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The thin layers in the ocean are temporally-coherent aggregations of phytoplankton with high concentrations at small vertical scales, presenting important hotspots of ecological activity. Lidar could identify thin phytoplankton layers at a large spatial scale due to its capabilities of profile detection with a high efficiency. However, studies that linked thin layers to environmental factors are few, which limits our understanding of the layer formation mechanism. This paper investigates the characteristics and formation conditions of thin phytoplankton layers in the northern Gulf of Mexico using airborne lidar. The results depict that the chlorophyll concentration determines the formation probability of the phytoplankton layer. The layer is mainly formed at concentrations less than 6 mg m−3 and mostly distributed at 2 mg m−3. In addition, layer thicknesses were within 5 m and layer depths were mainly in the range of 10–15 m. Layer depths in the nearshore region were shallower than those in the offshore region. We conclude that the characteristics and formation conditions of the thin phytoplankton layers depend on the nutrients and light that are related to the seabed topography, turbidity, eddies and upwelling. The findings of this paper will enhance the understanding of layer formation mechanisms. [ABSTRACT FROM AUTHOR]

During the period of 2018–2022, there were six named Loop Current Eddy (LCE) shedding events in the central Gulf of Mexico (GoM). LCEs form when a large anticyclonic eddy (AE) separates from the main Loop Current (LC) and propagates westward. In doing so, each LCE traps and advects warmer, saltier waters with lower Chlorophyll-a (Chl-a) concentrations than the surrounding Gulf waters. This difference in water mass permits the study of the effectiveness of using Chl-a from satellite-derived ocean color to identify LCEs in the GoM. In this work, we apply an eddy-tracking algorithm to Chl-a to detect LCEs, which we have validated against the traditional sea surface height-(SSH) based eddy-tracking approach with three datasets. We apply a closed-contour eddy-tracking algorithm to the SSH of two model products (HYbrid Coordination Ocean Model; HYCOM and Nucleus for European Modelling of the Ocean; NEMO) and absolute dynamic topography (ADT) from altimetry, as well as satellite-derived Chl-a data to identify the six named LCEs from 2018 to 2022. We find that Chl-a best characterizes LCEs in the summertime due to a basin-wide increase in the horizontal gradient of Chl-a, which permits a more clearly defined eddy edge. This study demonstrates that Chl-a can be effectively used to identify and track LC and LCEs in the GoM, serving as a promising source of information for regional data assimilative models. [ABSTRACT FROM AUTHOR]

The roles of the Loop Current (LC) and associated eddies in driving the circulation of the Gulf of Mexico (GoM) have been investigated for several decades from different perspectives. Nevertheless, a clear understanding of the relative contributions of the wind forcing and the Loop Current eddies (LCEs) to the GoM circulation and variability remain lacking. In this study, the roles of these two factors in sustaining the less well-known western GoM upper-layer (~1000 m) circulation are investigated with two numerical experiments using the HYbrid Coordinate Ocean Model (HYCOM). First, we examine the relative contributions of the wind and LCEs in setting the mean circulation in the western GOM. We then perform a vorticity balance to analyze the relative importance of the physical processes, including the wind stress, involved in sustaining the western GoM circulation. The results show that the wind stress contributes to a mean anticyclonic circulation in the central and northwestern Gulf, while in the southwestern subregion both wind and LCEs combine to induce a cyclonic circulation, highlighting the role of wind stress curl and topographic confinement. The vorticity balance analysis conducted in the upper layer of the western basin shows that planetary vorticity and stretching are primarily responsible for the balance in time scales longer than weeks, and their covariability are good indicators of LCEs entering the central and northwestern subregions. However, the southwestern subregion is primarily driven by vortex stretching. Mean advection of vorticity and planetary vorticity are also contributors to the time-averaged vorticity field. Since the wind stress is distributed over the upper layer of the GoM, direct input of vorticity in the regional vorticity balance is negligible, but it does play a role through the vortex stretching term. The results also suggest that wind forcing acts to produce larger, faster moving, and longer-lived anticyclonic eddies that impact the western Gulf and modulate the circulation over monthly timescales. [ABSTRACT FROM AUTHOR]

The 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. Plain Language Summary: Ocean eddies can be long‐lived and carry large amounts of heat and salt across ocean basins and marginal seas. This is the case of Loop Current rings (LCRs), which are large warm‐core eddies drifting through the Gulf of Mexico (GoM). Understanding how these eddies lose their energy is key to understand their longevity and transport properties. Here, we use a previously validated empirical method based on in situ observations to estimate the time evolution of LCRs energy using satellite observations. We show that LCRs decay continuously during their life cycle, contrary to the previously accepted idea that they decay when collapsing against the western GoM's continental shelf. LCRs typically lose three‐quarter of their energy before reaching any topographic obstacle. Using wind observations, we also show that wind‐current interactions are key to the energy loss of these eddies. Key Points: Time evolution of the energy of warm‐core rings in the Gulf of Mexico is assessed using empirical methods and satellite altimetryThe vast majority of mechanical energy (kinetic plus available potential) is lost early in the eddies' life cycles, far from the western boundaryWind‐current feed back effects play an important role in energy conversion and decay [ABSTRACT FROM AUTHOR]

