Loop Current Eddies Research Articles

Shelf break exchange events near the De Soto Canyon

Observations of currents, temperature, sea-surface height, sea-surface temperature and ocean color, derived from moorings, surface and deep drifters, hydrographic surveys, and satellites, are used to characterize shelf-slope exchange events near the apex of the De Soto Canyon in the northeast Gulf of Mexico. During the winter of 2012–2013, shelf-break time series showed a number of events where cold shelf water extruded over the slope. These events were largely consistent with slope eddies of both signs influencing shelf break currents. Larger-scale circulations, derived from the Loop Current and a separating Loop Current eddy, strongly influenced circulation over the De Soto slope during summer 2012, with flow patterns consistent with potential vorticity conservation over shoaling topography. Statistical investigation into shelf-slope exchange using large numbers of surface drifters indicated that export from the shelf is larger than vice-versa, and is more uniformly distributed along the shelf break. Import onto the shelf appears to favor a region just east of the Mississippi Delta, which is also consistent with the observed onshore transport of surface oil from the Deepwater Horizon disaster.

Predictability of the Loop Current Variation and Eddy Shedding Process in the Gulf of Mexico Using an Artificial Neural Network Approach

AbstractA novel approach based on an artificial neural network was used to forecast sea surface height (SSH) in the Gulf of Mexico (GoM) in order to predict Loop Current variation and its eddy shedding process. The empirical orthogonal function analysis method was applied to decompose long-term satellite-observed SSH into spatial patterns (EOFs) and time-dependent principal components (PCs). The nonlinear autoregressive network was then developed to predict major PCs of the GoM SSH in the future. The prediction of SSH in the GoM was constructed by multiplying the EOFs and predicted PCs. Model sensitivity experiments were conducted to determine the optimal number of PCs. Validations against independent satellite observations indicate that the neural network–based model can reliably predict Loop Current variations and its eddy shedding process for a 4-week period. In some cases, an accurate forecast for 5–6 weeks is possible.

Characterization of the uncertainty of loop current metrics using a multidecadal numerical simulation and altimeter observations

Satellite altimetry is routinely used to monitor Loop Current intrusion and eddy shedding in the Gulf of Mexico. Statistical estimates of the location and variability of the Loop Current vary significantly among published studies and it is not obvious whether these differences are caused by observational errors, different analysis methodologies, processing and gridding of altimeter data products, or the highly variable nature of the Loop Current system itself. This study analyzes the uncertainty of basic Loop Current statistical estimates derived from altimeter observations, i.e. the northern and western extent, the mean Loop Current eddy separation period, and the relationship between the Loop Current retreat latitude and eddy separation period. The robustness of these statistics is assessed using sea surface height data from a 1/25° free-running multidecadal numerical simulation of the Gulf of Mexico HYbrid Coordinate Ocean Model. A suite of sensitivity tests is performed to identify sources of uncertainty in the Loop Current statistics. The tests demonstrate that the Loop Current metrics from the altimeter fields are less sensitive to the choice of the reference sea surface height mean field or Loop Current front definition than to satellite sampling patterns. Analysis of the model and altimetry-derived sea surface height fields shows that the Loop Current variability changes between regimes of rapid and slow eddy formation cycles. This analysis leads to a discussion of the stationarity of the LC system. The mean separation period estimated from the altimeter fields for 1993–2010 is 8±1.8 months. This uncertainty is larger than the errors introduced by the satellite data processing and gridding technique, which is on the order of O (1 month). It is shown that the available altimetry observational record is not long enough at this time to be able to estimate the mean separation period within one-month uncertainty.

