From the deep ocean to the coasts and estuaries through the shelf: linking coastal response to a deep blow-out

As a marginal sea connected to neighboring basins through straits, the Gulf of Mexico (GoM) is dynamically and topographically complex. Physical processes are strongly influenced by the interaction of circulation in the GoM deep basin interior with the surrounding shelf areas of diverse morphologies that include deltas, estuaries, barrier islands and marshes. This was particularly evident during the 2010 Deepwater Horizon (DwH) incident, a deep blow-out close to the Northern GoM shelves, over an area strongly affected by the brackish river plume originated from the Mississippi Delta. The transport processes in the GoM are largely controlled by circulation from the wetlands, estuaries and deltas to the shallow and deep shelf areas, extending to shelf-open sea interactions across the shelf-break. Discharges from large northern GoM rivers also induce buoyancy-driven currents that further impact the transport of brackish waters and any substances (e.g., nutrients, sediments, pollutants) that they carry. For instance, Mississippi waters have been both observed and modeled (Androulidakis and Kourafalou, 2013; Androulidakis et al., 2015,Androulidakis et al., 2018) to travel in 3 distinct regimes of circulation and transport: westward (along the Texas-Louisiana shelf), eastward (along the Mississippi-Alabama-Florida shelf) and offshore, under the influence of the Loop Current (LC) and its associated eddies (Schiller et al., 2011; Schiller and Kourafalou, 2014; Androulidakis et al., 2014; Le Hénaff and Kourafalou, 2016; Androulidakis et al., 2019). When these Mississippi waters head southeast, they may even reach the Florida Keys and travel beyond the Straits of Florida, into the Atlantic Ocean (Hu et al., 2005). Along with the Mississippi plume, the wind-driven and LC driven currents largely impacted the transport of hydrocarbons during the 2010 DwH incident (e.g., Liu et al., 2011a,b; Walker et al, 2011; Weisberg et al., 2011; Kourafalou and Androulidakis, 2013, Weisberg et al., 2016). Androulidakis et al. (2018) showed that river-induced fronts and mesoscale eddies also contributed to the pathways of the oil, both over the shelves and offshore, that was continuously released from the Taylor Energy platform (Warren et al., 2014). Given the added effects of wave-induced Stokes drift, Weisberg et al. (2017) accounted for the transport of DwH surface oil from the deep-ocean to the continental shelf and onto the northern Gulf beaches. The specific physical conditions will be revisited, to illustrate the synergy between the evolution of the Loop Current / Florida Current (LC/FC) system and the rapidly changing shelf and coastal currents under the influence of river runoff and winds. Each of these physical factors had been studied prior to the DwH incident, but their combined effects on hydrocarbon pathways were not known.

Examples will be given on what has been learned through research under the Gulf of Mexico Research Initiative (GoMRI) in the last 10 years. The focus will be on transport processes in the GoM along the ocean continuum from the deep basin interior to the coastal ocean and wetland areas, and their relevance for oil transport and fate. Post-DwH studies have advanced related methodologies and tools. These include multi-platform observations and data analyses, in conjunction with high-resolution, data assimilative models for past simulations and predictions. Two examples on connectivity processes will be discussed: a) advances in the understanding of transport rates and pathways from the Mississippi Delta to the Florida Keys; b) new findings on the role of coastal circulation near Cuba on the evolution of the LC system and on oil fate from a potential oil spill in Cuban waters.

GoMRI-funded research provided unprecedented opportunities to study transport processes along the river-wetland-estuary-coastal-open ocean continuum and a number of important discoveries were made. Several studies took place during the GoMRI decade in smaller estuaries, showing that while riverine fronts are important locally, they can also contribute to regional transport. An example is the cross-shore transport of riverine waters from small estuaries in the northwestern GoM after spring floods in 2016 (Le Hénaff et al., 2019). This transport of waters rich in nutrients had significant ecological implications, as it played an important role in the precipitous episode of coral mortality in the Flower Garden Banks National Marine Sanctuary (off the Texas coast). GoMRI funded efforts have also focused on improving understanding of how small-scale physical processes modulate the coastal-offshore interactions in the GoM.