In contrast to the Loop Current and rings, much less is known about deep eddies (deeper than 1000 m) of the Gulf of Mexico. In this paper, results from a high-resolution numerical model of the Gulf are analyzed to explain their origin and how they excite topographic Rossby waves (TRWs) that disperse energy to the northern slopes of the Gulf. It is shown that north of Campeche Bank is a fertile ground for the growth of deep cyclones by baroclinic instability of the Loop Current. The cyclones have horizontal (vertical) scales of about 100 km (1000∼2000 m) and swirl speeds ∼0.3 m s-1. The subsequent development of these cyclones consists of two modes, A and B. Mode-A cyclones evolve into the relatively well-known frontal eddies that propagate around the Loop Current. Mode-A cyclone can amplify off the west Florida slope and cause the Loop Current to develop a “neck” that sometimes leads to shedding of a ring; this process is shown to be the Loop Current’s dominant mode of upper-to-deep variability. Mode-B cyclones are “shed” and propagate west-northwestward at speeds of about 2–6 km day-1, often in concert with an expanding loop or a migrating ring. TRWs are produced through wave–eddy coupling originating primarily from the cyclone birthplace as well as from the mode-B cyclones, and second, but for longer periods of 20∼30 days only, also from the mode-A frontal eddies. The waves are “channeled” onto the northern slope by a deep ridge located over the lower slope. For very short periods (≲10 days), the forcing is a short distance to the south, which suggests that the TRWs are locally forced by features that have intruded upslope and that most likely have accompanied the Loop Current or a ring. [ABSTRACT FROM AUTHOR]

Observations suggest the hypothesis that deep eddy kinetic energy (EKE) in the Gulf of Mexico can be accounted for by topographic Rossby waves (TRWs). It is presumed that the TRWs are forced by Loop Current (LC) pulsation, Loop Current eddy (LCE) shedding, and perhaps also by LCE itself. Although the hypothesis is supported by model results, such as those presented in Oey, the existence of TRWs in the model and how they can be forced by larger-scale LC and LCEs with longer-period vacillations have not been clarified. In this paper, results from a 10-yr simulation of LC and LCEs, with double the resolution of that used by Oey, are analyzed to isolate the TRWs. It is shown that along an east-to-west band across the gulf, approximately over the 3000-m isobath, significant EKE that accounts for over one-half of the total spectrum is contained in the 20-100-day periods. Bottom energy intensification exists in this east-west band with vertical decay scales of about 600-300 m decreasing westward. The decrease agrees with the TRW solution. The band is also located within the region where TRWs can be supported by the topographic slope and stratification used in the model and where wavenumber and frequency estimates are consistent with the TRW dispersion relation. The analysis indicates significant correlation between pairs of east-west stations, over distances of approximately 400 km. Contours of lag times suggest offshore (i.e., downslope) phase propagation, and thus the east-west band indicates nearly parabathic and upslope energy propagation. Ray tracing utilizing the TRW dispersion relation and with and without (for periods >43 days) ambient deep currents shows that TRW energy paths coincide with the above east-west high-energy band. It also explains that the band is a result of TRW refraction by an escarpment (with increased topographic gradient) across the central gulf north of the 3000-m isobath, and also by deep current and its cyclonic shear, and that ray... [ABSTRACT FROM AUTHOR]

Consistent with satellite‐tracked trajectories of drogued drifters, but at odds with Eulerian assessment of satellite‐altimetry measurements of sea‐surface height, we show that North Brazil Currents Rings (NBCRs) are incapable of bypassing the Lesser Antilles as structures that coherently transport material. We arrive at this conclusion by applying geodesic eddy detection on the altimetric data set over nearly its entire extent. While we detect northwestward translating NBCRs that can be classified as coherent Lagrangian eddies, they typically experience strong filamentation and complete loss of coherence prior to reaching the Lesser Antilles. Moreover, the filamented material hardly penetrates into the Caribbean Sea, let alone the Gulf of Mexico, and not without substantively mixing with the ambient fluid east of the archipelago. The nature of the inability of the de‐facto oceanographic Eulerian, streamline‐based eddy detection technique to produce a correct assessment of the connectivity between the tropical Atlantic and the Gulf of Mexico is rooted in its lack of objectivity. Plain Language Summary: Geodesic eddy detection is a Lagrangian (fluid‐parcel‐following) method that objectively, that is, in an observer‐independent manner, reveals eddies (fluid regions spinning about a common axis) with material (fluid) boundaries that defy the exponential stretching that arbitrary material loops typically experience in turbulent flow. Eddies instantaneously revealed as regions enclosed by streamlines in general do not have such a property: the topology of the streamlines depends on the observer viewpoint. Thus instantaneously closed streamlines cannot be guaranteed to hold and carry within material. Here we use geodesic eddy detection to demonstrate that North Brazil Current rings detected from their Eulerian footprints in the altimetric sea‐surface height field do not connect the tropical Atlantic Ocean and the Gulf of Mexico. Key Points: The topology of sea‐surface height (SSH) streamlines is observer dependent and hence are eddies from the AVISO+ META databaseGeodesic eddy detection objectively reveals eddies with stretching resisting material boundaries that also minimize diffusionNorth Brazil Current Ring contents hardly penetrate into the Caribbean without mixing with the ambient fluid east of the Lesser Antilles [ABSTRACT FROM AUTHOR]

It is first shown that wind in the Gulf of Mexico can delay the shedding of Loop Current eddies. A time-dependent, three-dimensional numerical experiment forced by a spatially and temporally constant westward wind stress within the Gulf is analyzed and then is compared with an otherwise identical no-wind run, and the result is confirmed with reduced-gravity experiments. It is shown that the wind produces westward transports over the northern and southern shelves of the Gulf, convergence in the west, and a returned (i.e., eastward) upper-layer flow over the deep central basin toward the Loop Current. The theory from T. Pichevin and D. Nof is then used to explain that the returned flow constitutes a zonal momentum flux that delays eddy shedding. Mass-balance analysis shows that wind also forces larger Loop Current and rings (because the delayed shedding allows more mass to be accumulated in them) and produces more efficient mass exchange between the Gulf and the Caribbean Sea. It is shown that eddies alone (without wind stress curl) can force a boundary current and downward flow in the western Gulf and a corresponding deep flow from the western Gulf to the eastern Gulf. [ABSTRACT FROM AUTHOR]

