence biological production and bottom‐layer oxygen consumption offshore. The ecological processes described in this study increase our understanding of how nearshore processes affect the development and maintenance of offshore hypoxia. Simulation models describe ecological processes controlling spatial patterns in production and respiration and effect on hypoxiaOffshore organic matter flux from highly productive nearshore areas supports increased respiration offshoreOffshore carbon flux is an important driver of oxygen demand and resulting hypoxia at intermediate depths where hypoxia commonly occurs

Three sets of Autonomous Temperature Line Acquisition Systems were deployed in the South China Sea. Gaps aside, the data covered nearly 3 years at the northern station and about 2 years farther south. Fluctuations ranged from episodic to interannual. Internal tides, more diurnal than semidiurnal, were active basinwide. Twelve typhoons passed during measurement periods. The most severe one, typhoon Babs in 1998, caused a temperature drop of over 7°C at 50 m. Despite strong monsoons, only near‐surface temperature showed clear seasonal variations. Intraseasonal variations induced by mesoscale eddy stood out much better at subsurface depths. Propagating eddies aside, some eddies were seasonal and nearly stationary. From daily archives of an eddy‐resolving, data‐assimilating ocean model (East Asian Seas Nowcast/Forecast System), we identified two paradigms leading to the generation of a persistent spring‐summer warm eddy in the central‐western basin. In normal years, a complete cyclonic gyre was driven by a strong winter northeast monsoon. Water piled up along the periphery of the South China Sea. Afterward, a warm eddy could be generated from west of Luzon Island and propagated westward while intensifying. Under a weak northeast monsoon, such as in El Niño years, piled‐up water tended to stay in the southern basin. When the wind relaxed in spring, warm water returned northward to form a warm eddy in the central‐western basin. Transition from SW to NE monsoon also often led to a warm eddy generation in southern latitudes, when the summer eastward jet departing from central Vietnam broke up.