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Realistic representation of monthly sea level anomalies in coastal regions has been a challenge for global ocean reanalyses. This is especially the case in coastal regions where sea levels are influenced by western boundary currents such as near the U.S. Atlantic Coast and the Gulf of Mexico. For these regions, most ocean reanalyses compare poorly to observations. Problems in reanalyses include errors in data assimilation and horizontal resolutions that are too coarse to simulate energetic currents like the Gulf Stream and Loop Current System. However, model capabilities are advancing with improved data assimilation and higher resolution. Here, we show that some current-generation ocean reanalyses produce monthly sea level anomalies with improved skill when compared to satellite altimetry observations of sea surface heights. Using tide gauge observations for coastal verification, we find the highest skill associated with the GLORYS12 and HYCOM ocean reanalyses. Both systems assimilate altimetry observations and have eddy-resolving horizontal resolutions (1/12°). We found less skill in three other ocean reanalyses (ACCESS-S2, ORAS5, and ORAP6) with coarser, though still eddy-permitting, resolutions (1/4°). The operational reanalysis from ECMWF (ORAS5) and their pilot reanalysis (ORAP6) provide an interesting comparison because the latter assimilates altimetry globally and with more weight, as well as assimilating ocean observations over continental shelves. We find these attributes associated with improved skill near many tide gauges. We also assessed an older reanalysis (CFSR), which has the lowest skill likely due to its lower resolution (1/2°) and lack of altimetry assimilation. ACCESS-S2 likewise does not assimilate altimetry, although its skill is much better than CFSR and only somewhat lower than ORAS5. Since coastal flooding is influenced by sea level anomalies, the recent development of skilful ocean reanalyses on monthly timescales may be useful for better understanding the physical processes associated with flood risks. [ABSTRACT FROM AUTHOR]

Meteorological data obtained from June to September 2005 to 2011 were used to analyze the dispersion of the Tecolutla, Cazones, and Tuxpan river plumes over the Lobos–Tuxpan reef system, one of the largest reef areas of the western Gulf of Mexico. The highest rainfall and river discharges in the area are observed during the summer season and September. The results suggest that the river's plumes can reach the Tuxpan reef under coastal winds blowing northward, inducing a coastal current flowing in the same direction. The river's plume gets dispersed around the Lobos–Tuxpan reef area (50 km away) due to the influence of an anticyclonic gyre, thus the plumes are classified as far-field plumes. The Kelvin numbers associated with the rivers are small, which allows the direction of the plumes to go northward, despite Coriolis dynamics. The rivers influence produce eutrophication, > 0.3 mg/m3 of chlorophyll-a, mainly around the Tuxpan reef cluster, but further north over the Lobos reef cluster, oligotrophic conditions prevail with chlorophyll-a concentrations below 0.15 mg/m3. Turbid waters from the rivers' plumes, obtained from the diffuse attenuation coefficient at 490 nm, reached the Tuxpan reef area with 0.1–0.2 m−1. This pattern was different during September when the rivers reached the entire area (Lobos and Tuxpan cluster), showing values ranging from 0.05 to 0.10 m−1. Correlation between the sea surface temperature, chlorophyll-a, and turbidity at the river´s mouths with the reef areas also suggest the river's influence at this location. This study could help future ecological studies investigating the relationship between the environment and the biological systems over the Lobos–Tuxpan reef system. [ABSTRACT FROM AUTHOR]