Sixteen months of observations from a surface-to-bottom mooring in the central Gulf of Mexico show that acoustic Doppler current profilers (ADCPs) are useful for directly measuring the vertical velocity within mesoscale anticyclonic eddies, such as those shed from the Loop Current; and combining simultaneous temperature measurements, vertical heat flux can also be estimated (as a covariance of both variables). There is evidence of significant and coherent signals of vertical velocity ∼2–3 mm s-1 and vertical heat (temperature) transport ∼10-3 °C m s-1 during the presence of three anticyclones. A simple analysis shows downward flow near the eddies’ centers above 350 m and essentially upward flow in the peripheries, but below 700-m depth the pattern is indeed the opposite; however, further study is necessary to determine the eddies’ interior structures. The observations also suggest the existence of a vertical convergence of heat somewhere around 600-m depth, and estimations of adiabatic heat flux suggest that part of the converged heat, which is not recirculated within the eddy, must escape from the eddy and flow upward along the isopycnals up to the surface layers. This is in good agreement with previous results that have suggested that an excess heat gained by the Gulf in the intermediate levels through exchanges with the Caribbean Sea must be exported to the upper layers by an upward mean heat flux. [ABSTRACT FROM AUTHOR]

The Gulf of Mexico circulation is modulated by a mesoscale current, the Loop Current (LC), and large anticyclonic eddies that detach from it. The LC dynamics are recurrent, and its evolution is in and from a few preferential states. This observation points to the existence of a low‐dimensional dynamical attractor. Building upon advancements in dynamical system theory, this work characterizes the average and instantaneous dimensions of such an attractor. The instantaneous dimension and its evolution in time are compared among an altimeter‐based data set, an ocean reanalysis and an operational hindcast. The LC complexity, measured by its dimension, differs among them, especially when the dimension is high. During shedding events, differences between datasets emerge in the second principal component. The information provided by this analysis is relevant to operational ocean forecasts and points to where improvement should occur. Plain Language Summary: We characterize the evolution of the large‐scale circulation of the Gulf of Mexico, which is dominated by the Loop Current, using a metric from dynamical system theory. Comparing results from an altimetry‐based sea surface height data set and model‐based products, it is found that the model misrepresents in part the set of states toward which a system tends to evolve. This is the case, especially when the Loop Current is in an intermediate configuration, not fully retracted toward Cuba or extended toward the Florida Panhandle. This analysis points to aspects that models should improve before ocean forecasts for the region can be extended successfully beyond 5 days. The forecast extension, which is theoretically supported by our work, would bring societal benefits to people working in the oil and fishery industry and those living on the coastlines of a basin often traversed by tropical storms and hurricanes. Key Points: A metric from dynamical system theory is applied to the Gulf of Mexico to characterize the attractor of the Loop CurrentModel sea surface height products (reanalysis and hindcast) portray a different local dimension than Gulf of Mexico Coastal Ocean Observing System (GCOOS) dataResults point to where improvement is needed for better ocean forecast skills [ABSTRACT FROM AUTHOR]

The Florida Current (FC) flows in the Straits of Florida (SoF) and connects the Loop Current in the Gulf of Mexico to the Gulf Stream (GS) in the western Atlantic Ocean. Its journey through the SoF is at time characterized by the formation and presence of mesoscale but mostly submesoscale frontal eddies on the cyclonic side of the current. The formation of those frontal eddies was investigated in a very high-resolution two-way nested simulation using the Regional Oceanic Modeling System (ROMS). Frontal eddies were either locally formed or originated from outside the SoF. The northern front of the incoming eddies was susceptible to superinertial shear instability over the shelf slope when the eddies were pushed up against the slope by the FC. Otherwise, incoming eddies could be advected, relatively unaffected by the current, when in the southern part of the straits. In the absence of incoming eddies, submesoscale eddies were locally formed by the roll-up of superinertial barotropically unstable vorticity filaments when the FC was pushed up against the shelf slope. The vorticity filaments were intensified by the friction-induced bottom-layer vorticity flux as previously demonstrated by Gula et al. in the GS. When the FC retreated farther south, negative-vorticity west Florida shelf waters overflowed into the SOF and led to the formation of submesoscale eddies by baroclinic instability. The instability regimes, that is, the submesoscale frontal eddies formation, appear to be controlled by the lateral "sloshing" of the FC in the SoF. [ABSTRACT FROM AUTHOR]

A previous study by the author concluded that either there were errors in the satellite results or that some long-term means were not in geostrophic balance. Ship-drift results are in good agreement with surface drifters, but these two do not agree with satellite sea surface heights (SSH). The agreement between the first two suggested the possibility that there could be errors in the SSH or that the mean surface flow is not in geostrophic balance. The present results, using the addition of a fourth long-term mean from hydrographic data, which agrees with the SSH, resolves the issue. The lack of agreement between different long-term means is from inadequate coverage in space and time in data from ship drifts and drifters. [ABSTRACT FROM AUTHOR]

A method for objectively extracting the displacement signals associated with coherent eddies from Lagrangian trajectories is presented, refined, and applied to a large dataset of 3770 surface drifters from the Gulf of Mexico. The method, wavelet ridge analysis, is a general method for the analysis of modulated oscillations, here modified to be more suitable to the eddy-detection problem. A means for formally assessing statistical significance is introduced, addressing the issue of false positives arising by chance from an unstructured turbulent background and opening the door to confident application of the method to very large datasets. Significance is measured through a frequency-dependent comparison with a stochastic dataset having statistical and spectral properties that match the original, but lacking organized oscillations due to eddies or waves. The application to the Gulf of Mexico reveals major asymmetries between cyclones and anticyclones, with anticyclones dominating at radii larger than about 50 km, but an unexpectedly rich population of highly nonlinear cyclones dominating at smaller radii. Both the method and the Gulf of Mexico eddy dataset are made freely available to the community for noncommercial use in future research. [ABSTRACT FROM AUTHOR]