This study addresses how knowledge acquired during GoMRI can advise future planning of scientific research to aid preparedness and response not only for the GoM, but for many offshore areas of oil exploration, as we advance the understanding and predictability of oil trajectories over pathways from the deep ocean to the coastal environment and vice versa.

The hydrodynamic simulations of the entire GoM were based on the data assimilative, high-resolution (1/50°, ~1.8 km) GoM Hybrid Coordinate Ocean Model (GoM-HYCOM 1/50; Le Hénaff and Kourafalou, 2016; Androulidakis et al., 2019), with atmospheric forcing from the European Centre for Medium-Range Weather Forecasts (ECMWF). This new model implementation is particularly suitable for these studies, with unique capabilities to represent plume dynamics and interaction with the LC system. A simulation from 2010 to 2018 was carried out with the high resolution GoM-HYCOM 1/50.

The MET Norway Open Drift model (Broström et al., 2011) is a 3-dimensional oil spill model that computes the transport, distribution, and weathering of an oil spill at sea. The model simulates the most significant processes that affect the motion of oil particles, such as surface spreading, evaporation, advection, dissolution, emulsification, vertical dispersion, and stranding of the oil particles along the shoreline. The model also incorporates oil thickness, a parameter that is generally lacking in oil spill modeling, even though small changes in oil thickness can have a profound effect on the volume of an oil slick and on its ability to be transformed and transported. A new model feature to initialize oil spill simulations from shape files (based on remote sensing of oil spills) has been implemented. The method was successfully tested on the site of the Taylor Energy platform in the northern GoM (Example 1) and Cuban waters (Example 2). The new National Oceanic and Atmospheric Administration (NOAA) open source oil database (http://www.response.restoration.noaa.gov/ADIOS) was implemented in Open Drift. Simulations were carried out with ocean forcing from the GoM-HYCOM 1/50 simulations, atmospheric and surface wave data from the ECMWF.

Satellite data sets include altimetry from AVISO (http://www.aviso.altimetry.fr) and ocean color maps from MODIS/Aqua chl-a images (http://optics.marine.usf.edu), with the Color Index (CI) values derived based on Hu (2011). Sea Surface Salinity (SSS) weekly estimates at 1 km resolution were derived from MODIS/Acqua (Chen and Hu, 2017). This novel MODIS SSS product provides much higher spatial resolution than the direct SSS measurements of the Aquarius satellite (~150 km). SAR imagery has capacity to detect oil under a wide range of wind speeds and regardless of the illumination conditions (Garcia-Pineda et al., 2009). Two images were collected at the end of April 2017 by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument on NASA's Terra spacecraft (resolution 15 m) and by the Worldview-2 multispectral satellite with an approximate spatial resolution of 1.5 m. The Worldview-2's high resolution allowed us to not only identify features related to thick oil with high detail but also showed the suspended matter contrast of the Mississippi River plume compared to the open sea clear waters.

A total of 14 GPS-Tracked drifters were deployed from research vessels within the Taylor Energy Site study area (approximately at 88.97°W, 28.93°N) in April 2017. The drifter trajectories described hydrocarbon pathways. Two types of drifters (drogued and undrogued), were employed to follow surface material pathways (like oil slicks) and distinguish them from possible subsurface (~0.5 m below surface) material pathways (like oil droplets suspended in the upper one meter of the water column).

Fronts due to river discharge are common in the GoM, especially in the northern shelf areas that receive large river inputs. Among them, the Mississippi is the largest river in North America (average mean discharge: 1.35×10⁴ m³ s⁻¹; Hu et al., 2005). Due to the contrast with the saltier waters of the GoM interior, strong fronts are induced as the river plumes carry lowsalinity waters and associated materials (such as nutrients, sediments and pollutants) both along-shore and offshore towards the GoM interior. The processes controlling river plume dynamics were known before from related GoMRI-funded studies. For instance, Schiller et al. (2011) discussed the development and evolution of the Mississippi plume, extending studies by Walker (1996) and Walker et al. (2005) among others. The fronts created by offshore branches of riverine waters are unique for the GoM and they can reach the Straits of Florida and further to coastal waters off Georgia following the Gulf Stream (Ortner et al., 1995; Hu et al., 2005), affecting optical, physical, and chemical properties of remote waters. The net offshore freshwater transport due to Mississippi plume and LC system interaction may reach ~35% annual average and ~10% long-term mean (Schiller and Kourafalou, 2014).

