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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]

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. [ABSTRACT FROM AUTHOR]

The Deep Water Horizon oil spill dramatically impacted the Gulf of Mexico from the seafloor to the surface. While dispersion of contaminants at the surface has been extensively studied, little is known about deep water dispersion properties. This study describes the results of the Deep Water Dispersion Experiment (DWDE), which consisted of the release of surface drifters and acoustically tracked RAFOS floats drifting at 300 and 1,500 dbar in the Gulf of Mexico. We show that surface diffusivity is elevated and decreases with depth: on average, diffusivity at 1,500 dbar is 5 times smaller than at the surface, suggesting that the dispersion of contaminants at depth is a significantly slower process than at the surface. This study also examines the turbulent regimes driving the dispersion, although conflicting evidences and large uncertainties do not allow definitive conclusions. At all depths, while the growth of dispersion and kurtosis with time supports the possibility of an exponential regime at very short time scales, indicating that early dispersion is nonlocal, finite size Lyapunov exponents support the hypothesis of local dispersion, suggesting that eddies of size comparable to the initial separation (6 km), may dominate the early dispersion. At longer time scales, the quadratic growth of dispersion is indicative of a ballistic regime, where a mean shear flow would be the dominating process. Examination of the along‐ and across‐bathymetry components of float velocities supports the idea that boundary currents could be the source for this shear dispersion. Plain Language Summary: The 2010 Deep Water Horizon oil spill has dramatically impacted the Gulf of Mexico's marine environment from the seafloor to the surface. While dispersion of contaminants at the surface has been extensively studied over the past decades, little is known about the deep water dispersion properties of the ocean, and the fate of deep contaminants is uncertain. This paper describes the results of the Deep Water Dispersion Experiment that took place in the western Gulf of Mexico, a deep water drilling operation area. The experiment consisted in the simultaneous release of surface drifters and floats drifting at 300 and 1,500 m, to assess the variations of dispersion properties with depth for the first time. It is shown that diffusivity is weaker in depth that at the surface, so that contaminants would spread less rapidly. Key Points: Pairs of surface drifters and RAFOS floats drifting at 300 and 1,500 dbar were released simultaneously in the Gulf of Mexico between 2016 and 2018Relative diffusivity at 300 and 1,500 dbar is on average 2 and 5 times weaker than at the surface, respectivelyTopography and sheared boundary currents might play a major role in float dispersion [ABSTRACT FROM AUTHOR]

Four ichthyoplankton cruises and backward tracking experiments were conducted to study the connectivity of coastal and neritic fish larvae over the continental slope and to the oceanic deep‐water region of the western Gulf of Mexico. Distribution patterns of larval abundance at oceanic stations showed higher abundance and the presence of larvae at oceanic stations during two cruises. Larval transport was simulated using outputs of a data assimilation model that represented the flow conditions during each cruise. Higher abundances of larvae of coastal and neritic species at oceanic stations agreed with offshore transport inferred from numerical experiments seeding particles over different spatial scales (stations vs. transects). Satellite images of surface chlorophyll were consistent with the circulation patterns indicated by the model, indicating filaments of shelf waters were transported toward the transects with higher larval abundances. Particle tracking experiments indicated that the northwestern shelf provinces of Perdido, Tamaulipas, and Texas were the main source of propagules to the oceanic region, while shelf provinces of northern Veracruz, Campeche, Yucatan, Louisiana, and Mississippi‐Alabama contributed much less. The length and intensity of the shelf front limited ichthyoplankton cross‐shelf exchange during some cruises, and mesoscale anticyclonic and cyclonic eddies advected larvae to the deep‐water region during others. The agreement between the spatial distribution of fish larvae and the simulated larval transport confirm that circulation models are a valuable tool for examining potential dispersal pathways of neritic species, as long as similar spatial and temporal scales as the ones used in this study are considered. [ABSTRACT FROM AUTHOR]

A large set of historical surface drifter data from the Gulf of Mexico – 3770 trajectories spanning 28 years and more than a dozen data sources – are collected, uniformly processed and quality controlled, and assimilated into a spatially and temporally gridded dataset called GulfFlow. This dataset is available in two versions, with 1/4 ∘ or 1/12 ∘ spatial resolution respectively, both of which have overlapping monthly temporal bins with semimonthly spacing and which extend from the years 1992 through 2020. Together these form a significant resource for studying the circulation and variability in this important region. The uniformly processed historical drifter data from all publicly available sources, interpolated to hourly resolution, are also distributed in a separate product called GulfDriftersOpen. Forming a mean surface current map by directly bin-averaging the hourly drifter data is found to lead to severe artifacts, a consequence of the extremely inhomogeneous temporal distribution of the drifters. Averaging instead the already monthly-averaged data in GulfFlow avoids these problems, resulting in the highest-resolution map of the mean Gulf of Mexico surface currents yet produced. The consolidated drifter dataset is freely available at 10.5281/zenodo.3985916 , while the gridded products are available for noncommercial use only (for reasons discussed herein) at 10.5281/zenodo.3978793. [ABSTRACT FROM AUTHOR]

