Loop Current Rings Research Articles

The Energy Decay of Warm‐Core Eddies in the Gulf of Mexico

AbstractThe Gulf of Mexico (GoM) is home to some of the most energetic eddies in the ocean. Warm‐core rings detach from the Loop‐Current and drift through the basin, transporting large amounts of heat and salt. These eddies, known as Loop Current rings (LCRs) have a crucial role in the GoM's dynamics and in the weather of the eastern US, and this role is largely conditioned by their longevity and decay properties. Here, we use an empirical method to estimate the energy evolution of all LCRs detached since 1993. We found that, contrary to the commonly accepted idea that LCRs conserve their energy as they drift through the GoM and decay suddenly against the western platform, LCRs' energy decay is faster in the eastern basin, and they typically lose three‐quarter of their energy before encountering the continental shelf. We also show that wind‐current feedback contributes to the energy decay and conversion.

Reconstructing the Three-Dimensional Structure of Loop Current Rings from Satellite Altimetry and In Situ Data Using the Gravest Empirical Modes Method

The three-dimensional structure of Gulf of Mexico’s warm-core rings, detaching from the Loop Current, is investigated using satellite altimetry and a large set of ARGO float profiles. Reconstruction of the Loop Current rings (LCRs) vertical structure from sea surface height observations is made possible by the use of the gravest empirical modes method (GEM). The GEMs are transfer functions that associate a value of temperature and salinity for each variable pair {dynamic height; pressure}, and are computed by estimating an empirical relationship between dynamic height and the vertical thermohaline structure of the ocean. Between 1993 and 2021, 40 LCRs were detected in the altimetry and their three-dimensional thermohaline structure was reconstructed, as well as a number of dynamically relevant variables (geostrophic and cyclogeostrophic velocity, relative vorticity, potential vorticity, available potential energy and kinetic energy density, etc.). The structure of a typical LCR was computed by fitting an analytical stream function to the LCRs dynamic height signature and reconstructing its vertical structure with the GEM. The total heat and salt contents and energy of each LCR were computed and their cumulative effect on the Gulf of Mexico’s heat, salt and energy balance is discussed. We show that LCRs have a dramatic impact on these balances and estimate that residual surface heat fluxes of −13 W m−2 are necessary to compensate their heat input, while the fresh water outflow of the Mississippi river approximately compensates for their salt excess input. An average energy dissipation of O [10−10–10−9] W kg−1 would be necessary to balance their energy input.

On the Evolution of the Gulf of Mexico Loop Current Through Its Penetrative, Ring Shedding and Retracted States

How the Gulf of Mexico Loop Current (LC) transitions from penetrative to ring shedding to retracted configurations remains an active research topic. Satellite altimetry suggests a set of states, some more persistent than others, identified as Campeche Bank Anchoring, West Florida Shelf Anchoring, Extended, and LC Ring, plus transitional phases. Histograms show that the most frequent states are Campeche Bank Anchoring and West Florida Shelf Anchoring. Two canonical evolutions are defined. The first is a transition from Campeche Bank Anchoring to West Florida Shelf Anchoring. The second is a transition from Campeche Bank Anchoring to Extended to LC Ring. Common to both are cyclonic eddies. The first canonical transition requires cyclones formed both east and west of the LC, acting as pincers far enough south to dislodge the LC from the Campeche Bank. The second has a single cyclone, formed on either side of the LC, that cuts through either from the east, thereby not dislodging the LC from the Campeche Bank, or from the west, but too far north to dislodge the LC. Notable is the importance of the Campeche Bank and Florida escarpments. Cyclones may generate either by instability or downslope perturbation, but whether these lead to protracted penetration, or just jostling from one anchoring state to another, depends on where they are generated and how they combine to shed a LC Ring. Thus, these findings suggest that furthering understanding and prediction requires observations and analyses over the two escarpment regions and the LC domain south of about 25°N.