Potential impact of climate change on the Intra-Americas Sea: Part-1. A dynamic downscaling of the CMIP5 model projections

This study examines the potential impact of anthropogenic greenhouse warming on the Intra-Americas Sea (IAS, Caribbean Sea and Gulf of Mexico) by downscaling the Coupled Model Intercomparison Project phase-5 (CMIP5) model simulations under historical and two future emission scenarios using an eddy-resolving resolution regional ocean model. The simulated volume transport by the western boundary current system in the IAS, including the Caribbean Current, Yucatan Current and Loop Current (LC), is reduced by 20-25% during the 21st century, consistent with a similar rate of reduction in the Atlantic Meridional Overturning Circulation (AMOC). The effect of the LC in the present climate is to warm the Gulf of Mexico (GoM). Therefore, the reduced LC and the associated weakening of the warm transient LC eddies have a cooling impact in the GoM, particularly during boreal spring in the northern deep basin, in agreement with an earlier dynamic downscaling study. In contrast to the reduced warming in the northern deep GoM, the downscaled model predicts an intense warming in the shallow (≤200m) northeastern shelf of the GoM especially during boreal summer since there is no effective mechanism to dissipate the increased surface heating. Potential implications of the regionally distinctive warming trend pattern in the GoM on the marine ecosystems and hurricane intensifications during landfall are discussed. This study also explores the effects of 20th century warming and climate variability in the IAS using the regional ocean model forced with observed surface flux fields. The main modes of sea surface temperature variability in the IAS are linked to the Atlantic Multidecadal Oscillation and a meridional dipole pattern between the GoM and Caribbean Sea. It is also shown that variability of the IAS western boundary current system in the 20th century is largely driven by wind stress curl in the Sverdrup interior and the AMOC.

Clustering of Loop Current patterns based on the satellite-observed sea surface height and self-organizing map

The self-organizing map is used to investigate variations of the Loop Current (LC) in the Gulf of Mexico from 1992 to 2013 based on satellite-observed sea surface height data. It is found that LC variations can be characterized by three spatial patterns: normal, extension and retraction. The corresponding temporal variations confirm that LC eddy shedding generally occurs during the transition from the extension to retraction patterns. On the weekly time scale, the wind stress curl (WSC) in the Caribbean Sea has a major influence on LC eddy shedding. The increase of Caribbean WSC from June to November favours more frequent LC eddy shedding during that period. On the interannual time scale, there is also a potential linkage between the frequency of LC eddy shedding and El Niño activities.

Cyclonic Eddies in the Gulf of Mexico: Observations by Underwater Gliders and Simulations by Numerical Model

AbstractCirculation in the Gulf of Mexico (GoM) is dominated by the Loop Current (LC) and by Loop Current eddies (LCEs) that form at irregular multimonth intervals by separation from the LC. Comparatively small cyclonic eddies (CEs) are thought to have a controlling influence on the LCE, including its separation from the LC. Because the CEs are so dynamic and short-lived, lasting only a few weeks, they have proved a challenge to observe. This study addresses that challenge using underwater gliders. These gliders’ data and satellite sea surface height (SSH) are used in a four-dimensional variational (4DVAR) assimilation in the Massachusetts Institute of Technology (MIT) general circulation model (MITgcm). The model serves two purposes: first, the model’s estimate of ocean state allows the analysis of four-dimensional fields, and second, the model forecasts are examined to determine the value of glider data. CEs have a Rossby number of about 0.2, implying that the effects of flow curvature, cyclostrophy, to modify the geostrophic momentum balance are slight. The velocity field in CEs is nearly depth independent, while LCEs are more baroclinic, consistent with the CEs origin on the less stratified, dense side of the LCE. CEs are formed from water in the GoM, rather than the Atlantic water that distinguishes the LCE. Model forecasts are improved by glider data, using a quality metric based on satellite SSH, with the best 2-month GoM forecast rivaling the accuracy of a global hindcast.

