Search

This study critically examines oil spill simulations for a hypothetical 15 day deepwater blowout spill scenario off the east coast of South Africa, coincident with the upper reaches of the Agulhas Current. Our aim is to quantify the impact of submesoscale circulations on the fate of the oil by comparing a submesoscale permitting (2.5 km) versus a mesoscale resolving (7.5 km) ocean model. The bimodal nature of the Agulhas Current plays a crucial role in the fate of the oil. Off the east coast, the relative stability of the current limits shoreline impacts, while downstream the increased variability of the current and its meandering is associated with the generation of submesoscale circulations that drive entrainment of oil onto the continental shelf. Oil entrainment is therefore enhanced in the submesoscale permitting model, and the predicted oiling of South Africa's south coast is found to vary in concert with seasonally varying winds. • We model a 15 day deepwater blowout spill scenario in the Agulhas Current. • Submesoscale ocean features enhance oil entrainment onto the continental shelf. • Shoreline oiling on the south coast is enhanced due to submesoscale variability. • The stability of the Agulhas Current off the east coast restricts shoreline oiling. • In total >2000 km of shoreline are at risk of impact. [ABSTRACT FROM AUTHOR]

A set of nine regional ocean model simulations at various horizontal (from 1 to 9 km) and vertical (from 25 to 150 layers) resolutions with different vertical mixing parameterizations is carried out to examine the transport and mixing of a passive tracer released near the ocean bottom over the continental slope in the northern Gulf of Mexico. The release location is in proximity to the Deepwater Horizon oil well that ruptured in April 2010. Horizontal and diapycnal diffusivities are calculated and their dependence on the model set-up and on the representation of mesoscale and submesoscale circulations is discussed. Horizontal and vertical resolutions play a comparable role in determining the modeled horizontal diffusivities. Vertical resolution is key to a proper representation of passive tracer propagation and – in the case of the Gulf of Mexico – contributes to both confining the tracer along the continental slope and limiting its vertical spreading. The choice of the tracer advection scheme is also important, with positive definiteness in the tracer concentration being achieved at the price of spurious mixing across density surfaces. In all cases, however, the diapycnal mixing coefficient derived from the model simulations overestimates the observed value, indicating an area where model improvement is needed. [ABSTRACT FROM AUTHOR]

• Mesoscale eddies impact the marine ecosystem of the North Pacific and its marginal Seas. • Impacts vary with time and regions. Knowns and unknowns are summarized. • How climate change will modify mesoscale processes remains a key open challenge. Physical transport dynamics occurring at the ocean mesoscale (∼20 km – 200 km) largely determine the environment in which biogeochemical processes occur. As a result, understanding and modeling mesoscale transport is crucial for determining the physical modulations of the marine ecosystem. This review synthesizes current knowledge of mesoscale eddies and their impacts on the marine ecosystem across most of the North Pacific and its marginal Seas. The North Pacific domain north of 20°N is divided in four regions, and for each region known, unknowns and known-unknowns are summarized with a focus on physical properties, physical-biogeochemical interactions, and the impacts of climate variability and change on the eddy field and on the marine ecosystem. [ABSTRACT FROM AUTHOR]

The predictability of the mesoscale circulation in the Gulf of Mexico is evaluated using an ensemble of four integrations for the period 2000–2008 using the Regional Ocean Modeling System (ROMS). In all four runs ROMS is forced by identical, monthly varying, heat and momentum fluxes. We explore the role of initial conditions, boundary conditions, atmospheric forcing, and resolution in the Mississippi plume area, on the potential predictability of the Gulf circulation at scales of 20 km or greater. The potential for predictability varies regionally and seasonally. The modeled circulation is mainly atmospherically forced in the coastal areas and dominated by chaotic mesoscale activity in the central portion of the basin. The mesoscale circulation in the top 1000 m of the water column does not correlate with the one below it except for a limited number of small areas. The potential for predicting the circulation at depths deeper than 1000 m is limited by the intrinsic variability of the eddy field and by the unavailability of a continuous monitoring system that extends below the surface. [ABSTRACT FROM AUTHOR]