The Loop Current (LC) system has long been assumed to be close to geostrophic balance despite its strong flow and the development of large meanders and strong frontal eddies during unstable phases. The region between the LC meanders and its frontal eddies was shown to have high Rossby numbers indicating nonlinearity; however, the effect of the nonlinear term on the flow has not been studied so far. In this study, the ageostrophy of the LC meanders is assessed using a high-resolution numerical model and geostrophic velocities from altimetry. A formula to compute the radius of curvature of the flow from the velocity field is also presented. The results indicate that during strong meandering, especially before and during LC shedding and in the presence of frontal eddies, the centrifugal force becomes as important as the Coriolis force and the pressure gradient force: LC meanders are in gradient-wind balance. The centrifugal force modulates the balance and modifies the flow speed, resulting in a subgeostrophic flow in the LC meander trough around the LC frontal eddies and supergeostrophic flow in the LC meander crest. The same pattern is found when correcting the geostrophic velocities from altimetry to account for the centrifugal force. The ageostrophic percentage in the cyclonic and anticyclonic meanders is 47% ± 1% and 78% ± 8% in the model and 31% ± 3% and 78% ± 29% in the altimetry dataset, respectively. Thus, the ageostrophic velocity is an important component of the LC flow and cannot be neglected when studying the LC system. [ABSTRACT FROM AUTHOR]

Numerical circulation modeling and observational studies have been conducted to understand the Loop Current (LC) system behaviors in the Gulf of Mexico (GoM). One of the factors that may influence the LC are upstream eddies from within the Caribbean Sea. By combining satellite altimetry, sea surface salinity and ocean color data, we demonstrate that mesoscale eddies from the western tropical Atlantic Ocean can eventually make their way to the Gulf of Mexico and likely affect the LC. In addition, our study shows that freshwater of Amazon and Orinoco River origin trapped within mesoscale eddies can also enter the GoM, potentially affecting the GoM stratification. This study provides insights into understanding variations of the LC system and showcases the roles of mesoscale eddies in connecting the open ocean and regional seas. Plain Language Summary: The Loop Current (LC) is the dominant large‐scale oceanic process in the Gulf of Mexico (GoM). However, the mechanism for variations of the LC system is still unsolved. Here, we show that some mesoscale eddies originated in the tropical Atlantic Ocean can pass through the Caribbean Sea and eventually enter the GoM. These remotely generated eddies could be an important upstream factor affecting the behavior of the LC. Also, freshwater and other materials (e.g., chlorophyll) trapped in the eddies could reach the GoM as well. In addition to advancing the understanding of the LC system, this study provides an explicit example showing eddies can serve as a route connecting regional seas and the open ocean. Key Points: Some eddies from the Atlantic Ocean can ultimately reach the Gulf of Mexico and likely affect the Loop CurrentFreshwater of Amazon and Orinoco River origin and other materials trapped in eddies could reach the Gulf of MexicoWeakening and strengthening of the long‐propagating eddies are mostly related to the variation of bathymetry [ABSTRACT FROM AUTHOR]

Turbulence parameters have been estimated from microstructure velocity and temperature measurements in an anticyclonic Loop Current Eddy in the Gulf of Mexico for the very first time. Measurements were performed during a mission in October 2016, using a G2 Webb Research Slocum ocean glider instrumented with a microstructure sensors suite. Well‐defined turbulent kinetic energy dissipation rate patterns in the eddy and the surface mixed layer were clearly identified from both the velocity and temperature microstructure data. Higher dissipation rates were observed at the flanks and beneath the eddy core (up to 10−7 W/kg), while the core interior appeared to be very quiescent (~10−11 W/kg). Double diffusive thermohaline intrusions were detected on the side of the eddy, while the region below the eddy core was more conducive for salt fingering. Heat and salt fluxes associated with these processes suggest that a life span of such an eddy is ~1.5 years. Plain Language Summary: Mixing intensity in the ocean is often observed using microstructure measurements that target small‐scale processes such as microvelocity shear and temperature fluctuations. This study targets a ubiquitous and important feature of the Gulf of Mexico, namely, Anticyclonic Loop Current Eddies. Here we report, for the first time, on physical processes in such an eddy surveyed with a state‐of‐the‐art microstructure sensors suite mounted on an autonomous ocean glider. Key Points: Temperature microstructure data allow reliable estimation of very low turbulent kinetic energy dissipation ratesGlider‐based microstructure measurements were conducted for the first time in a large mesoscale anticyclonic eddyDouble diffusion processes associated with eddy dynamics were observed with microstructure and CTD data [ABSTRACT FROM AUTHOR]

The evaluation of the ocean energy balance is crucial for improving the fundamental understanding of the mechanisms sustaining ocean circulation. Based on the outputs of the ROMS ocean model, the energy cycle, eddy-mean flow interactions, and energy pathways of the deep Gulf of Mexico (GoM) have been investigated in this study. The theoretical framework for the analysis is based on the energy equations for the time-mean and time-varying flow, where some of the terms were split into their horizontal and vertical components to monitor the energy pathways. Of the energy maintaining deep kinetic energy (KE), approximately 75% is transferred from the upper layer to the deep layer by vertical pressure work (PW), about6%by the horizontal PW through the Yucatan and Florida straits, and ;19% is generated through the processes related to baroclinic instabilities. The mean circulation generates eddies in the upper layer, while eddies drive mean circulation in the deep layer. Energy is transferred downward in the eastern and western part of the Gulf, upward in the deep western-central part, and a strong westward energy transport can be observed below 2000-m depth. [ABSTRACT FROM AUTHOR]