A key consequence in climate change is the warming of deep waters, away from the faster warming rates of near-surface subtropical and tropical waters. Since surface and near-surface oceanic temperatures have been measured far more frequently in time and space than deep waters (>2000 m), deep measurements become quite valuable. Semi-enclosed basins, such as the Gulf of Mexico, are of particular interest as the waters below sills that connect with the neighboring oceans have residence times much longer than upper layers. Within the western Gulf of Mexico, near-bottom measurements at ~3500-m depths at four sites show a stable linear warming trend of ~16 ± 2 m°C decade−1 for the period 2007–18, and CTD data from eight oceanographic cruises occurring from 2003 to 2019 show a trend of ~18 ± ~2 m°C decade−1 from the bottom to ~2000 m below the surface. The bottom geothermal heat flux is a contributing factor to be considered in the warming and renewal of such waters, but it has not changed over millennia and is therefore unlikely to be the cause of the observed trend. The densest waters that spill into the Gulf of Mexico, over the Yucatan Channel sill, must mix substantially during their descent and in the near-bottom interior, losing their extreme values. A simple box model connects the observed warming, well within the Gulf interior, with that expected in the densest waters that spill from the North Atlantic into the Cayman Basin through Windward Passage and suggests that the source waters at the entrance to the Caribbean have been warming for at least 100 years. [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]

A large set of historical surface drifter data from the Gulf of Mexico—3761 trajectories spanning 27 years and more than a dozen data sources—are collected, uniformly processed and quality controlled, and assimilated into a spatially and temporally gridded dataset called GulfFlow. This dataset is available in two versions, with one-quarter degree or one-twelfth degree spatial resolution respectively, both of which have overlapping monthly temporal bins with semimonthly spacing, and extend from the years 1992 through 2019. Together these form a significant resource for studying the circulation and variability in this important region. The uniformly processed historical drifter data interpolated to hourly resolution from all publicly available sources are also distributed in a separate product called GulfDriftersOpen. Forming a mean surface current map by directly bin-averaging the hourly drifter data is found to lead to severe artifacts, a consequence of the extremely inhomogeneous temporal distribution of the drifters. Averaging instead the already monthly-averaged data in GulfFlow avoids these problems, resulting in the highest-resolution map of the mean Gulf of Mexico surface currents yet produced. The consolidated drifter dataset is freely available from https://doi.org/10.5281/zenodo.3985916 (Lilly and Pérez-Brunius, 2020a), while the gridded products are available for noncommercial use at https://doi.org/10.5281/zenodo.3978793 (Lilly and Pérez-Brunius, 2020b), the latter being freely available for noncommercial use only for reasons discussed herein. [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]

The seasonal cycle of transport through the Yucatan Channel is estimated from 59 months of direct mooring measurements and 23 years of a transport proxy from AVISO sea level across the channel. Both exhibit a seasonal cycle with a maximum in summer (July–August) but have a minimum in March for the mooring and in November for AVISO data. The annual and semiannual harmonics explain respectively 19% (~32%) and 6% (~4%) of the subinertial variance of the moored (proxy) transports. Seasonal variations of zonal wind stress and anticyclonic wind stress curl over the Cayman Sea appear to be positively correlated with transport in Yucatan Channel and the northward extension of the Loop Current during the summer, agreeing to some extent with modeling results previously reported. Transport increments during summer coincide with enhanced regional easterly winds and anticyclonic wind stress curl in 60% of the cases (of 23 years). However, this connection is not as tight as model results suggest during winter. The summer correlation only appears to be valid in a broad statistical sense since it is modulated by large interannual and higher-frequency variability. Moored time series confirm previous results that the transport signal on the western side of the channel is quite different from the total Yucatan Channel transport and that eddy kinetic energy at higher frequencies (50–100 days) dominates the variability and is characterized by a relatively low net transport signal, with flow of opposite signs on each side of the channel. [ABSTRACT FROM AUTHOR]