The Gulf of Mexico ecosystem is a hotspot for biological diversity and supports a number of industries, from tourism to fishery production to oil and gas exploration, that serve as the economic backbone of Gulf coast states. The Gulf is a natural hydrocarbon basin, rich with stores of oil and gas that lie in reservoirs deep beneath the seafloor. The natural seepage of hydrocarbons across the Gulf system is extensive and, thus, the system׳s biological components experience ephemeral, if not, frequent, hydrocarbon exposure. In contrast to natural seepage, which is diffuse and variable over space and time, the 2010 Macondo oil well blowout, represented an intense, focused hydrocarbon infusion to the Gulf׳s deepwaters. The Macondo blowout drove rapid shifts in microbial populations and activity, revealed unexpected phenomena, such as deepwater hydrocarbon plumes and marine “oil snow” sedimentation, and impacted the Gulf׳s pelagic and benthic ecosystems. Understanding the distribution and fate of Macondo oil was limited to some degree by an insufficient ability to predict the physical movement of water in the Gulf. In other words, the available physical oceanographic models lacked critical components. In the past six years, much has been learned about the physical oceanography of the Gulf, providing transformative knowledge that will improve the ability to predict the movement of water and the hydrocarbons they carry in future blowout scenarios. Similarly, much has been learned about the processing and fate of Macondo hydrocarbons. Here, we provide an overview of the distribution, fate and impacts of Macondo hydrocarbons and offer suggestions for future research to push the field of oil spill response research forward. [ABSTRACT FROM AUTHOR]

The distribution of surface nutrients along the salinity gradient in the Mississippi-Atchafalaya River outflow region was examined during four cruises, including two simultaneous cruises, conducted in the northern Gulf during the summer of 2010 and 2011, and in late spring of 2012. The new, extensive data set covers the salinity gradient from 11 to 37 psu (practical salinity unit) in a year of extraordinarily high river discharge (2011), with few samples from a year of average (2010) and below average (2012) river outflow. The overall surface concentrations of nitrate+nitrite, orthophosphate and silicate are compared to those recorded in cruises spanning the 1985 – 2009 interval. Using Monte Carlo simulations to test the statistical significance, we found that surface orthophosphate and nitrate+nitrite concentrations are approximately three and two fold smaller, respectively, in the 2010–2012 period compared to the previous years. Changes in silicate concentrations were, in most cases, not significant, and their assessment complicated by different measurement techniques and potential preservation artifacts. The weighted river loading of these nutrients was, on the other hand, very high in the latest period when samples mostly covered 2011. The well-known negative correlation between nutrient concentrations and salinity at the ocean surface is confirmed in the most recent data. The area surrounding the Mississippi River mouth is characterized by inorganic N:P ratios greater than 30:1 that decrease to values typically less than 10:1 at about 100 km from of the mouth. Overall our analysis suggests that surface nutrient concentrations in the northern Gulf of Mexico cannot be described with any good accuracy by a linear model based on river discharge alone. [ABSTRACT FROM AUTHOR]

This study examines the mesoscale and submesoscale circulations along the continental slope in the northern Gulf of Mexico at depths greater than 1000 m. The investigation is performed using a regional model run at two horizontal grid resolutions, 5 km and 1.6 km, over a 3 year period, from January 2010 to December 2012. Ageostrophic submesoscale eddies and vorticity filaments populate the continental slope, and they are stronger and more abundant in the simulation at higher resolution, as to be expected. They are formed from horizontal shear layers at the edges of highly intermittent, bottom-intensified, along-slope boundary currents and in the cores of those currents where they are confined to steep slopes. Two different flow regimes are identified. The first applies to the De Soto Canyon that is characterized by weak mean currents and, in the high-resolution run, by intense but few submesoscale eddies that form near preferentially along the Florida continental slope. The second is found in the remainder of the domain, where the mean currents are stronger and the circulation is highly variable in both space and time, and the vorticity field is populated, in the high-resolution case, by numerous vorticity filaments and short-lived eddies. Lagrangian tracers are deployed at different times along the continental shelf below 1000 m depth to quantify the impact of the submesoscale currents on transport and mixing. The modeled absolute dispersion is, on average, independent of horizontal resolution, while mixing, quantified by finite-size Lyapunov exponents and vertical relative dispersion, increases when submesoscale processes are present. Dispersion in the De Soto Canyon is smaller than in the rest of the model domain and less affected by resolution. This is further confirmed comparing the evolution of passive dye fields deployed in De Soto Canyon near the Macondo Prospect, where the Deepwater Horizon rig exploded in 2010, and at the largest known natural hydrocarbon seep in the northern Gulf, known as GC600, located a few hundred kilometers to the west of the rig wellhead. [ABSTRACT FROM AUTHOR]