A recently developed tool, the multiscale window transform, along with the theory of canonical energy transfer is used to investigate the roles of multiscale interactions and instabilities in the Gulf of Mexico Loop Current (LC) eddy shedding. A three-scale energetics framework is employed, in which the LC system is reconstructed onto a background flow window, a mesoscale eddy window, and a high-frequency eddy window. The canonical energy transfer between the background flow and the mesoscale windows plays an important role in LC eddy shedding. Barotropic instability contributes to the generation/intensification of the mesoscale eddies over the eastern continental slope of the Campeche Bank. Baroclinic instability favors the growth of the mesoscale eddies that propagate downstream to the northeastern portion of the well-extended LC, eventually causing the shedding by cutting through the neck of the LC. These upper-layer mesoscale eddies lose their kinetic energy back to the background LC through inverse cascade processes in the neck region. The deep eddies obtain energy primarily from the upper layer through vertical pressure work and secondarily from baroclinic instability in the deep layer. In contrast, the canonical energy transfer between the mesoscale and the high-frequency frontal eddy windows accounts for only a small fraction in the mesoscale eddy energy balance, and this generally acts as a damping mechanism for the mesoscale eddies. A budget analysis reveals that the mesoscale eddy energy gained through the instabilities is balanced by horizontal advection, pressure work, and dissipation. [ABSTRACT FROM AUTHOR]

Three simulations of the circulation in the Gulf of Mexico (the "Gulf") using different numerical general circulation models are compared with results of recent large-scale observational campaigns conducted throughout the deep (>1500 m) Gulf. Analyses of these observations have provided new understanding of large-scale mean circulation features and variability throughout the deep Gulf. Important features include cyclonic flow along the continental slope, deep cyclonic circulation in the western Gulf, a counterrotating pair of cells under the Loop Current region, and a cyclonic cell to the south of this pair. These dominant circulation features are represented in each of the ocean model simulations, although with some obvious differences. A striking difference between all the models and the observations is that the simulated deep eddy kinetic energy under the Loop Current region is generally less than one-half of that computed from observations. A multidecadal integration of one of these numerical simulations is used to evaluate the uncertainty of estimates of velocity statistics in the deep Gulf computed from limited-length (4 years) observational or model records. This analysis shows that the main deep circulation features identified from the observational studies appear to be robust and are not substantially impacted by variability on time scales longer than the observational records. Differences in strengths and structures of the circulation features are identified, however, and quantified through standard error analysis of the statistical estimates using the model solutions. [ABSTRACT FROM AUTHOR]

We analyze high-resolution (1 km) simulations of the western Pacific, Gulf of Mexico, and Arabian Sea to understand submesoscale eddy dynamics. A mask based on the Okubo–Weiss parameter isolates small-scale eddies, and we further classify those with |ζ/f| ≥ 1 as being submesoscale eddies. Cyclonic submesoscale eddies exhibit a vertical depth structure in which temperature anomalies from the large-scale background are negative. Peak density anomalies associated with cyclonic submesoscale eddies are found at a depth approximately twice the mixed layer depth (MLD). Within anticyclonic submesoscale eddies, temperature anomalies are positive and have peak density anomalies at the MLD. The depth–depth covariance structure for the cyclonic and anticyclonic submesoscale eddies have maxima over a shallow region near the surface and weak off diagonal elements. The observed vertical structure suggests that submesoscale eddies have a shallower depth profile and smaller vertical correlation scales when compared to the mesoscale phenomenon. We test a two-dimensional submesoscale eddy dynamical balance. Compared to a geostrophic dynamical balance using only pressure gradient and Coriolis force, including velocity tendency and advection produces lower errors by about 20%. In regions with strong tides and associated internal waves (western Pacific and Arabian Sea), using the mixed layer integrated small-scale steric height within the dynamical equations produces the lowest magnitude errors. In areas with weak tides (Gulf of Mexico), using small-scale sea surface height (SSH) produces the lowest magnitude errors. Recovering a submesoscale eddy with the correct magnitude and rotation requires integration of small-scale specific volume anomalies well below the mixed layer. [ABSTRACT FROM AUTHOR]

Two glider transects in the Gulf of Mexico reveal fine-vertical-scale thermohaline structures within a Loop Current eddy (LCE). Partially compensating temperature and salinity anomalies are shown to organize as thin layers below the eddy and near its edges. The anomalies have vertical scales ranging from 2 to 60m and extend laterally over distances up to 120 km. These structures are evident in synthetic acoustic reflectivity derived from the glider data and are reminiscent of the intense layering observed in seismic imagery around meddies, Agulhas rings, and warm-core Kuroshio rings. The observed layers are aligned with the geostrophic streamfunction rather than isopycnals and develop preferentially in zones of intense vertical shear. These observations suggest that tracer stirring by the eddy's vertically sheared azimuthal flow might be an important process for their generation. In an attempt to rationalize this process, high-resolution quasigeostrophic simulations were performed using an idealized anticyclonic ring for the initial conditions. As the vortex destabilizes, layering rapidly develops in the model, resulting in structures similar to those found in the observation data. Passive tracer experiments also suggest that the layers form through differential advection of the tracer field by the vertically sheared flow associated with the LCE. [ABSTRACT FROM AUTHOR]

The emergence of coherent Lagrangian swirls (CLSs) among submesoscale motions in the ocean is illustrated. This is done by applying recent nonlinear dynamics tools for Lagrangian coherence detection on a surface flow realization produced by a dataassimilative submesoscale-permitting ocean general circulation model simulation of the Gulf of Mexico. Both mesoscale and submesoscale CLSs are extracted. These extractions prove the relevance of coherent Lagrangian eddies detected in satellitealtimetry- based geostrophic flow data for the arguably more realistic ageostrophic multiscale flow. [ABSTRACT FROM AUTHOR]