Coincident Lagrangian observations of coastal circulation with surface drifters and dye tracer were collected to better understand small-scale physical processes controlling transport and dispersion over the inner shelf in the Gulf of Mexico. Patches of rhodamine dye and clusters of surface drifters at scales of O(100) m were deployed in a cross-shelf array within 12 km from the coast and tracked for up to 5 h with airborne and in situ observations. The airborne remote sensing system includes a hyperspectral sensor to track the evolution of dye patches and a lidar to measure directional wavenumber spectra of surface waves. Supporting in situ measurements include a CTD with a fluorometer to inform on the stratification and vertical extent of the dye and a real-time towed fluorometer for calibration of the dye concentration from hyperspectral imagery. Experiments were conducted over a wide range of conditions with surface wind speed between 3 and 10 m s−1 and varying sea states. Cross-shelf density gradients due to freshwater runoff resulted in active submesoscale flows. The airborne data allow characterization of the dominant physical processes controlling the dispersion of passive tracers such as freshwater fronts and Langmuir circulation. Langmuir circulation was identified in dye concentration maps on most sampling days except when the near surface stratification was strong. The observed relative dispersion is anisotropic with eddy diffusivities O(1) m2 s−1. Near-surface horizontal dispersion is largest along fronts and in conditions dominated by Langmuir circulation is larger in the crosswind direction. Surface convergence at fronts resulted in strong vertical velocities of up to −66 m day−1. [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]

The large-scale circulation of the bottom layer of the Gulf of Mexico is analyzed, with special attention to the historically least studied western basin. The analysis is based on 4 years of data collected by 158 subsurface floats parked at 1500 and 2500 m and is complemented with data collected by current meter moorings in the western basin during the same period. Three main circulation patterns stand out: a cyclonic boundary current, a cyclonic gyre in the abyssal plain, and the very high eddy kinetic energy observed in the eastern Gulf. The boundary current and the cyclonic gyre appear as distinct features, which interact in the western tip of the Yucatan shelf. The persistence and continuity of the boundary current is addressed. Although high variability is observed, the boundary flow serves as a pathway for water to travel around the western basin in approximately 2 years. An interesting discovery is the separation of the boundary current over the northwestern slope of the Yucatan shelf. The separation and retroflection of the along-slope current appears to be a persistent feature and is associated with anticyclonic eddies whose genesis mechanism remains to be understood. As the boundary flow separates, it feeds into the westward flow of the deep cyclonic gyre. The location of this gyre-named the Sigsbee Abyssal Gyre-coincides with closed geostrophic contours, so eddy-topography interaction via bottom form stresses may drive this mean flow. The contribution to the cyclonic vorticity of the gyre by modons traveling under Loop Current eddies is discussed. [ABSTRACT FROM AUTHOR]

Abstract: The upper layer circulation in the Bay of Campeche is analyzed with three years of data recorded by surface drifters, current meter moorings, and satellite altimetry. The measurements show that the mean cyclonic circulation observed by previous authors extends below 1000m, and that its size and location are delimited by the particular topography of the region: a deep basin to the west, and a shallower and gentle sloping submarine fan to the east. An Empirical Orthogonal Function analysis and large correlations of the surface flow with the deeper currents suggest that the topographic constraint is the result of potential vorticity conservation for an equivalent barotropic flow. The variability of the surface currents in the western basin is mostly due to changes in the size, form, position and intensity of the cyclonic gyre due to its interaction with northern Gulf of Mexico eddies, particularly Loop Current Eddies traveling the southern route towards the western boundary. By contrast, the eastern basin is characterized by a weak northward drift, with the occasional generation of anticyclones in the southeastern boundary, the genesis of which remains to be understood. This suggests that the variability in the eastern basin is mostly driven by locally generated disturbances, rather than by an influx of northern Gulf of Mexico eddies. Strong northward flows in the central and eastern basins result from the flow convergence between locally generated anticyclones and the cyclonic gyre. [Copyright &y& Elsevier]

The article presents a study on barnacle distributions in two adjacent coastal lagoons of Bahía Honda, a bay on the Pacific coast of Panama. It discusses details of the experiments to determine the probable causes for differences in the two lagoons and the role of season and salinity in barnacle distribution. It highlights the effects of natural stressors on population and community dynamics of nearshore species with a two-phase life cycle.