The processes and instabilities occurring at the ocean surface in the northern Gulf of Mexico between 96.3°W–86.9°W and 25.4°N–30.7°N are investigated with a regional model at submesoscale-permitting horizontal grid resolution (i.e., HR with dx = 1.6 km) over a three-year period, from January 2010 to December 2012. A mesoscale-resolving, lower resolution run (LR, with dx = 5 km) is also considered for comparison. The HR run is obtained through two-way nesting within the LR run. In HR quantities such local Rossby number, horizontal divergence, vertical velocity, and strain rate are amplified in winter, when the mixed layer is deepest, as found in other basins. In the model configuration considered this amplification occurs in surface waters over the continental slope and off-shore but not over the shelf. Submesoscale structures consist of a mixture of fronts and eddies generated by frontogenesis and mixed layer instabilities, with elevated conversion rates of available potential energy (APE) into eddy kinetic energy (EKE). In all quantities a secondary maximum emerges during the summer season, when the mixed layer depth (MLD) is shallowest, barely 15–20 m. The secondary peak extends to the coast and is due to the intense lateral density gradients created by the fresh water inflow from the Mississippi River system. Submesoscale structures in summer consist predominately of fronts, as observed in the aftermath of the 2010 Deepwater Horizon oil spill, and their secondary circulations are impeded due to the limited depth of the mixed layer. Freshwater river input is key to the submesoscale activity in summer but modulates it also in winter, as shown with a sensitivity run in which the riverine inflow is absent. Implications for transport studies in regions characterized by intense freshwater fluxes and for submesoscale parameterizations are discussed. [ABSTRACT FROM AUTHOR]

The aim of this work is to investigate the Arabian Sea response to changes in the South Tropical Atlantic (STA) sea surface temperatures (SSTs). A series of recent studies have shown that the atmospheric circulation and SSTs in the Indian Ocean, and particularly in the Arabian Sea, are affected by STA SST anomalies via a simple Gill–Matsuno mechanism. Here, we use a regional ocean model coupled with a nutrient–phytoplankton–zooplankton–detritus (NPZD) ecosystem model to analyze the impact of the tropical Atlantic SST anomalies on the IO circulation and ecosystem variability. The STA teleconnection to the Indian Ocean develops as follows: Cold SST anomalies in the Gulf of Guinea during boreal summer cause strengthening of the Somali Jet, upwelling favorable winds, cold SST anomalies and a shallower than usual thermocline in the Arabian Sea. The enhanced upwelling in the Arabian Sea, in turn, causes an increase in phytoplankton concentrations. The opposite sequence is verified for warm SST anomalies in the STA region. For a 1°C STA anomaly, the increase/decrease in productivity represents by September up to 19% of the surface phytoplankton climatological values in the model, and up to 13% in the observations. The STA teleconnection contributes to the interannual variability in the Arabian Sea in boreal summer as much as the El Niño Southern Oscillation and the Indian Ocean Dipole. [ABSTRACT FROM AUTHOR]

Abstract: The South China Sea is a marginal basin with a complex circulation influenced by the East Asian Monsoon, river discharge and intricate bathymetry. As a result, both the mesoscale eddy field and the near-inertial energy distribution display large spatial variability and they strongly influence the oceanic transport and mixing. With an ensemble of numerical integrations using a regional ocean model, this work investigates how the temporal resolution of the atmospheric forcing fields modifies the horizontal and vertical velocity patterns and impacts the transport properties in the basin. The response of the mesoscale circulation in the South China Sea is investigated under three different forcing conditions: monthly, daily and 6-hourly momentum and heat fluxes. While the horizontal circulation does not display significant differences, the representation of the vertical velocity field displays high sensitivity to the frequency of the wind forcing. If the wind field contains energy at the inertial frequency or higher (daily and 6-hourly cases), then submesoscale fronts, vortex Rossby waves and near inertial waves are excited as ageostrophic expression of the vigorous eddy field. Those quasi- and near-inertial waves dominate the vertical velocity field in the mixed layer (vortex Rossby waves) and below the first hundred meters (near inertial waves) and they are responsible for the differences in the vertical transport properties under the various forcing fields as quantified by frequency spectra, vertical velocity profiles and vertical dispersion of Lagrangian tracers. [Copyright &y& Elsevier]