Fourteen autonomous profiling floats, equipped with CTDs, were deployed in the deep eastern and western basins of the Gulf of Mexico over a four-year interval (July 2011-August 2015), producing a total of 706 casts. This is the first time since the early 1970s that there has been a comprehensive survey of water masses in the deep basins of the Gulf, with better vertical resolution than available from older ship-based surveys. Seven floats had 14-day cycles with parking depths of 1500 m, and the other half from the U.S. Argo program had varying cycle times. Maps of characteristic water masses, including Subtropical Underwater, Antarctic Intermediate Water (AAIW), and North Atlantic Deep Water, showed gradients from east to west, consistent with their sources being within the Loop Current (LC) and the Yucatan Channel waters. Altimeter SSH was used to characterize profiles being in LC or LC eddy water or in cold eddies. The two-layer nature of the deep Gulf shows isotherms being deeper in the warm anticyclonic LC and LC eddies and shallower in the cold cyclones. Mixed layer depths have an average seasonal signal that shows maximum depths (~60 m) in January and a minimum in June-July (~20 m). Basin-mean steric heights from 0-50-m dynamic heights and altimeter SSH show a seasonal range of ~12 cm, with significant interannual variability. The translation of LC eddies across the western basin produces a region of low homogeneous potential vorticity centered over the deepest part of the western basin. [ABSTRACT FROM AUTHOR]

Summer ichthyoplankton surveys were conducted in the northern Gulf of Mexico from 2007 to 2010 to characterize the distribution and abundance of tuna larvae. Larval assemblages of tunas were comprised of four genera: Thunnus, Auxis, Euthynnus, and Katsuwonus. Thunnus were the most abundant and four species were detected; T. atlanticus [blackfin tuna], T. obesus [bigeye tuna], T. albacares [yellowfin tuna], and T. thynnus [bluefin tuna]. Intra- and inter-annual variability in the distribution and abundance of Thunnus species were observed with higher densities in 2008 and 2009, with a decline in abundance observed in 2010. Distribution and abundance of Thunnus larvae were influenced by physical and chemical conditions of the water mass, notably sea surface temperature and salinity. Distinct species-specific habitat preferences were observed and the location of mesoscale oceanographic features influenced larval abundance with higher densities of T. atlanticus, T. obesus, and T. albacares near anticyclonic (warm core) regions and the Loop Current, while T. thynnus was observed in higher densities near cyclonic (cold core) regions. This study demonstrates that spatial and temporal variability in the location of mesoscale oceanographic features may be important to partitioning nursery habitat among Thunnus species. [ABSTRACT FROM AUTHOR]

Four numerical simulations are used to characterize the impact of submesoscale circulations on surface Lagrangian statistics in the northern Gulf of Mexico over 2 months, February and August, representative of winter and summer. The role of resolution and riverine forcing is explored focusing on surface waters in regions where the water column is deeper than 50 m. Whenever submesoscale circulations are present, the probability density functions (PDFs) of dynamical quantities such as vorticity and horizontal velocity divergence for Eulerian and Lagrangian fields differ, with particles preferentially mapping areas of elevated negative divergence and positive vorticity. The stronger the submesoscale circulations are, the more skewed the Lagrangian distributions become, with greater differences between Eulerian and Lagrangian PDFs. In winter, Lagrangian distributions are modestly impacted by the presence of the riverine outflow, while increasing the model resolution from submesoscale permitting to submesoscale resolving has a more profound impact. In summer, the presence of riverine-induced buoyancy gradients is the key to the development of submesoscale circulations and different Eulerian and Lagrangian PDFs. Finite-size Lyapunov exponents (FSLEs) are used to characterize lateral mixing rates. Whenever submesoscale circulations are resolved and riverine outflow is included, FSLEs slopes are broadly consistent with local stirring. Simulated slopes are close to −0.5 and support a velocity field where the ageostrophic and frontogenetic components contribute stirring at scales between about 5 and 7 times the model resolution and 100 km. The robustness of Lagrangian statistics is further discussed in terms of their spatial and temporal variability and of the number of particles available. [ABSTRACT FROM AUTHOR]

Velocity data from a mooring array deployed northeast of the Campeche Bank (CB) show the presence of subinertial, high-frequency (below 15 days) velocity fluctuations within the core of the northward flowing Loop Current. These fluctuations are associated with the presence of surface-intensified Loop Current frontal eddies (LCFEs), with cyclonic vorticity and diameter < 100 km. These eddies are well reproduced by a high-resolution numerical simulation of the Gulf of Mexico, and the model analysis suggests that they originate along and north of the CB, their main energy source being the mixed baroclinic-barotropic instability of the northward flow along the shelf break. There is no indication that these high-frequency LCFEs contribute to the LC eddy detachment in contrast to the low-frequency LCFEs (periods > 30 days) that have been linked to Caribbean eddies and the LC separation process. Model results show that wind variability associated with winter cold surges are responsible for the emergence of high-frequency LCFEs in a narrow band of periods (6-10 day) in the region of the CB. The dynamical link between the formation of these LCFEs and the wind variability is not direct: (i) the large-scale wind perturbations generate sea level anomalies on the CB as well as first baroclinic mode, coastally trapped waves in the western Gulf of Mexico; (ii) these waves propagate cyclonically along the coast; and (iii) the interaction of these anomalies with the Loop Current triggers cyclonic vorticity perturbations that grow in intensity as they propagate downstream and develop into cyclonic eddies when they flow north of the Yucatan shelf. [ABSTRACT FROM AUTHOR]

A 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. [ABSTRACT FROM AUTHOR]