Influence of frontal cyclone evolution on the 2009 (Ekman) and 2010 (Franklin) Loop Current eddy detachment events

Abstract. The anticyclonic Loop Current Eddy (LCE) shedding events are strongly associated with the evolution of Loop Current Frontal Eddies (LCFEs) over the eastern Gulf of Mexico (GoM). A numerical simulation, in tandem with in situ measurements and satellite data, was used to investigate the Loop Current (LC) evolution and the surrounding LCFE formation, structure, growth and migration during the Eddy Ekman and Eddy Franklin shedding events in the summers of 2009 and 2010, respectively. During both events, northern GoM LCFEs appeared vertically coherent to at least 1500 m in temperature observations. They propagated towards the base of the LC, where, together with the migration of Campeche Bank (southwest GoM shelf) eddies from south of the LC, contributed to its "necking-down". Growth of Campeche Bank LCFEs involved in Eddy Franklin was partially attributed to Campeche Bank waters following upwelling events. Slope processes associated with such upwelling included offshore exports of high positive potential vorticity that may trigger cyclone formation and growth. The advection and growth of LCFEs, originating from the northern and southern GoM, and their interaction with the LC over the LCE detachment area favor shedding conditions and may contribute to the final separation of the LCE.

Using physical oceanography to improve transmission loss calculations in undersampled environments

The vertical sound speed profile (SSP) is a critical input to any acoustic propagation model. However, even when measured SSPs are available they are frequently noisy “snapshots” of the SSP at a single moment in time and space and do not fully capture changes such as solar heating and wind-driven mixing that can significantly affect shallow water propagation on time scales of less than a day. Furthermore, SSPs measured in the field may not extend to the ocean bottom and are often based on measured profiles of temperature with an implicit assumption of constant salinity. In April–May 2013, the Target and Reverberation Experiment (TREX) was conducted in the Northeastern Gulf of Mexico near Panama City, Florida, a region strongly affected by local wind forcing, freshwater inputs, and the presence of a warm-core Gulf of Mexico Loop Current eddy ("Eddy Kraken") offshore of the experimental site. “Synthetic” SSPs were constructed for the trial area by combining knowledge of the physical oceanography and water masses in the area with the measured SSPs that were available. Transmission loss was modelled using both synthetic and measured SSPs and the results will be compared with measured transmission loss.

Oceanographic conditions in the Gulf of Mexico in July 2010, during the Deepwater Horizon oil spill

Circulation in the Gulf of Mexico (GOM) is dominated by mesoscale features that include the Loop Current (LC), Loop Current Rings (LCRs), and smaller frontal eddies. During May–June 2010, while oil was still flowing from the Macondo well following the Deepwater Horizon (DWH) platform explosion on April 20, 2010, drifter trajectories, satellite observations, and numerical simulations indicated a potential for direct connectivity between the northern Gulf and the Florida Straits via the LC system. This pathway could have potentially entrained particles, including northern GOM contaminants related to the oil spill, carrying them directly towards the coastal ecosystems of south Florida and northern Cuba. To assess this connectivity, and to evaluate the potential oil impacts on economically important GOM fisheries, an interdisciplinary shipboard survey was conducted in the eastern Gulf during July 2010. Analysis of the resulting hydrographic data confirmed that: (1) by July 2010 a large LCR had become separated from the main LC by a cyclonic eddy resulting in the loss of a direct transport mechanism from the northern GOM to the Florida Straits, leaving only indirect pathways available to potential contaminants; and (2) with the exception of four hydrographic stations occupied within 84km of the wellhead, no evidence of oil was found during the survey on the surface or within the water column. These results corroborated analysis of satellite altimetry observations of the GOM surface circulation and verified official surface oil coverage forecasts where they intersected with the survey track. This cruise sampled the LC, LCR, and frontal eddies to a depth of 2000m, with the results suggesting that any oil entrained by circulation features in prior months had either been weathered, consumed by bacteria, dispersed to undetectable levels, or was only present in unsurveyed areas. The assembled subsurface measurements represent one of only a few data sets collected across the dominant GOM mesoscale circulation features at a time when there was great concern about the potential long-range spreading of DWH related contaminants. Direct observations such as these are critical for the assessment of particle trajectory and circulation models utilized during the spill, and for the improvement of future numerical forecast products.