New findings during the GoMRI decade greatly advanced the previous studies, establishing that the river-induced density fronts play an important role in along-shore and cross-shore transport, acting both as barriers on material transport and as convergence zones that guide transport along frontal lines. Based on satellite and model derived surface fields of oil patches and GoM salinity, Kourafalou and Androulidakis (2013) have shown the influence of river-induced fronts on hydrocarbon transport during the 2010 DwH incident. They showed that the distinct river plume domains to the east and west of the Mississippi Delta influenced the variability of the observed surface oil patches, following the variability in river discharge and wind stress. These results were further established with a multi-platform observational study (Androulidakis et al., 2018) that measured the impact of river-induced fronts on the transport and fate of waters that originated from oil covered areas near the Mississippi Delta, where oil patches were observed by satellite and in situ methods. Drifters released in such waters were well aligned with the river plume fronts and traveled in three distinct directions: westward along the Louisiana-Texas shelf, northeastward along the Mississippi-Alabama-Florida shelf and southward (offshore), toward the GoM interior. These regimes correspond to the major buoyancy-driven currents were modified by other shelf flows (especially wind-driven circulation) and offshore influence (LC and associated eddies).

The riverine fronts that extend to the GoM interior and the related cross-basin transport processes were further elucidated in GoMRI studies. Androulidakis et al. (2019) used ship survey profiles, satellite observations and high-resolution model fields (GoMHYCOM 1/50; Le Hénaff and Kourafalou, 2016) to quantify the three-dimensional structure of such offshore (southward) branches, concentrating on the double episode of 2015 (branches west and east of the LC). Figure 1 shows examples of such episodes during both extended (a,b) and retracted (c,d) phases of the LC, also captured by the MODIS ocean color images and Sea Surface Salinity (SSS) derived both from the GoM-HYCOM 1/50 model and from MODIS data, based on a novel algorithm to extract SSS from ocean color data (Chen and Hu, 2017). During the 2015 event (Figure 1e and 1f) two distinctive offshore pathways were detected, east and west of the LC under the effects of cyclonic (LC Frontal Eddies) and anticyclonic (LC Eddy; LCE) eddies. Le Hénaff and Kourafalou (2016) described the offshore pathways of Mississippi waters that took place far from the Delta under low discharge conditions in 2014.

The continuous oil leaking at the Taylor Energy MC-20 site is producing an oil spill very close to the Mississippi Delta that forms pathways influenced by the shelf ocean dynamics (Androulidakis et al., 2018). The river discharge and moreover the prevailing circulation patterns of the brackish plume play a significant role on the hydrocarbons fate and pathways in the GoM. A case where the low salinity waters together with oil and drifters deployed in Taylor site propagated offshore towards the South was observed at the end of April 2017 (Figure 2). The regional circulation over the northern-central GoM was characterized by strong southward currents due to the synergy of the LCE and a cyclone contributing to the formation of offshore oil pathways.

An idealized numerical experiment for the Taylor oil spill in mid-September 2011 is presented in Figure 3. The major portion of hydrocarbons were trapped between the river plume front (34 isohaline) and the coasts, while a significant quantity was dragged westward along the downstream current. However, a portion of riverine waters with higher salinity (36 isohaline) was carried offshore, towards the central GoM. A southward oil branch was also simulated along the offshore propagating brackish waters. The anticyclonic LCE was completely separated from the LC body, which was still well extended over the central region. A cyclonic eddy (LC Frontal Eddy: LCFE) was also located between the two major features, keeping the LCE away from the main LC. The synergy of these counter-rotating eddies led both low salinity waters and oil patches towards the South similarly to the low salinity pathways described in Figure 1. The ocean color distribution (Figure 3) confirms the shedding of the LCE from the LC over the central-northern GoM and the offshore southward extension of Mississippi waters, supporting the good performance of the ocean model.