Circulation 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Although the upper-layer dynamics of the Loop Current and eddies in the Gulf of Mexico are well studied, the understanding of how they are coupled to the deep flows is limited. In this work, results from a numerical model are analyzed to classify the expansion, shedding, retraction, and deep-coupling cycle (the Loop Current cycle) according to the vertical mass flux across the base of the Loop. Stage A is the ''Loop reforming'' period, with downward flux and deep divergence under the Loop Current. Stage B is the ''incipient shedding,'' with strong upward flux and deep convergence. Stage C is the ''eddy migration,'' with waning upward flux and deep throughflow from the western Gulf into the Yucatan Channel. Because of the strong deep coupling between the eastern and western Gulf, the Loop''s expansion is poorly correlated with deep flows through the Yucatan Channel. Stage A is longest and the mean vertical flux under the Loop Current is downward. Therefore, because the net circulation around the abyssal basin is zero, the abyssal gyre in the western Gulf is cyclonic. The gyre''s strength is strongest when the Loop Current is reforming and weakest after an eddy is shed. The result suggests that the Loop Current cycle can force a low-frequency [[time scales ∼∼ shedding periods; O(months)]] abyssal oscillation in the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

The Gulf of Mexico (GOM) receives heat from the Caribbean Sea via the Yucatan--Loop Current (LC) system, and the corresponding ocean heat content (OHC) is important to weather and climate of the continental United States. However, the mechanisms that affect this heat influx and how it is distributed in the Gulf have not been studied. Using the Princeton Ocean Model, the authors show that a steady, uniform westward wind in the Gulf increases (∼∼100 KJ cm−−2) the upper OHC (temperature T > 18°°C) of the Gulf. This is because wind increases the water exchange between the Gulf and the Caribbean Sea, and the heat input into the Gulf is also increased, by about 50 TW. The westward heat transport to the western Gulf is ∼∼30 TW, and a substantial portion of this is due to wind-induced shelf currents, which converge to produce downwelling near the western coast. Finally, eddies are effective transporters of heat across the central Gulf. Wind forces larger LC and rings with deeper isotherms. This and downfront-wind mixing on the southern side of anticyclonic rings, northward spread of near-zero potential vorticity waters, and downwelling on the northern shelf break result in wide and deep eddies that transport large OHCs across the Gulf. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) Katrina and Rita moved as major hurricanes over energetic geostrophic ocean features in the Gulf of Mexico. Increased and reduced oceanic mixed layer (OML) cooling was measured following the passage of both storms over cyclonic and anticyclonic geostrophic relative vorticity ζg, respectively. This contrasting thermal response is investigated here in terms of the evolution of the storms’ near-inertial wave wake in geostrophic eddies. Observational data and ray-tracing techniques in realistic geostrophic flow indicate that TC-forced OML near-inertial waves are trapped in regions of negative ζg, where they rapidly propagate into the thermocline. These anticyclonic-rotating regimes coincided with the distribution of reduced OML cooling because rapid downward dispersion of near-inertial energy reduced the amount of kinetic energy available to increase vertical shears at the OML base. By contrast, TC-forced OML near-inertial waves were stalled in upper layers of cyclonic circulations, which strengthened vertical shears and entrainment cooling. Upgoing near-inertial energy propagation dominated inside a geostrophic cyclone that interacted with Katrina; the salient characteristics of these upward-propagating waves were the following: (i) they were radiated from the ocean interior because of geostrophic adjustment following upwelling–downwelling processes; (ii) rather than with the buoyancy frequency, they amplified horizontally as they encountered increasing values of f + ζg/2 during upward propagation; and (iii) they produced episodic vertical mixing through shear instability at a critical layer underneath the OML. To improve the prediction of TC-induced OML cooling, models must capture geostrophic features and turbulence closures must represent near-inertial wave processes such as dispersion and breaking between the OML base and the thermocline. [ABSTRACT FROM AUTHOR]

Recent measurements over the sill in the Yucatan Channel indicate that the deepest flows between the Caribbean Sea and the Gulf of Mexico, those that take place below the sill level at the Florida Straits, have zero mean net mass transport but carry significant amounts of heat and oxygen. The heat flux associated with the mean exchange exports approximately 150 GW from the deep Gulf toward the Caribbean and may be related to the formation of the Yucatan Undercurrent. The eddy heat transfer is also significantly different from zero and exports on average an additional 60 GW. This eddy transfer is attributable mostly to events that last from a few days to about 1.5 months, during which colder water from deeper levels in the Caribbean (beneath 2000 m) flows over the sill within a bottom boundary layer close to 200 m thick. The colder water is also very rich in oxygen, and the deep exchange sustains the near-bottom oxygen maximum in the Gulf of Mexico, whence that cold water must slide down the northern slope of the Yucatan Sill. Estimates of oxygen transport by diffusion from the deep water into the overlying intermediate water (∼50 m3 s-1) and the oxygen consumption reported in the literature (∼100 m3 s-1) are balanced by the rates of mean and eddy transfers over the sill (∼150 m3 s-1). The near-bottom mass transport [∼0.32 Sv (1 Sv ≡ 106 m3 s-1)] measured across the deepest portion of the central Yucatan Channel suggests a residence time for the deep waters of the Gulf of about 250 yr. [ABSTRACT FROM AUTHOR]

A two-layer, intermediate equations model that, uniquely, allows for the intersection of the bathymetry with the layer interface is used to study the interaction of isolated Loop Current Eddy (LCE)-type anticyclones with western-boundary topography. Two idealized topography configurations representative of the Gulf of Mexico (GoM) coastal topography at 25° and 23°N are studied; the 25°N topography configuration is characterized by a relatively wide shelf and a narrow continental slope, and the 23°N configuration is characterized by a relatively narrow shelf and a wide continental slope. The physical mechanism that has the most significant effect on the evolution of the LCE in both topographic configurations is the interaction of the LCE with cyclones formed directly to its north through the process of off-shelf advection of potential vorticity in the upper layer. The LCE interaction with those cyclones that are generated through this mechanism results in the LCE becoming elliptic and rotating clockwise with its center following a cyclic trajectory. The amplitude of the cyclic motion produced by LCE interactions with cyclones is controlled by a different physical mechanism. The mechanism consists of the LCE interacting with deep eddies that can be generated beneath the LCE over regions of flat topography adjacent to the continental slope. The deep eddies are generated by stretching and compression of the lower layer by a rotating elliptic LCE. The net effect of these eddies is to amplify significantly the cyclic motion of the LCE. The width of the continental slope is the critical parameter controlling the strength of the LCE interaction with deep eddies and, therefore, the amplitude of the cyclic motion. The characteristic pattern of LCE evolution seen in the numerical experiments can be identified in some observed cases of LCE interaction with the western boundary in the GoM. [ABSTRACT FROM AUTHOR]