The Dynamical Structure of a Warm Core Ring as Inferred from Glider Observations and Along-Track Altimetry

This study investigates the vertical structure of the dynamical properties of a warm-core ring in the Gulf of Mexico (Loop Current ring) using glider observations. We introduce a new method to correct the glider’s along-track coordinate, which is, in general, biased by the unsteady relative movements of the glider and the eddy, yielding large errors on horizontal derivatives. Here, we take advantage of the synopticity of satellite along-track altimetry to apply corrections on the glider’s position by matching in situ steric height with satellite-measured sea surface height. This relocation method allows recovering the eddy’s azimuthal symmetry, precisely estimating the rotation axis position, and computing reliable horizontal derivatives. It is shown to be particularly appropriate to compute the eddy’s cyclo-geostrophic velocity, relative vorticity, and shear strain, which are otherwise out of reach when using the glider’s raw traveled distance as a horizontal coordinate. The Ertel potential vorticity (PV) structure of the warm core ring is studied in details, and we show that the PV anomaly is entirely controlled by vortex stretching. Sign reversal of the PV gradient across the water column suggests that the ring might be baroclinically unstable. The PV gradient is also largely controlled by gradients of the vortex stretching term. We also show that the ring’s total energy partition is strongly skewed, with available potential energy being 3 times larger than kinetic energy. The possible impact of this energy partition on the Loop Current rings longevity is also discussed.

Genesis, evolution, and apocalypse of Loop Current rings

We carry out assessments of the life cycle of Loop Current vortices, so-called rings, in the Gulf of Mexico by applying three objective (i.e., observer-independent) coherent Lagrangian vortex detection methods on velocities derived from satellite altimetry measurements of the sea-surface height (SSH). The methods reveal material vortices with boundaries that withstand stretching or diffusion or whose fluid elements rotate evenly. This involved a technology advance that enables framing vortex genesis and apocalypse robustly and with precision in a truly parameter-free fashion. We find that the stretching- and diffusion-withstanding assessments produce consistent results, which show large discrepancies with Eulerian assessments that identify vortices with regions instantaneously filled with streamlines of the SSH field. The even-rotation assessment, which is vorticity-based, is found to be quite unstable, suggesting life expectancies much shorter than those produced by all other assessments.

On the Mean Flow in the Western Gulf of Mexico and a Reappraisal of Errors in Ship-Drift Data

AbstractShip-drift data in the Gulf of Mexico have led to a perplexing result, that the near-surface flow in the west has a north–south mean, of the east–west flow, ~5–10 cm s−1 into a closed basin. Ship-drift data have been used in the past hundred years under the assumption that they are reasonably accurate; the present study examines that assumption carefully, finding that the standard deviation of individual observations is typically ~20 cm s−1. In a monthly mean composed of order 400 observations or more, as examined here, the standard error of the mean will be reduced accordingly. In the southern part of the western Gulf of Mexico, the observed upper-layer flow is clearly to the west and is consistent with our expectations. In the northern part, however, the apparent flow as reported by ship drift in deep water is not significantly different from zero. Thus, the puzzling result remains: three different datasets in the southern half of the basin clearly show flow to the west, with speeds of 10 cm s−1 or more, yet there is no clear evidence of a near-surface return flow back to the east. The convergent wind stress forces downwelling of the upper layer; its return flow could be at some intermediate depth. The transport to the west from Loop Current rings is possibly returned in a deep boundary flow driven by the rectification of deep topographic Rossby waves.

In Situ Measurements of Circulation Features Influencing Cross‐Shelf Transport Around Northwest Cuba