Abstract: The effects of the 1997–1998 and 2002–2004 El Niño on the upper waters in the continental shelf and slope regions off northwestern Baja California are explored with data from eight cruises taken in late spring from 1998 to 2004 and the summers of 1997 and 1998. Geostrophic velocities were calculated referenced to a specific volume anomaly surface separating the generally southward flowing California Current waters from the waters advected to the north by the California Undercurrent. The resulting fields show equatorward flow near the surface except in the summer of 1997, when a poleward jet was found in the upper 40dbar. This shallow jet advected anomalously warm and salty waters characteristic of the 1997–1998 El Niño, with its core found within 20–30km from the coast. By spring of 1998, the waters brought into the region by the jet had mixed across the pycnoline with the salty California Undercurrent waters below, resulting in high salinity levels on the density surfaces corresponding to the otherwise fresh California Current waters (25–26). By contrast, the 2002–2004 El Niño stands out for the very fresh and cold waters found on the same density surfaces in late spring of 2003 and 2004, marking a pronounced presence of subarctic waters. The fresh conditions found on the latter years represent a nearshore expression of the anomalous intrusion of subarctic waters observed 50–150km from the coast of Southern California and Punta Eugenia, reported from July 2002 until April 2003. Our results suggest that the presence of this intrusion has continued to influence the region at least until May 2004. [Copyright &y& Elsevier]

To obtain a description of the hydrographic state of the North Atlantic Current–subpolar front (NAC–SPF) system, historical hydrographic data from the subpolar North Atlantic are projected into a baroclinic streamfunction space, resulting in three-dimensional Gravest Empirical Mode (GEM) fields for temperature and specific volume anomaly, parameterized by pressure, dynamic height, and day of the year. From the specific volume anomaly GEM, the corresponding potential vorticity field is calculated. These fields are constructed for 12 subregions, chosen to follow the mean path of the NAC–SPF system. Analysis of the seasonal potential vorticity cycle of the GEM fields shows that the main mechanism for the formation of Subpolar Mode Water is winter convection. The GEM fields are also used to obtain the approximate location of formation sites for the different Subpolar Mode Water classes. The evolution of the mean fields for the waters is studied along baroclinic streamlines of the NAC–SPF system. This shows that cross-frontal mixing, between the cold and fresh subpolar waters and the salty and warm waters coming north from the subtropics via the Gulf Stream, is the dominant mechanism for the light-to-dense transformation process of the NAC–SPF waters that enter the western subpolar region. On the other hand, a combination of atmospheric cooling, vertical mixing during wintertime convection, and entrainment of the saltier waters found on the northeastern subtropical gyre is the main factor transforming the NAC–SPF waters that enter the eastern subpolar gyre. This suggests that an influx along the eastern margin of salty water from the European Basin plays a significant role in the transformation of the NAC–SPF waters that continue their way toward the Nordic seas. [ABSTRACT FROM AUTHOR]

The flow of subtropical waters carried into the northern North Atlantic Ocean by the North Atlantic Current– subpolar front system (NAC–SPF) is an important component of the meridional overturning circulation. These waters become colder and denser as they flow through the subpolar region, both by mixing with the colder subpolar waters and by atmospheric cooling. The relative roles of these two processes remain to be quantified, and the mechanisms driving lateral mixing need to be better understood. To address those questions, a new methodology is developed to estimate the mean absolute transports of mass and heat for the top 1000 dbar in the region of the NAC–SPF for the time period 1993–2000. The transports are obtained by combining historical hydrography with isopycnal RAFOS float data from the area. The mean absolute transport potential field shows an NAC–SPF “pipe,” defined by two bounding transport potential contours. This pipe transports 10.0 ± 3.5 Sv (Sv ≡ 106 m3 s-1) (top 1000 dbar) from the subtropics into the eastern subpolar North Atlantic. In contrast to earlier studies, the northward-flowing NAC follows a distinct meandering path, with no evidence of permanent branches peeling off the current before reaching the “Northwest Corner.” As the current enters the Northwest Corner, it loses its tight structure and maybe splits into two or more branches, which together constitute the eastward flow along the SPF. The eastward flow between the Northwest Corner and the Mid-Atlantic Ridge is not as tightly defined because of the meandering and/or eddy shedding of the branches constituing the SPF. As the flow approaches the Mid-Atlantic Ridge, it converges to cross above the Charlie–Gibbs and Faraday Fracture Zones. The mean absolute temperature transport (top 1000 dbar) by the 10-Sv pipe was estimated across 10 transects crossing the NAC–SPF. Because the mean mass flux is constant in the pipe, variations in the mean temperature transports result from lateral exchange and mixing across the pipe's side walls and from air–sea fluxes across the surface of the pipe. The NAC–SPF current loses 0.18 ± 0.05 PW on its transit through the region, most of the loss occuring upstream of the Northwest Corner. The heat loss is 10 times the corresponding heat lost to the atmosphere. We conclude that cross-frontal exchange induced by the steep meanders of the northward-flowing NAC is the main mechanism by which heat is lost along the current in the region between the “Tail of the Grand Banks” and the Mid-Atlantic Ridge. [ABSTRACT FROM AUTHOR]