The species composition, distribution, and abundance of the hyperiid amphipods collected in March 1993 across a Gulf of Mexico cold-core ring (CCR) were analyzed. Day and night samples were collected by oblique tows (100 m to surface) with a plankton net. Hyperiids were represented by 56 species, 21 of which have not been recorded previously in gulf waters. The local oceanic community differs from that reported from adjacent neritic and neritic-oceanic areas of the Northwestern Tropical Atlantic. Overall, hyperiids were more abundant within (59% of total catch in org./1000 m3) than outside the CCR (41%). All inside CCR stations were sampled at night. Night outside vs. night inside CCR hyperiid faunas showed important differences in terms of species richness, composition and density. Cluster analysis indicated that day sta. 5 on the edge but outside the CCR was more similar to those stations inside the CCR (nighttime samples) than to the other daytime samples. Moreover, all the stations outside the CCR were clustered together independently of their day or night origin. It is suggested that the differences found were more related to differential conditions related to the CCR than to diel vertical migration. The analysis of three congeneric pairs with inverse CCR-related abundance and with known or inferred migratory patterns strengthened the idea that these pairs are probably separated by thermal preferences; also, their vertical migratory patterns seem to be abnormal inside the CCR. A relatively higher concentration of immature stages inside the CCR supports the idea that the enriched CCR waters constitute areas of increased production. Furthermore, this higher productivity enhances the chances of hyperiids to find their hosts, the gelatinous zooplankters, which are also more abundant inside the CCR. [ABSTRACT FROM AUTHOR]

The evolution and propagation of large anticyclonic eddies similar to the Loop Current eddies found in the Gulf of Mexico are studied in a two-layer intermediate equation numerical model. It is found that the propagation of these eddies is governed primarily by the β effect, lower-layer flow driven by the surface eddies, and the image effect. As the eddies move westward under the influence of the β effect, they drive deep circulation that typically includes a cyclone near the eddies' trailing edge. This cyclone accelerates the eddy southward, leading to a path substantially different from that found in reduced-gravity models of similar surface eddies. When the eddy encounters a continental slope, the lower-layer flow is dispersed by topographic Rossby waves, and the eddy loses its southward propagation component. If the continental slope is backed by a shelf that is wider than the eddy's radius, a deep anticyclone will form over the shelf as the eddy approaches the shelf break. This anticyclone will drive the surface eddy rapidly southward until it crosses the shelf break and encounters the boundary, where the eddy will turn northward under the influence of the image effect. Eddies initialized over a zonal slope, which cannot develop the deep circulation that accelerates the eddy southward because of topographic Rossby wave dispersion, propagate nearly zonally until they encounter the western boundary. Over a northern slope such as that found in the Gulf of Mexico, these eddies will retroflect and propagate westward under the influence of the image effect if the northwest corner shelf is narrow, but they will turn southward along the western boundary if the northwest corner shelf is wide. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

During August of the 2020 Atlantic Hurricane Season, the Gulf of Mexico (GoM) was affected by two subsequent storms, Hurricanes Marco and Laura. Hurricane Marco entered the GoM first (22 August) and was briefly promoted to a Category 1 storm. Hurricane Laura followed Marco closely (25 August) and attained Category 4 status after a period of rapid intensification. Typically, hurricanes do not form this close together; this study aims to explain the existence of both hurricanes through the analysis of air-sea fluxes, local thermodynamics, and upper-level circulation. The GoM and its quality of warm, high ocean heat content waters proved to be a resilient and powerful reservoir of heat and moisture fuel for both hurricanes; however, an area of lower ocean heat content due to circulation dynamics was crucial in the evolution of both Marco and Laura. An analysis of wind shear further explained the evolution of both hurricanes. Furthermore, a suite of satellite observations and ocean model outputs were used to evaluate the biophysical modulations in the GoM. The cold core eddy (CCE) and Mississippi River surface plume had the greatest biophysical oceanic responses; the oceanic modulations were initialized by Marco and extended temporally and spatially by Laura. Reduced sea surface temperatures (SST), changes in sea surface salinity (SSS), and changes in Chlorophyll-a (Chl-a) concentrations are related to translation speeds, and respective contributions of hurricane winds and precipitation are evaluated in this work. [ABSTRACT FROM AUTHOR]

The presence of seabirds was related to offshore surface eddies and the freshwater plume of the Mississippi River in the northern Gulf of Mexico during autumn, winter, spring, and summer 1992–1993. Skuas (Stercorarius spp.) were the most common bird seen in autumn, whereas skuas and gulls (Larus spp.) were the most common in winter. Few birds were seen in the spring, and terns (Sterna spp.) were the most common birds seen in late summer. During summer, terns were associated with the freshwater plume of the Mississippi River. In the winter, herring (Larus argentatus) and laughing gulls (L. artricilla) were associated with areas with steep thermoclines, while pomarine skuas (Stercorarius pomarinus) were found in areas of low productivity. Herring and laughing gulls in the winter, all birds in the spring, and terns in the summer were more common outside eddies. Only pomarine skuas in the winter were seen more commonly inside warm-core eddies. [ABSTRACT FROM PUBLISHER]