Investigating the sensitivity of hurricane intensity and trajectory to sea surface temperatures using the regional model WRF

The influence of sea surface temperature (SST) anomalies on the hurricane characteristics are investigated in a set of sensitivity experiments employing the Weather Research and Forecasting (WRF) model. The idealised experiments are performed for the case of Hurricane Katrina in 2005. The first set of sensitivity experiments with basin-wide changes of the SST magnitude shows that the intensity goes along with changes in the SST, i.e., an increase in SST leads to an intensification of Katrina. Additionally, the trajectory is shifted to the west (east), with increasing (decreasing) SSTs. The main reason is a strengthening of the background flow. The second set of experiments investigates the influence of Loop Current eddies idealised by localised SST anomalies. The intensity of Hurricane Katrina is enhanced with increasing SSTs close to the core of a tropical cyclone. Negative nearby SST anomalies reduce the intensity. The trajectory only changes if positive SST anomalies are located west or north of the hurricane centre. In this case the hurricane is attracted by the SST anomaly which causes an additional moisture source and increased vertical winds.

Submesoscale impacts on horizontal and vertical transport in the Gulf of Mexico

[1] Impacts of submesoscale processes on transport are investigated numerically in an energetic mesoscale flow with an ocean model run at two horizontal resolutions, 1 and 5 km. The focus is the northwestern Gulf of Mexico, where the Loop Current eddies are surrounded by smaller vortices. By increasing the horizontal resolution, the number and strength of submesoscale eddies and vorticity filaments within the mixed layer increase dramatically and with them the vertical velocities. Inside the coherent eddies and at their peripheries, increased vertical velocities for increasing resolution are associated to near-inertial motions and they are not limited to the mixed layer, but are found at all depths. Horizontal velocities, on the contrary, are similar. Lagrangian isobaric tracers are deployed close to the surface and at 100 m, and three-dimensional, neutrally buoyant particles are released close to the surface, at the base of the mixed layer and at 100 m. The modeled horizontal dispersion curves for each deployment depth are independent of the kind of particles and of horizontal resolution. Close to the ocean surface, however, convergence zones, generated by submesoscale ageostrophic motions and resolved at 1 km resolution, influence the details of the tracer distributions. Vertical dispersion increases by several folds for increasing resolution at all depths explored, with the largest differences found close to the surface. Therefore, submesoscales processes play a fundamental role in driving vertical transport in eddy-dominated flows, both within and below the mixed layer, for times comparable to the Eulerian time scale.

An Analysis of Loop Current Frontal Eddies in a ⅙° Atlantic Ocean Model Simulation

Abstract The Loop Current frontal eddies (LCFEs) refer to cyclonic cold eddies moving downstream along the outside edge of the Loop Current in the eastern Gulf of Mexico. They have been observed by in situ measurements and satellite imagery, mostly downstream of the Campeche Bank continental shelf. Their evolution, simulated by a primitive equation ⅙° and 37-level Atlantic Ocean general circulation numerical model, is described in detail in this study. Some of the simulated LCFEs arise, with the passage through the Yucatan Channel of a Caribbean anticyclonic eddy, as weak cyclones with diameters less than 100 km near the Yucatan Channel. They then grow to fully developed eddies with diameters on the order of 150–200 km while moving along the Loop Current edge. Modeled LCFEs have a very coherent vertical structure with isotherm doming seen from 50- to ~1000-m depth. The Caribbean anticyclone and LCFE are two predominant features in this numerical model simulation, which account for 22% and 10%, respectively, of the short-term (period less than 100 days) temperature variance at 104.5 m in the complex empirical orthogonal function (CEOF) analysis. The source water inside the LCFEs that are generated by Caribbean anticyclonic eddy impingement can be traced back, using a backward-in-time Lagrangian particle-tracking method, to the western edge of the Caribbean Current in the northwest Caribbean Sea and to coastal waters near the northern Yucatan Peninsula. The model results indicating a pairing of anticyclonic and cyclonic eddies within and north of the Yucatan Channel are supported by satellite altimetry measurements during February 2002 when several altimeters were operational.