Abstract: The variability of the surface eddy kinetic energy (EKE) in the Labrador Sea is investigated with a suite of numerical integrations using a regional ocean model. Simulations are performed over the period 1980–2001 and are compared to satellite observations over the last 9 years. The surface EKE pattern in the basin is dominated by a region along the West coast of Greenland where eddies, mainly anticyclonic, are formed by instability of the main currents flowing over the continental slope, consistent with previous idealized results. Here the interannual changes are linked to the shear of the incoming boundary current system imposed as boundary condition to the model domain. The highly variable strength of the East Greenland current at the northeast boundary, derived from the Simple Ocean Data Assimilation (SODA) reanalysis, strongly influences the vortex formation. In the center of the Labrador Sea, where deep convection occurs, a statistically significant portion of the modeled interannual surface EKE variability is correlated with the local atmospheric forcing, and both heat and wind fluxes play an important role and can be adopted as predictors at a lag of 2–3months. The Arctic Oscillation index can also be used as a remote indicator of the atmospheric fluxes, but with lower skill than local measurements. In contrast the North Atlantic Oscillation index does not correlate significantly with the surface EKE at intraseasonal and interannual scales. The analysis of altimeter data over the 1993–2001 supports the existence of this asymmetry between the regime locally forced by the atmosphere in the central basin, and the regime remotely forced by the incoming boundary current along the west Greenland coast. Those results have important implications for monitoring and predicting the surface eddy kinetic energy variability in the Labrador Sea. [Copyright &y& Elsevier]

Deepwater corals are found on hard grounds of the continental shelf and slope of all ocean basins, where they enhance the abundance and biodiversity of invertebrates and fishes. Despite their essential role in deep-sea ecosystems, the knowledge of the factors that promote or impede connectivity among discrete coral communities remains elusive. Logistical challenges prevent a direct quantification of the essential factors affecting connectivity, such as timing of spawning, time spent in the water column, and settlement behavior, as well as the details of the physical environment and its variability. This study argues that an integrated framework including population genetic approaches and physical models of the ocean circulation with larval particle tracking capabilities can enhance our understanding. Genetic approaches allow determination of general time-integrated patterns of dispersal distance and direction in virtually any species, while transport models can refine understanding of the processes behind the observed dispersal patterns. Here, this integrated approach is applied towards investigating the connectivity of a deepwater coral, Callogorgia delta , along the upper continental slope of the northern Gulf of Mexico. The circulation in the basin is simulated by a regional ocean model at 1 km horizontal resolution, which is sufficiently detailed to allow for the generation and evolution of submesoscale eddies and vorticity filaments, and for a reliable representation of major bathymetric features. Building upon data from four sites spanning about 250 km of distance and 400 m of depth, it is concluded that depth differences on scales of tens to at most few hundreds of meters are sufficient to limit C. delta connectivity among sites. This result has important implications in the development of restoration and preservation strategies of deepwater corals in the Gulf of Mexico and calls for carefully accounting for the depth dimension in these efforts. • Deepwater coral connectivity is logistically hard to quantify. • Genetic approaches help determining time-integrated patterns of dispersal distance/direction. • Transport models can refine understanding of the processes behind observed patterns. • An integrated framework (population genetic + modeling) enhances understanding. • In the Gulf of Mexico deepwater coral, C. Delta , shows depth separation due to limited transport. [ABSTRACT FROM AUTHOR]

• A high-resolution (1 km) model study of Red Snapper larvae dispersal. • Buoyancy and ontogenetic vertical migration are pivotal behaviors. • Ocean advection can shape larvae dispersal. • Limited connectivity between areas west and east of the Mississippi Delta. Advancements in computing power and improved biophysical dispersal models, have enhanced our ability to realistically simulate distributions and behaviors of fish larvae. In this study, a 1 km high-resolution ocean model capable of capturing the ocean's mesoscale and sub-mesoscale motions is integrated with a biophysical dispersal model that considers a range of larval behaviors. Together they are used to investigate the dispersal and connectivity of Red Snapper (Lutjanus campechanus) larvae, a key species for both commercial and recreational fisheries in the northern Gulf of Mexico (GOM). We quantify how various larval behaviors influence the spatiotemporal dispersal, connectivity and settling of Red Snapper larvae focusing on egg buoyancy, larvae swimming capability, and ontogenetic vertical migration. Alongside habitat preferences, the ocean advection of Red Snapper larvae is crucial in shaping their dispersal patterns. Moreover, our simulations suggest different settling and connectivity characteristics between the eastern and western GOM. These results, indicate the need to divide these regions into distinct entities for stock management, rather than treating them as a unified stock as conventionally done. [ABSTRACT FROM AUTHOR]