This work presents new evidence for conducive conditions for the existence of overturning instabilities in the mixed layer across a mesoscale anticyclonic Loop Current Eddy (LCE) and surrounding cyclonic eddies in the Gulf of Mexico (GoM). The LCE was intensively sampled using four gliders during 12 months. The LCE is characterized by a strong anomaly of fresh and warm water in the center and strong strain at the periphery. Mixed layers prone to overturning instabilities are diagnosed from the gradients of buoyancy. In the GoM, the reduction of potential vorticity (PV) in the mixed layer is driven by surface cooling by year‐round persistent negative turbulent heat fluxes, eventually reaching −700 W m−2 during extreme winter wind events. Lateral buoyancy gradients associated with submesoscale filaments and fronts at the periphery of the LCE enhance the frictional torque supplied by the wind stress when aligned with the geostrophic jet current. Negative PV in the mixed layer occurs simultaneously with negative turbulent heat fluxes and events of Ekman buoyancy fluxes that reduce the stratification. These regions are susceptible to mixed‐layer gravitational‐symmetric instability preferentially in winter. Extreme Ekman equivalent heat flux (≤−10,000 W m−2) associated with wind stress of 1.2 N m−2 coincides with the development of inertial‐symmetric instability across anticyclonic vorticity. These observations might have several implications for the restratification processes, kinetic energy budget, and biogeochemical cycles inside LCEs and can contribute to the formation of low‐PV waters that could feed mode water formation inside the GoM. Plain Language Summary: Recent computational simulations of ocean circulation have shown intense vertical velocities associated with the so‐called "submesoscale" currents as eddies, fronts, and filaments, occurring at horizontal scales of 1–10 km and life spans of 1 day. Fine‐scale temperature and salinity measurements obtained using autonomous underwater vehicles, called gliders, in the Gulf of Mexico (GoM), are used to identify the times and locations where overturning instabilities modulate the ocean–atmosphere exchanges. These instabilities are enhanced inside eddies ubiquitous in the Gulf and at their periphery forced by strong winds during winter. A high‐resolution model of the circulation of the GoM confirms the observations for contrasting wind conditions. These submesoscale diagnostics might be relevant to understand the transfer of kinetic energy from larger to smaller scales, the life cycle of eddies, and the vertical distribution of nutrients and biological activity in the sunlit upper ocean, which includes the atmospheric carbon dioxide sequestration. Key Points: The core of the Warm Core Ring is more (less) susceptible to gravitational instabilities during winter (summer)Favorable conditions for symmetrically unstable mixed layer are captured by the glider observations during a down‐front wind eventAt the periphery of Warm Core Ring, observations of negative potential vorticity suggest favorable conditions to symmetric instabilities [ABSTRACT FROM AUTHOR]

Satellite and in‐situ measurements are used in this study to investigate a possible link between the Florida Current transport (FCT) and temperature changes in the equatorial Pacific. Consistent with the geostrophic nature of the current, the FCT variability shows good correspondence with the changes in sea surface height differences (∆SSH) between the eastern (SSHeast) and western (SSHwest) sides of the Florida Straits. While the variability of SSHwest is mostly associated with seasonal and shorter timescale fluctuations of ∆SSH and FCT, changes in SSHeast are strongly related to the interannual variability of ∆SSH and FCT. A significant correlation is found between the FCT and the Oceanic Niño Index (ONI) on interannual timescales, which explains 21% of the interannual FCT variance. The connection of ONI with FCT is through its impact on SSHeast, associated with the anomalous convergence/divergence in the Caribbean region and the Bahamas forced by ONI‐induced wind stress curl changes. Plain Language Summary: Variations in the Florida Current transport (FCT) have been linked to widespread weather and climate phenomena. Various driving mechanisms for the FCT variability have been proposed, but none can fully explain its wide spectrum variability. In this study, we analyzed in‐situ and satellite observations and found a linear link between the FCT and temperature changes in the equatorial Pacific on interannual timescales. A warming condition in the equatorial Pacific results in low pressure anomalies in the Gulf of Mexico and high pressure anomalies extended into the Caribbean Sea from the tropical Atlantic. This atmospheric pressure pattern is associated with anticyclonic winds over the Caribbean Sea and the Bahamas that drive anomalous oceanic convergence and, therefore, cause higher sea levels in those regions. A higher sea level near the Bahamas often steepens the sea level slope across the Florida Straits, which corresponds to a stronger FCT. A cooling condition in the equatorial Pacific will have the opposite effect, resulting in a reduced FCT. Key Points: Interannual variations in Florida Current transport are highly correlated with sea level at eastern side of the Florida Straits (SSHeast)Changes in SSHeast exhibit a linear link to warming and cooling conditions in the equatorial PacificThis connection is through the wind stress curl changes remotely forced from the equatorial Pacific [ABSTRACT FROM AUTHOR]

• Processing of three different kinds of drifters to extract the geostrophic component. • The use of drifters leads to a higher improvement than an additional altimeter. • Improvement is of 1% in SLA and 10% in meridional velocities. • Both components of the velocity vector have similar accuracy. Knowledge of ocean surface geostrophic circulation has been greatly improved due to satellite observations. However, it is essential to use different observation sources (satellites and in situ data) in order to improve accuracy and resolution. This study combines along-track Sea Level Anomalies (SLA) with geostrophic velocity estimated from surface drifters to map SLA and associated geostrophic current anomalies in the Gulf of Mexico. Firstly, substantial pre-processing is needed on the drifter data to extract the geostrophic component of the signal in order to be consistent with the physical content provided by altimetry. This step includes estimating and removing the Ekman current, Stokes drift and wind slippage. Three kinds of drifters are used: – Drifters belonging to Woods Hole Group, a CLS Group company that launches their own drifters in the Gulf of Mexico. – Drifters launched in the framework of the Lagrangian Submesoscale ExpeRiment (LASER) campaign (January-April 2016). – Drifters from Atlantic Oceanographic and Meteorological Laboratory (AOML). Secondly, drifters and along-track SLA from Jason2, HY2, SARAL and Cryosat-2 are combined through multivariate objective analysis to map a daily time series of SLA and associated geostrophic current anomalies from 01/09/2015 to 30/04/2016. Finally, comparisons with independent data reveal the improved agreement of maps combining both altimetry and drifter data, especially for the meridional component of geostrophic current. [ABSTRACT FROM AUTHOR]