AbstractWe analyzed circulation processes sampled in the Gulf of Mexico in May 2016 by the National Oceanic and Atmospheric Administration (NOAA) Ship Nancy Foster. This data set is one of the first in situ surveys in Cuban waters available to the international community. Along northwest Cuba, these data suggested coastal upwelling and revealed, for the first time, a ~50 km diameter Cuban ANticyclonic (CubAN) eddy and a ~25 km diameter cyclonic eddy, which together advected upwelled waters offshore. The CubAN eddy was associated with downwelling, and the cyclonic eddy with upwelling. At the western tip of Cuba, local currents were predominantly anticyclonic, presumably due to the proximity of the retracted Loop Current, with limited export of coastal waters. Conversely, additional data from two cruises when the Loop Current was extended showed cyclonic circulation within upwelling filaments extending far offshore. These processes are important, as they can potentially entrain marine organism larvae from local reefs into the Loop Current system and to other reef ecosystems of the region. They might also affect the transport of pollutants, as hydrocarbons in case of a spill in Cuban waters. The 2016 cruise took place after the shedding of a Loop Current Ring, which involved an unusually large (~250 km) cyclonic frontal eddy. The eddy signature was observed down to 1,200 m depth, deeper than the Loop Current. A surface drifter revealed a low relative vorticity (0.19 f) inside the eddy. Along its southern edge, filaments exported from the Campeche Bank were associated with high relative chlorophyll a at 3,060 m depth.

Medium‐Term Forecasting of Loop Current Eddy Cameron and Eddy Darwin Formation in the Gulf of Mexico With a Divide‐and‐Conquer Machine Learning Approach

AbstractThe Loop Current (LC) is the dominant circulation system in the Gulf of Mexico. A long‐term prediction of the LC system (LCS) behavior is critical for understanding the Gulf of Mexico oceanography and ecosystem, and for mitigating outcomes of anthropogenic and natural disasters. In early 2018, the National Academies of Science, Engineering, and Medicine posed a challenge to the research community to develop systems that can forecast the movement of the LCS over longer periods of time than the current state of art. In this paper, a Recurrent Neural Network, the Long Short‐Term Memory (LSTM) network, is applied to predict the LC evolution and the LC ring formation. The LSTM model is trained to learn patterns hidden in sea surface height (SSH) time series. To reduce the memory demand owing to the use of high spatial resolution SSH data set, the region of interest is partitioned into nonoverlapping subregions. After partitioning, an LSTM network is trained to predict the SSH in each subregion. A smoothing function is then applied to reduce discontinuities of the SSH predictions across the partition boundaries, hence error propagation. It is shown that such a machine learning model is capable of predicting the LCS SSH evolution 9 weeks in advance within 40 km in terms of the LCS frontal distance errors. Furthermore, it is shown that the model predicted the timing and general location of eddy Darwin's shedding event 12 weeks in advance, and eddy Cameron's detachment and reattachment 8 weeks in advance.

Enduring Lagrangian coherence of a Loop Current ring assessed using independent observations

Ocean flows are routinely inferred from low-resolution satellite altimetry measurements of sea surface height assuming a geostrophic balance. Recent nonlinear dynamical systems techniques have revealed that surface currents derived from altimetry can support mesoscale eddies with material boundaries that do not filament for many months, thereby representing effective transport mechanisms. However, the long-range Lagrangian coherence assessed for mesoscale eddy boundaries detected from altimetry is constrained by the impossibility of current altimeters to resolve ageostrophic submesoscale motions. These may act to prevent Lagrangian coherence from manifesting in the rigorous form described by the nonlinear dynamical systems theories. Here we use a combination of satellite ocean color and surface drifter trajectory data, rarely available simultaneously over an extended period of time, to provide observational evidence for the enduring Lagrangian coherence of a Loop Current ring detected from altimetry. We also seek indications of this behavior in the flow produced by a data-assimilative system which demonstrated ability to reproduce observed relative dispersion statistics down into the marginally submesoscale range. However, the simulated flow, total surface and subsurface or subsampled emulating altimetry, is not found to support the long-lasting Lagrangian coherence that characterizes the observed ring. This highlights the importance of the Lagrangian metrics produced by the nonlinear dynamical systems tools employed here in assessing model performance.