Comparison between environmental characteristics of larval bluefin tuna Thunnus thynnus habitat in the Gulf of Mexico and western Mediterranean Sea

Despite being well adapted for feeding in cold water on their North Atlantic feeding grounds, Atlantic bluefin tuna undertake long migrations to reach warm, low productivity spawn - ing grounds in the Gulf of Mexico and Mediterranean Sea. Environmental conditions within spawning areas have been presumed to benefit larval survival, through appropriate feeding conditions, and enhanced larval retention and growth rates. However, field collections and studies to explore the potential mechanisms are rare. In this study, a comparison of the environmental characteristics of both spawning sites was completed using standardized environmental data and modeling methods. Predictive models of larval occurrence were constructed using historical larval collections, and environmental variables from both in situ and remotely sensed sources. Results showed that larvae on both spawning grounds were most likely to be found in warm (23 to 28°C), low chlorophyll areas with moderate current velocities and favorable regional retention condi- tions. In the Gulf of Mexico, larvae were located in offshore waters outside of the Loop Current and warm eddies, while in the western Mediterranean, larval occurrences were associated with the confluence of inflowing Atlantic waters and saltier resident surface waters. Although our results suggested common themes within preferred spawning grounds on both sides of the Atlantic Ocean, the ecological processes governing larval survival and eventual recruitment are yet to be fully understood.

Adjoint sensitivity studies of loop current and eddy shedding in the Gulf of Mexico

[1] Adjoint model sensitivity analyses were applied for the loop current (LC) and its eddy shedding in the Gulf of Mexico (GoM) using the MIT general circulation model (MITgcm). The circulation in the GoM is mainly driven by the energetic LC and subsequent LC eddy separation. In order to understand which ocean regions and features control the evolution of the LC, including anticyclonic warm-core eddy shedding in the GoM, forward and adjoint sensitivities with respect to previous model state and atmospheric forcing were computed using the MITgcm and its adjoint. Since the validity of the adjoint model sensitivities depends on the capability of the forward model to simulate the real LC system and the eddy shedding processes, a 5 year (2004–2008) forward model simulation was performed for the GoM using realistic atmospheric forcing, initial, and boundary conditions. This forward model simulation was compared to satellite measurements of sea-surface height (SSH) and sea-surface temperature (SST), and observed transport variability. Despite realistic mean state, standard deviations, and LC eddy shedding period, the simulated LC extension shows less variability and more regularity than the observations. However, the model is suitable for studying the LC system and can be utilized for examining the ocean influences leading to a simple, and hopefully generic LC eddy separation in the GoM. The adjoint sensitivities of the LC show influences from the Yucatan Channel (YC) flow and Loop Current Frontal Eddy (LCFE) on both LC extension and eddy separation, as suggested by earlier work. Some of the processes that control LC extension after eddy separation differ from those controlling eddy shedding, but include YC through-flow. The sensitivity remains stable for more than 30 days and moves generally upstream, entering the Caribbean Sea. The sensitivities of the LC for SST generally remain closer to the surface and move at speeds consistent with advection by the high-speed core of the current, while sensitivities to SSH generally extend to deeper layers and propagate more slowly. The adjoint sensitivity to relative vorticity deduced from the sensitivities to velocity fields suggests that advection of cyclonic (positive) relative vorticity anomalies from the YC or the LCFEs accelerate the LC eddy separation. Forward model perturbation experiments were performed to complement and check the adjoint sensitivity analysis as well as sampling the predictability and nonlinearity of the LC evolution. The model and its adjoint can be used in four-dimensional variational assimilation (4D-VAR) to produce dynamically consistent ocean state estimates for analysis and forecasts of the circulation of the GoM.