In the North Pacific, variations in isopycnal depth influence the biogeochemical characteristics of the water column and the aragonite saturation horizon (ZΏa) during interannual events. The objective of this study was to assess the effect of the 2011 La Nina conditions on ZΏa in Todos Santos Bay (Baja California, Mexico) and surrounding waters. The results showed that ZΏa variability was modulated by the intensity of interannual conditions and by the water masses that were present in the region. Subarctic Water predominated in the upper 200 m with anomalous characteristics, such as low temperature and low salinity. Also, isopycnals shoaled toward the coast and ZΏa was thus ~30 m in the nearshore area, in contrast with the oceanic region, where ZΏa was ~150 m. Prior to this study, there were no records of ZΏa in Todos Santos Bay, nor were there any records of its shallowness. [ABSTRACT FROM AUTHOR]

Abstract We evaluated the factors that control the variability in dissolved cadmium (Cd d), phosphate (PO 4 3−) and the Cd d /PO 4 3− ratio in the upper 1000 m of the deep region of the Gulf of Mexico (GoM), a marginal sea where trace element data are limited. These variables were compared to data from studies of adjacent areas (in the Western Central Atlantic), related to physical processes (mesoscale structures and upwelling events) occurring inside the GoM, and to dinoflagellate and diatom abundances in surface waters of this region. Vertical distributions of Cd d concentrations and the Cd d /PO 4 3− ratio displayed the typical nutrient profile that has been extensively reported for different oceans. Cd d and PO 4 3− concentrations in the GoM were higher than those in the Western Central Atlantic. The slope of the Cd d vs. PO 4 3− relationship for samples within the Loop Current (LC) in the GoM was similar (242 pmol μmol−1, R 2 = 0.96) to that calculated for the Western Central Atlantic waters (same depth range) that reach the GoM through the LC (235 pmol μmol−1, R 2 = 0.98). The linear regression for all the GoM samples, though having a similar slope (249 pmol μmol−1), presented a larger deviation (R 2 = 0.91). We propose that this deviation is due to spatial inhomogeneity that can be explained by 1) preferential remineralization of PO 4 3− vs. Cd d at subsurface waters in some regions, 2) Cd d inputs from the north, transported by mesoscale processes, and 3) interactions with the continental slope at the minimum O 2 layer. Local changes in the relationship between Cd d and PO 4 3− were traced with Cd*, which represents the deviation of measured Cd to that expected by the deep-water Cd d vs. PO 4 3− relationship. Cd* presented mainly negative values at surface and subsurface waters (~150 m) indicating preferential uptake of Cd d vs. PO 4 3− by the phytoplankton at the surface and preferential remineralization of PO 4 3− vs. Cd d from the organic matter at the subsurface waters. At deeper waters (~400–1000 m) Cd d was preferentially remineralized. In order to understand the inhomogeneity of the horizontal distribution of Cd d , PO 4 3− and the Cd d /PO 4 3− ratio in surface waters, three regions were defined considering their biological and physical characteristics. These were: I) the Oligotrophic region (in the northwest GoM) where the variability in the Cd d /PO 4 3− ratio was largely explained by salinity, II) the Upwelling region (off the coast of the Yucatan Peninsula) where the variability in the Cd d /PO 4 3− ratio was principally due to the diatom abundance and, III) Semi-permanent Cyclonic Gyre region (Bay of Campeche) where the variability in Cd d and the Cd d /PO 4 3− ratio were primarily influenced by the upwelling generated by the cyclonic gyre, and the variability in PO 4 3− was likely due to the consumption of this nutrient by phytoplankton. In summary, physical processes, river inputs, and phytoplankton abundance appear to play an important role in explaining the spatial variation in the Cd d /PO 4 3− ratio. Highlights • Dissolved Cd (Cd d) and PO 4 3− are enriched in the Gulf of Mexico (GoM). • Cd d and PO 4 3− concentration distributions showed high spatial variation in the GoM. • The GoM and Western Central Atlantic waters exhibit similar Cd d vs. PO 4 3− slopes. • Biological and mesoscale processes control the Cd d /PO 4 3− ratio in the GoM. [ABSTRACT FROM AUTHOR]