Coupling between the surface and near‐bottom currents in the Gulf of Mexico (GoM) has been reported in many case studies. However, geographical variations of this coupling need more examination. In this study, surface geostrophic currents derived from satellite‐observed sea surface height and subsurface currents from a collection of deep ocean moorings are used to examine the surface and bottom coupling in different parts of the GoM. The short‐period (30–90 days) fluctuations generated by the Loop Current (LC) and the LC eddies (LCEs) have a more vertically coherent structure and stronger deep ocean expressions than the long‐period fluctuations (>90 days). In addition, the strength of the coupling is modulated by the long‐period variations of the LC and LCE sheddings. Moreover, the surface and bottom coupling varies geographically. In the LC region, the surface fluctuations along the eastern side of the LC are important in causing the bottom current fluctuations through baroclinic instability under the LC and through traveling topographic Rossby waves (TRWs) north of the LC. In the central deep GoM, the bottom currents are affected by the upper fluctuations of the northern LC through both local baroclinic instability and remote TRW propagation. In the northwestern GoM, the bottom current fluctuations are largely related to the remote surface variability from the west side of the LC by TRWs propagating northwestward. This study will help us better understand mechanisms of the bottom current fluctuations that are important for the dispersal of deep ocean materials and properties. Plain Language Summary: Bottom currents in the Gulf of Mexico (GoM) are important in many ways, such as transporting fish larvae and spilled oil. However, in contrast to surface currents that are easily derived from the abundant satellite observations, our understanding of the bottom currents is very limited. In this study, the relationships between surface and near‐bottom current fluctuations in different parts of the GoM are investigated by combining a collection of historical near‐bottom current measurements and satellite data. The results show that the bottom current variability is linked to local processes such as current instability or remote processes through planetary wave propagation. The surface and bottom coupling is mainly related to the short‐period variability generated along the Loop Current and the Loop Current eddies and has a significant influence on the bottom currents. Although the long‐period fluctuations have weak bottom expressions, they can modulate the strength of the short‐period coupling. Studying the coupling between the surface and bottom current fluctuations in the GoM can advance our understanding of mechanisms of the bottom currents that are important for the deep ocean ecosystem and industrial operations. Key Points: Surface and bottom currents in the Gulf of Mexico can be coupled locally by baroclinic instability and remotely by topographic Rossby wavesFluctuations generated along the Loop Current have stronger deep ocean expressions at the period of 30–90 days than at longer periodsSurface and bottom coupling in the Gulf of Mexico varies geographically and is modulated by the Loop Current and the Loop Current eddies [ABSTRACT FROM AUTHOR]

One-year subinertial flow profiles and near-bottom temperatures were recorded in a tropical coral reef of the southwestern Gulf of Mexico to determine whether baroclinicity affects flow distributions. The subinertial flow at four sites was predominantly influenced by three empirical modes of variability. The first mode (explaining 78% of the variance) was mostly associated to wind stress (r = 0.62). Its temporal variability had greatest energy during the northerly wind season (October–May), related to atmospheric cold fronts, at periods of 2–10 days and at 20–30 days. Mode 2 (with 6% of the variance) was related to sea-level slopes, linked also to northerly wind events; this mode induced an anticyclonic circulation around the reefs, although a less intense cyclonic circulation was also observed. Mode 3 (with 5% of the variance) was associated to a baroclinic pressure gradient that drove bidirectional flow profiles: Near-surface flows moving in opposite direction to near-bottom flows. Mode 3 dominated in the vertically stratified summer (June–September), the rainy season, but it was also occasionally observed during northerly winds. Near-bottom summer intrusions of relative cold water were baroclinically driven, linked to Mode 3, and unrelated to Ekman dynamics. Modes 2 and 3 of variability described additional flow distributions relative to the behavior already proposed for the reefs on the southwestern continental shelf of the Gulf of Mexico. • The first three EOF modes of velocity explained ∼90% of the variability around the Veracruz Reef System. • Currents driven by northerly winds were associated with semigeostrophic dynamics (∼80% of the variance). • Anticyclonic circulation around the reef area was related to northerly winds. • Bidirectional flow related with the baroclinic pressure gradient was a common structure during summer season. • Relatively colder water intrusion from the northwest near the bottom was observed in summer. [ABSTRACT FROM AUTHOR]