State estimates and forecasts of the loop current in the Gulf of Mexico using the MITgcm and its adjoint

[1] An ocean state estimate has been developed for the Gulf of Mexico (GoM) using the MIT general circulation model and its adjoint. The estimate has been tested by forecasting loop current (LC) evolution and eddy shedding in the GoM. The adjoint (or four-dimensional variational) method was used to match the model evolution to observations by adjusting model temperature and salinity initial conditions, open boundary conditions, and atmospheric forcing fields. The model was fit to satellite-derived along-track sea surface height, separated into temporal mean and anomalies, and gridded sea surface temperature for 2 month periods. The optimized state at the end of the assimilation period was used to initialize the forecast for 2 months. Forecasts explore practical LC predictability and provide a cross-validation test of the state estimate by comparing it to independent future observations. The model forecast was tested for several LC eddy separation events, including Eddy Franklin in May 2010 during the deepwater horizon oil spill disaster in the GoM. The forecast used monthly climatological open boundary conditions, atmospheric forcing, and run-off fluxes. The model performance was evaluated by computing model-observation root-mean-square difference (rmsd) during both the hindcast and forecast periods. The rmsd metrics for the forecast generally outperformed persistence (keeping the initial state fixed) and reference (forecast initialized using assimilated Hybrid Coordinate Ocean Model 1/12° global analysis) model simulations during LC eddy separation events for a period of 12 months.

Empirical Models of the Loop Current Eddy Detachment/Separation Time in the Gulf of Mexico

Eddies pinching off the Loop Current can have a significant impact on offshore exploration and production activities in the Gulf of Mexico. Advance knowledge of an eddy shedding event would be of considerable value to the offshore industry and coastal and ocean engineering community that serves the offshore industry. However, imperfect understanding of the dynamics of the Loop Current eddy pinch-off has made reliable forecasting of such events elusive. Two empirical, phenomenological models are presented of the Loop Current eddy detachment/separation times based on the retreat latitude of the Loop Current and its rate of intrusion prior to eddy shedding, and the volume flow rate of the outflow in the Yucatan Channel. The models do a fairly good job of hindcasting past separation and detachment events for which observational data are available. The models also showed some skill when applied to forecasting the detachment of eddy Franklin in early June 2010, which occurred not long after the Deepwater Horizon rig explosion. However, because of the inherent limitations of the observational database available and used (e.g., its less than ideal temporal coverage and interpretational errors) and the very complexity of the eddy shedding process, the author cautions that the methodology is by no means conclusively proven. However, it is one worth continuing to explore as more and more increasingly accurate data and steadily improving understanding of the underlying physics become available in the coming decades.

A downscaling method for simulating deep current interactions with topography – Application to the Sigsbee Escarpment

A nesting approach is applied to dynamically downscale the deep circulation from a basin-scale model in regions of complicated topography where deep dynamics may be poorly resolved. The method is applied to nest a high vertical and horizontal resolution Navy Coastal Ocean Model (NCOM) domain covering the north-central Gulf of Mexico within a HYbrid Coordinate Ocean Model (HYCOM) Gulf of Mexico domain. The northwestern Gulf of Mexico has a very steep topographic feature, the Sigsbee Escarpment, over which localized bottom-intensified currents with short cross-isobath length scales have been observed in water depths between 1500m and 3000m. It has been hypothesized that these intense currents are related to the presence of the Loop Current or Loop Current Eddies, strong upper ocean mesoscale circulation features in the Gulf. A modeling system is required that can resolve the short length scales of topography and the currents, resolve the vertical trapping of the currents, and realistically simulate the mesoscale upper and deep ocean circulation features. The multi-model nesting approach described here simulates these intense currents with characteristics very similar to observations, and demonstrates the connectivity to the larger scale ocean circulation features.

Loop Current Growth and Eddy Shedding Using Models and Observations: Numerical Process Experiments and Satellite Altimetry Data