Observational evidence for atmospheric modulation of the Loop Current migrations

Recent modeling studies on the shedding of Loop Current rings suggest that the intensification of the dominant zonal wind field delays the detachment of rings and affects the Loop Current migrations. The atmospheric modulation of the Loop Current migrations is analyzed here using reanalysis winds and altimetry-derived observations. A newly developed methodology is applied to locate the Loop Current front, and a wavelet-based semblance analysis is used to explore correlations with atmospheric forcing. The results show that weakening (intensification) of the zonal wind stress in the eastern Gulf of Mexico is related with the Loop Current excursions to the north (south). Semblance analyses confirm negative correlations between the zonal wind stress and the Loop Current migrations during the past 20 years. The intrusions of the Loop Current might involve an increase of the Yucatan Transport, which would balance the westward Rossby wave speed of a growing loop and delay the ring shedding. The results of this study have consequences for the interpretation of the chaotic processes of ring detachment and Loop Current intrusions, which might be modulated by wind stress.

Do Assimilated Drifter Velocities Improve Lagrangian Predictability in an Operational Ocean Model?

Abstract The Lagrangian predictability of general circulation models is limited by the need for high-resolution data streams to constrain small-scale dynamical features. Here velocity observations from Lagrangian drifters deployed in the Gulf of Mexico during the summer 2012 Grand Lagrangian Deployment (GLAD) experiment are assimilated into the Naval Coastal Ocean Model (NCOM) 4D variational (4DVAR) analysis system to examine their impact on Lagrangian predictability. NCOM-4DVAR is a weak-constraint assimilation system using the indirect representer method. Velocities derived from drifter trajectories, as well as satellite and in situ observations, are assimilated. Lagrangian forecast skill is assessed using separation distance and angular differences between simulated and observed trajectory positions. Results show that assimilating drifter velocities substantially improves the model forecast shape and position of a Loop Current ring. These gains in mesoscale Eulerian forecast skill also improve Lagrangian forecasts, reducing the growth rate of separation distances between observed and simulated drifters by approximately 7.3 km day−1 on average, when compared with forecasts that assimilate only temperature and salinity observations. Trajectory angular differences are also reduced.

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.

Revisiting the relationship between Loop Current rings and Florida Current transport variability

It is suggested that internal ocean variability plays a significant role in Florida Current transport variability on interannual time scales. A clear relationship is found between different stages within a ring shedding cycle of the Loop Current in the Gulf of Mexico and minima in the Florida Current transport in mesoscale eddy‐permitting ocean model simulations. Available observations are generally in agreement with such a relation but too sparse to give full evidence. Before the Loop Current intrudes far into the Gulf of Mexico a coherent eddy within the Loop Current partly blocks the inflow from the Caribbean Sea through Yucatan Channel leading to a minimum in the Florida Current transport. Such a blocking situation typically occurs over a period of 1–2 months. The irregular blocking leads to interannual variability explaining large parts of the variability of the Florida Current transport in the model simulations, even exceeding atmospheric forcing variability on the considered time scales. Model simulations without ring shedding produce significantly less variability in Florida Current transport. Simulations without interannual variability in the surface forcing show almost as large (or even larger) interannual Florida Current transport changes as without forcing variability. Adding a seasonal cycle can lead to a beat frequency which might be important for the decadal variability.

Hyperbolicity in temperature and flow fields during the formation of a Loop Current ring

Abstract. Loop Current rings (LCRs) are among the largest mesoscale eddies in the world ocean. They arise when bulges formed by the Loop Current in the Gulf of Mexico close off. The LCR formation process may take several weeks, and there may be several separations and reattachments before final separation occurs. It is well established that this period is characterized by a persistent saddle point in the sea surface height field, as seen in both model and satellite data. We present here a detailed study of this saddle region during the formation of Eddy Franklin in 2010, over multiple days and at several depths. Using a data-assimilating Gulf of Mexico implementation of the HYbrid Coordinate Ocean Model (HYCOM), we compare the vertical structure of the currents and temperature fields on 5 and 10 June 2010. Finite-time Lyapunov exponents (FTLE) are computed from the surface down to 200 m to estimate the location of relevant transport barriers. Several new features of the saddle region associated with LCR formation are revealed: the ridges in the FTLE fields are shown to be excellent surrogates for the manifolds delineating the material flow structures with only slight degradation at depth. The intersection of the ridges representing stable and unstable manifolds drops nearly vertically through the water column at both times; remarkably, the material boundary shapes are maintained even as they are advected. Moreover, velocity stagnation points and saddle points in the temperature field are consistently found near the intersections at all depths, and their geographic positions are also nearly constant with depth.