Abstract: The Colorado University Princeton Ocean Model (CUPOM) is used here to study the early stages in the life of Millennium, a mesoscale anticyclonic ring that detached from the Loop Current on April 2001 and lasted for more than 100days. The numerical near-surface velocity field for the Gulf of Mexico is validated with the altimetry geostrophic velocities. The first 30days of numerical data, before Millennium interacts with other mesoscalar features, are closely examined both from Eulerian and Lagrangian perspectives. During this time Millennium had a near-constant rotation period of 6.5days, and particles do not leave the ring. Nevertheless, the distributions of temperature, salinity, and angular velocity confirm the existence of significant (possibly numerical) radial diffusion. Polar-coordinate phase plots for temperature–salinity anomalies and tangential–radial velocities, at several depths, illustrate the presence of an evolving oscillating pattern. Radial and tangential velocities change in phase, associated with vertical displacements of the isothermal and isohaline surfaces. A simple diffusion model with an effective diffusion coefficient of 200m2 s−1 is appropriate to grossly simulate the temporal evolution of angular velocity within Millennium. [Copyright &y& Elsevier]

Eddies and fronts created by geostrophic balance between counter-rotating currents are hydrodynamic processes that concentrate or dilute plankton and suspended material by physical processes of accumulation, retention, or dispersion. Over Campeche Canyon, in the southern Gulf of Mexico, observations revealed an anticyclonic eddy, a cyclonic current off the continental shelf, and an associated thermal front in their common boundary. Along this front there was high Trichodesmium spp. occurrence. Three water masses were found in the upper 120 m layer: Caribbean Tropical Surface Water (CTSW), Caribbean Subtropical Underwater (CSUW), and Gulf Common Water (GCW). The frontal zone between the warm core and cyclonic circulation was observed below the hot summertime mixed layer. The presence of Trichodesmium spp. was associated with the front, indicating its influence on the distribution of this diazotrophic cyanobacterium that enters the Campeche Canyon region via modified CTSW. The associated phytoplankton was dominated by tropical oceanic species of diatoms and dinoflagellates, especially species of Ceratium. [ABSTRACT FROM AUTHOR]

Data assimilation methods often use an ensemble to represent the background error covariance. Two approaches are commonly used; a simple one with a static ensemble, or a more advanced one with a dynamic ensemble. The latter is often non-practical due to its high computational requirements. Some recent studies suggested using a hybrid covariance, which is a linear combination of the covariances represented by a static and a dynamic ensemble. Here, the use of the hybrid covariance is first extensively tested with a quasi-geostrophic model and with different analysis schemes, namely the Ensemble Kalman Filter (EnKF) and the Ensemble Square Root Filter (ESRF). The hybrid covariance ESRF (ESRF-OI) is more accurate and more stable than the hybrid covariance EnKF (EnKF-OI), but the overall conclusions are similar regardless of the analysis scheme used. The benefits of using the hybrid covariance are large compared to both the static and the dynamic methods with a small dynamic ensemble. The benefits over the dynamic methods become negligible, but remain, for large dynamic ensembles. The optimal value of the hybrid blending coefficient appears to decrease exponentially with the size of the dynamic ensemble. Finally, we consider a realistic application with the assimilation of altimetry data in a hybrid coordinate ocean model (HYCOM) for the Gulf of Mexico, during the shedding of Eddy Yankee (2006). A 10-member EnKF-OI is compared to a 10-member EnKF and a static method called the Ensemble Optimal Interpolation (EnOI). While 10 members seem insufficient for running the EnKF, the 10-member EnKF-OI reduces the forecast error compared to the EnOI, and improves the positions of the fronts. [ABSTRACT FROM AUTHOR]

Dynamical systems theory is employed to study the irregular Loop Current in the Gulf of Mexico using a short database of shedding periods and north–south positions of the current. Two independent tests based on these data suggest that the Loop Current is not chaotic but behaves as a nonlinear driven and dampened oscillator with a very short memory. It is suggested that this current varies around a limit-cycle elliptical attractor. It was found that the amplitude and period of the oscillation vary at time scales of 3–5 yr, a time scale that agrees with those of the North Atlantic Oscillation (NAO) and/or ENSO; however, it is proposed that NAO provides the link between these systems. The proposed mechanism is the ITCZ changes caused by NAO, which affects the wind strength and the transport across the Yucatan Channel. A forecasting scheme that allows for prediction of the next eddy-shedding period from knowledge of the last shedding event, a condition caused by the short memory of the system, is provided. [ABSTRACT FROM AUTHOR]