Abstract Recent studies on Loop Current’s variability in the Gulf of Mexico suggest that the system may behave with some regularity forced by the biannually varying trade winds. The process is analyzed here using a reduced-gravity model and satellite data. The model shows that a biannual signal is produced by vorticity and transport fluctuations in the Yucatan Channel because of the piling up and retreat of warm water in the northwestern Caribbean Sea forced by the biannually varying trade wind. The Loop grows and expands with increased northward velocity and cyclonic vorticity of the Yucatan Current, and eddies are shed when these are near minima. Satellite sea surface height (SSH) data from 1993 to 2010 are analyzed. These show, consistent with the reduced-gravity experiments and previous studies, a (statistically) significant asymmetric biannual variation of the growth and wane of Loop Current: strong from summer to fall and weaker from winter to spring; the asymmetry being due to the asymmetry that also exists in the long-term observed wind. The biannual signal is contained in the two leading EOF modes, which together explain 47% of the total variance, and which additionally describe the eddy shedding and westward propagation from summer to fall. The EOFs also show connectivity between Loop Current and Caribbean Sea’s variability by mass and vorticity fluxes through the Yucatan Channel.

A Lagrangian approach to the Loop Current eddy separation

Abstract. Determining when and how a Loop Current eddy (LCE) in the Gulf of Mexico will finally separate is a difficult task, since several detachment re-attachment processes can occur during one of these events. Separation is usually defined based on snapshots of Eulerian fields such as sea surface height (SSH) but here we suggest that a Lagrangian view of the LCE separation process is more appropriate and objective. The basic idea is very simple: separation should be defined whenever water particles from the cyclonic side of the Loop Current move swiftly from the Yucatan Peninsula to the Florida Straits instead of penetrating into the NE Gulf of Mexico. The properties of backward-time finite time Lyapunov exponents (FTLE) computed from a numerical model of the Gulf of Mexico and Caribbean Sea are used to estimate the "skeleton" of flow and the structures involved in LCE detachment events. An Eulerian metric is defined, based on the slope of the strain direction of the instantaneous hyperbolic point of the Loop Current anticyclone that provides useful information to forecast final LCE detachments. We highlight cases in which an LCE separation metric based on SSH contours (Leben, 2005) suggests there is a separated LCE that later reattaches, whereas the slope method and FTLE structure indicate the eddy remains dynamically connected to the Loop Current during the process.

Loop Current eddy merger exposed by satellites during Gulf of Mexico oil spill

The clockwise flow of water that extends northward into the Gulf of Mexico known as the Loop Current, and its associated eddies, regularly produces strong currents of 2–4 knots in the northern Gulf. Cyclonic (i.e., counterclockwise rotating) eddies, migrating along its outer margin, are difficult to study due to their rapid and unpredictable growth and propagation, as well as persistent cloud cover. We have found that night-time midinfrared satellite images obtained every 30 minutes from geostationary satellites used to quantify sea surface temperature, superimposed with daily updated gridded sea surface height data based on several satellite altimeters, allows us to track the Loop Current and cyclonic eddiesmore effectively than previous methods.1, 2 During the Deepwater Horizon oil spill, we assessed daily changes in the motion of the Loop Current, its cyclonic eddies (called Loop Current frontal eddies), and detached anticyclonic eddies by analyzing daily-updatedmaps of sea surface temperatures and heights. We used ship-board acoustic Doppler current profiler current measurements and time-series positions from satellite-tracked Global Positioning System (GPS) buoys to validate our satellite data interpretations.3 Surface oil is often mapped using readily available passive radiometric optical imaging, such as ‘true-color’ images from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) sensors (see Figure 1). During the Deepwater Horizon oil spill these data were useful, but not totally reliable due to solar interference with the optical sensors. Synthetic aperture radar (SAR) sensors proved more valuable, as the backscatter signal is very sensitive to surface oil and is usable in all weather, day Figure 1. Top: Moderate Resolution Imaging Spectroradiometer (MODIS) ‘true-color’ image depicting the offshore entrainment of the Deepwater Horizon oil toward the Loop Current front on 17May 2010. Middle: Radarsat-2 synthetic aperture radar (SAR) image also depicting the surface oil on 17May 2010. Bottom: Geostationary Operational Environment Satellite East (GOES-East) sea surface temperature image depicting the Loop Current and the large merged cyclone north of the Loop Current on 17–19 May 2010. Superimposed on the color-coded sea surface temperatures are the oiled area, traced from the