Figure 4 exemplifies the relevant submesoscale processes that have been investigated through modelling studies (see Bracco et al., 2019 for a review) and observational campaigns (e.g. Poje et al., 2014; D'Asaro et al., 2018). Such processes at scales of few hundreds of meters to few kilometres, with time scales ranging from hours to few days, control the evolution of the density gradients induced by the riverine water, with non-geostrophic frontal instabilities and baroclinic mixed-layer instabilities aiding the formation of lateral convergence zones and submesoscale eddies, respectively (Zhong and Bracco, 2013; Huntley et al., 2015; Barkan et al., 2017a,b; Choi et al., 2017), that converge and trap oil and any surface confined tracer, from plastic to algae (Zhong et al., 2012). These interactions display a strong seasonal cycle: the freshwater input to the GoM enhances submesoscale currents by introducing lateral buoyancy gradients at the ocean surface in late spring and summer, but suppresses them by contributing to decreasing the depth of the turbulent mixed-layer in winter (Luo et al., 2016; Barkan et al., 2017a). Submesoscale circulations are key to the advection of low salinity waters around the LC periphery and its detached eddies. When freshwater is most abundant, low-salinity waters can be advected quickly and efficiently southward to the Straits of Florida if the Loop Current extends into De Soto Canyon, while a retracted Loop Current state is usually indicative of a greater percentage of riverine water confined close to the coast (Brokaw et al., 2019).

The salinity gradients shown at the bottom of Figure 4 track the freshwater distribution in winter and summer in a simulation performed with the Regional Ocean Modeling System (ROMS) at 1 km horizontal resolution (see Bracco et al. 2019 for details of the model set-up). Relative Vorticity (RV; middle row) and lateral divergence (top row) normalized by the Coriolis parameter highlight the non-geostrophic nature of the flow, with the local Rossby number being proportional to the absolute value of the normalized vorticity.

The predominant structures differ between the two seasons, with submesoscale eddies being more abundant in winter, (deep mixed-layer), and non-geostrophic fronts being prevalent in summer along the density (salinity-dominated, for the GoM) gradients. These structures shape the mixing characteristics as seen in the Lagrangian tracers released in the two seasons.

The FC variability (position, strength and volume) has been also found to be strongly related to the mesoscale dynamics that prevail in the northern (Kourafalou and Kang, 2012) and southern (Kourafalou et al., 2017) parts of the Straits, controlling the connectivity between the Straits and remote regions in the GoM and Atlantic Ocean. Over the northern part of the Straits and along the southeast U.S. continental shelf, cyclonic cold-core eddies may evolve from the Dry Tortugas to the Florida Keys island chain (Lee, 1975; Kourafalou and Kang, 2012). Over the southern part of the Straits and along the northwestern coast of Cuba, anticyclonic warm-core mesoscale eddies (namely CubANs: Cuban ANticyclones; Kourafalou et al., 2017) may either form and remain inside the retracted LC (CubAN “A”), or propagate eastward along the coast as independent eddies (CubAN “B”). CubAN eddies may also have significant biochemical implications directly along the Cuban coast and indirectly, via the FC variability, over environmentally protected regions at other parts of the Straits of Florida (e.g. Pulley Ridge, Dry Tortugas and Florida Keys coral reefs).