Loop Current excursions and ring detachments during 1993–2009

Linkages between the variability of Loop Current (LC) surface dynamics, LC ring detachments, and the mean sea height anomaly in the Gulf of Mexico (GOM) are explored using a new methodology that locates the LC fronts and detects the shedding of LC rings. Based on satellite altimetry observations and dynamic height gradient, this methodology allows the determination of the dynamic structure in the region from 1993 to 2009. Northward penetration of the LC was found to be seasonal, with a tendency to increase during the spring and peaking in summer. Whereas northward oscillations exhibit ranges of 4 degrees of latitude, the range of westward oscillations is 6 degrees of longitude. Using the newly developed methodology, 28 LC rings are identified and described during 1993–2009. Starting in 2003, the LC is located more to the north on average, and the average number of LC rings formed per year increases. Since 2003, a significant increase in sea height residuals in the GOM has been observed, exhibiting a linear trend of 2.78 ± 0.26 cm/decade for the period 1993–2009. It is hypothesized here that the increase observed in sea height residuals is linked to the increase in mesoscale activity (LC northward intrusions and number of rings shed) obtained from satellite altimetry observations. Results shown here complement previous observational studies in the region, cover a longer time span, and define objectively the locations of the LC front and the shedding of the rings.

Challenges of Drilling and Completion Operations of Deep Wells in Ultradeepwater Zones in the Gulf of Mexico

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 125111, ’Challenges of Drilling and Completion Operations of Deep Wells in Ultradeepwater Zones in the Gulf of Mexico,’ by J.C. Cunha, SPE, and O. Moreira, Petrobras America, and G.H. Azevedo, SPE, B.C.M. Pereira, and L.A.S. Rocha, Petrobras S.A, prepared for the 2009 SPE Annual Technical Conference and Exhibition, New Orleans, 4-7 October. Challenging wells are being drilled in deepwater zones in the US Gulf of Mexico (GOM). Many are in water depths of 8,000 ft or more, with several target reservoirs at 30,000 ft and deeper. Completion challenges include completing multiple zones while trying to minimize future expensive workover operations. Practical experiences dealing with these problems and suggestions to make them manageable are presented. Introduction Exploration, appraisal, and production in areas such as Mississippi Canyon, Atwater Valley, Walker Ridge, Keathley Canyon, and others present inherent difficulties associated with deepwater drilling along with new difficulties in very deep zones that demand careful planning to be reached without compromising safety, well integrity, formation evaluation, and estimated budget. Challenges include difficult-to-find seafloor locations to place the production wells or drilling center. The complex directional drilling requires planning, follow-up, and feedback that must be fine tuned to reduce nonproductive time (NPT) and improve well-construction performance. A “Plan/Do/Check/Act” (PDCA) routine should be implemented. The completion design may include stimulating deep, high-pressured, tight reservoirs. Completing the Lower Tertiary section of deep wells in the GOM proves to be as challenging as the drilling. With few analogous wells drilled and no production history to date, validation of any completion design and the corresponding engineering solution is affected by uncertainties. Weather-related events are a major source of NPT. Ocean currents associated with the Loop Current and Eddies may cause massive losses of time. Moreover, the always unpredictable hurricane season has caused such large losses that many operators will delay an operation rather than try to drill during hurricane season. Weather—Be Prepared To Be Surprised The GOM Loop Current, a stream of warm ocean water that flows northward from the Yucatan Peninsula, loops clockwise and eventually exits through the Florida Straits. The Eddy, or Loop Current ring, occurs when a stream of water “breaks” (shears) and separates from the main stream. The Eddy can be up to 250-km diameter and more than 1000 m deep. Once formed, it travels west across the GOM as it rotates clockwise, completing a rotation once every 10 days. It may take from 2 months to 1 year for an Eddy to dissipate. In a recent operation in the Walker Ridge quadrant, an anchored rig in the presence of strong Loop Current lost significant time while trying to perform mooring operations. A total of 224.5 hours in 13 days of operations (72%) was lost waiting on weather. This type of delay can postpone rig operations and delay development and production. It is recommended that a rigorous monitoring and forecasting service be used to enable accurate predictions of the possible effects of Loop Current and Eddies on planned offshore operations.