Here, we investigate the relation between these anticyclonic eddies and LC evolution in 2010, focusing on the DwH period. In particular, we examine if the release of type “B” CubANs is related to the elongated LC, detected during the accident in April 2010. The Sea Surface Height (SSH) differences between 10 and 20 April are presented in Figure 5a. Two large areas with negative (−0.30 m) and positive (0.30 m) differences are revealed at 83.5°W and 82.5°W (~23.5°N), respectively. These two regions are the only ones with significantly high differences inside the Straits during this 10-day period, which was marked by the formation and eastward evolution of a type “B” CubAN, under the influence of a cyclone at the southeastern part of the LC. The satellite altimetry data on 20 April support the strong cyclonic activity between the LC and the CubAN in the western part of the Straits (Figure 5b). The ocean color images also show the presence of this eddy dipole (cyclone and anticyclonic CubAN) along the Cuban coast on 21 April (Figure 5d); the spatial distribution of CI clearly shows the cyclonic circulation between the LC (west) and the type “B” CubAN (east). This eddy dipole was absent on 7 April, when the LC/FC system was dominant along the Cuban coast (Figure 5c); as shown with the dark blue areas (low CI), the strength of the LC/FC system is reduced inside the Straits on 21 April. Two additional areas with anticyclonic intensification were detected over the northern (27°N) and western (88°W) fronts of the LC, where the SSH increased by 50 cm in only 10 days (Figure 5a).

A clear northward extension of the LC took place during this short period, associated with the evolution of mesoscale processes over the western Straits of Florida. The darker blue areas in the ocean color images confirm the change in the LC northern location between the two dates, showing its extension toward the central northern Gulf on 21 April 2010. The evolution of CubAN eddies inside the Straits of Florida also affects the fate and pathways of oil that might be released at potential oil exploration sites inside the Exclusive Economic Zone of Cuba (Figure 6a). Oil spill scenarios based on GoM-HYCOM 1/50 ocean simulations and Open Drift oil spill simulations were evaluated in conjunction with the ocean dynamics inside the Straits. The impact of the anticyclonic activity (CubANs) on oil presence inside the Straits, evaluated in Area 1 (Figure 6a), is described in Figure 6b. The presence of anticyclonic eddies (high negative RV values) coincides with the large number of oil particles inside the Straits. Especially in 2016, the oil was dominant over the areas almost during the entire year, while the RV revealed very long period of high negative values. The correlation coefficient between the two variables (anticyclonic activity and oil presence) is negative (counter correlation) and high (r=−0.56). Kourafalou et al. (2017), based on satellite data and numerical simulations, showed that the highest occurrence frequency of CubAN eddies was observed in 2016 (more than 80% of the year) as derived from a long period of 13 years (2004–2016). It is concluded that anticyclonic eddies along the Cuban coast may delay the exit of oil towards the Atlantic under the dominant influence of the FC throughout the Straits of Florida.

We showed that the transport and fate of the oil spills over areas covered by river plumes are strongly affected by the plume dynamics and especially the accompanying density river-induced fronts. Therefore, the efficient detection and evolution of the river plume spreading either by observations (in situ or satellite) or by numerical simulations is essential during the response period after an oil spill accident. The operational use of high resolution coupled hydrodynamic and oil drift models with detailed river input would improve the simulation and prognosis of an oil spill in the marine environment. Moreover, the regional circulation in the Gulf of Mexico (GoM), including Loop Current (LC), Florida Current (FC), LC rings and other mesoscale cyclonic and anticyclonic features, controls the connectivity between the remote regions of the Gulf and the exit to the Atlantic Ocean. Important new findings include the connectivity between remote coastal regions, as deep oceanic currents can facilitate the cross-marginal transport of materials not only locally, but also regionally.

These findings form a broader and more challenging view for the management of coastal marine resources that should be integrated for preparedness and response. We also showed that the fate of potential oil spills in the Straits of Florida is strongly related to the local circulation processes prevailing over the area. Thus, the operational platforms for oil spill forecasting should consist of modeling attributes that cover the broader large-scale circulation but with high resolution to take into account the details of the local processes. During an oil spill, multi-platform observations can provide vital information about all environmental aspects, both oceanic and meteorological, that control the fate and pathways of the released hydrocarbons, especially over dynamically complicated regions, such as the deltas and the adjacent coastal areas and shelves.

The above synthesis demonstrates how knowledge acquired during GoMRI can advise future planning of scientific research to aid preparedness and response, not only for the GoM, but for many offshore areas of oil exploration. The broader implication is the advancement of the understanding and predictability of oil trajectories over pathways from the deep to the coastal environment and vice versa.