A Trigger Mechanism for Loop Current Ring Separations

Abstract The Loop Current in the Gulf of Mexico sheds large anticyclonic rings on an irregular basis. The authors attempt to show what actually triggers the ring separations. Pulses of increased transport through the Florida Straits, as observed by the cable data, are observed prior to each ring separation. This finding is consistent over all separation events observed in the satellite altimetry record. The pulses of transport occur approximately two to four weeks before the rings separate. The increase in transport is usually accompanied by a corresponding increase in offshore sea level, suggesting forcing from the open ocean. The delay times between the pulses of increased transport and ring separations can be shown to be significantly correlated with the length of the Loop Current. Mean sea levels over the Caribbean and Gulf also peak before the separations, on average.

Modelling the early evolution of a Loop Current ring

The Colorado University Princeton Ocean Model (CUPOM) is used here to study the early stages in the life of Millennium, a mesoscale anticyclonic ring that detached from the Loop Current on April 2001 and lasted for more than 100 days. The numerical near-surface velocity field for the Gulf of Mexico is validated with the altimetry geostrophic velocities. The first 30 days of numerical data, before Millennium interacts with other mesoscalar features, are closely examined both from Eulerian and Lagrangian perspectives. During this time Millennium had a near-constant rotation period of 6.5 days, and particles do not leave the ring. Nevertheless, the distributions of temperature, salinity, and angular velocity confirm the existence of significant (possibly numerical) radial diffusion. Polar-coordinate phase plots for temperature–salinity anomalies and tangential–radial velocities, at several depths, illustrate the presence of an evolving oscillating pattern. Radial and tangential velocities change in phase, associated with vertical displacements of the isothermal and isohaline surfaces. A simple diffusion model with an effective diffusion coefficient of 200 m 2 s − 1 is appropriate to grossly simulate the temporal evolution of angular velocity within Millennium.

Death of three Loop Current rings

Death of three Loop Current rings

Climatology of Ocean Features in the Gulf of Mexico Using Satellite Remote Sensing Data

AbstractA climatology of various ocean features in the Gulf of Mexico (GOM) was developed using the combination of satellite remote sensing and in situ data that spanned periods as long as 32 years. Twelve separate statistics were created, some of which described characteristics of the Loop Current, while others are involved with warm core rings that separate from the Loop Current and cold core rings. These statistics examined the frequency with which the Loop Current was found in the GOM, the orientation of the Loop Current, the frequency of intrusion of Loop Current water onto the west Florida shelf and into the GOM common water region, ring separation period from the Loop Current, ring dissipation, ring speed, ring path, frequency of ring water in the western GOM, and the frequency of warm and cold core rings (WCRs and CCRs, respectively) in the GOM. The results indicate that CCRs were principally responsible for mass and heat redistribution in the eastern GOM (EGOM) and WCRs are responsible for mass and heat redistribution in the western GOM (WGOM). The average period for WCR separation from the Loop Current was 11 months and the range from 5 to 19 months. WCRs moved through the WGOM most often using the central path (i.e., their trajectory was found between 24° and 26°N latitude) and they decreased to about 55% of their initial size when they reach the western wall of the GOM. CCRs were most often found in the EGOM, and their frequency of occurrence in the EGOM surpassed that of WCRs anywhere in the GOM.