Reply on RC3

The dynamics associated with the Loop Current (LC) variability in the Gulf of Mexico (GoM) are studied using a 5‐year, free‐running numerical simulation with the Hybrid Coordinate Ocean Model (HYCOM). The dynamics of major GoM circulation features are represented: the extension of the LC and the associated anticyclonic, warm core Loop Current Eddies (LCEs) and cyclonic Loop Current Frontal Eddies (LCFEs). The study focuses on the dynamics of the LCFEs and their role during the LCEs shedding, which dramatically affects the GoM circulation. We analyze several characteristics of the LC frontal dynamics. Modeled LCFEs have a coherent vertical structure, which extends to the deep layers of the GoM. They may split in two separate upper and lower layer eddies. Deep and surface remnants from different frontal eddies are able to align to form new, coherent structures. LCFEs intensify along the extended LC northern edge when flowing over the deep northern GoM shelf slope that forms the Mississippi Fan, through a “promontory effect” in which the incoming cyclone aggregates positive potential vorticity anomalies in lower layers, leading to the intensification of the whole vortex structure. LCFEs may also expand further along the LC path by horizontal vortex merging, when they are blocked between the LC and the northeast corner of the continental shelf in the GoM. The intensification and merging due to topographic effects explain the enlarged frontal eddies observed on the eastern side of the Loop Current. These larger eddies further migrate along the LC front and may play a role in the shedding sequence.

Cyclonic activity in the eastern Gulf of Mexico: Characterization from along-track altimetry and in situ drifter trajectories

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.

As the oil and gas industry migrates towards deepwater exploration, the need to characterize the deep-water environments of the Gulf of Mexico has become increasingly important. This study focused on developing remote sensing tools to study Loop Current Frontal Eddies (LCFEs), which are cyclonic features found along the periphery of the Loop Current (LC). These features have proven problematic for study as they often move rapidly and they do not always have a distinct thermal signature. They play an integral but not completely understood role in separation of warm core rings from the LC and have been discussed as potential trigger mechanisms for high-energy events within the water column and near the bottom along the Sigsbee Escarpment. They are biological oases along the margin of the oligotrophic LC waters. The over-arching goal of this project was to develop and apply new remote sensing methodologies to improve upon the surveillance of ocean features and the understanding of circulation processes in the Gulf of Mexico (GoM). The more specific project goal was to apply these new techniques to improve understanding of the basic characteristics of LCFE cyclones, specifically where they develop, how they evolve as they travel around the LC, frequency of occurrence, and their impacts on surface and deeper circulation in areas of oil and gas operations. Satellite data included GOES sea surface temperatures (SST), sea surface height (SSH) data from several operational sensors, and satellite-tracked drifters. The new techniques provided metrics on LCFE motion, size, frequency and intensity. Extensive use was made of Hovmoller time/distance graphs comprised of daily de-clouded SST composite data (from GOES-8 and GOES-12) for tracking the LCFEs around the entire perimeter of the LC. The 17-cm SSH contour was used as a useful benchmark for the location of the LC on a daily basis. The along track SSH data was essential for determining intensity and size of LCFEs as they evolved along the LC margin. Case study analyses of selected events provided important additional information on LCFE variability during extreme events and their affects on circulation through the water column. These new techniques can be used in real-time to improve on surveillance and modeling capabilities of mesoscale circulation in the GoM.

Impacts of mesoscale eddies on the nitrate distribution in the deep-water region of the Gulf of Mexico

Eddy‐topography (ET) interactions are important in determining the path and evolution of oceanic eddies, including Loop Current Eddies (LCE) in the Gulf of Mexico (GOM). We use the Hybrid Coordinate Ocean Model and satellite altimetry data to investigate the ET interactions and the impact on LCE pathway evolution in the GOM. Satellite altimetry reveals that LCEs translate dominantly westward in the central GOM and strongly collide and reflect against topography near the continental slope in the northern and western GOM. The result is the frequent generation of an anticyclone‐cyclone (AC) pair in conjunction with the LCEs. In the absence of lateral or surface boundary forcing but including realistic topography, simulations initialized with idealized eddies at various locations in the GOM reveal the following results. Southward eddy reflection from the northern slope occurs when a cyclone drastically strengthens east of the anticyclone because of the ET collision. The prevailing westward propagation in the central GOM occurs because the cyclone is very dispersive toward nearby topographic features, causing a reduced southward component of drift and/or moves to the south, forming a meridional AC pair, causing an enhanced westward component. ET collision is strongest over the northwestern slope (north of 24°N) because of the eddy colliding relatively normal to the steep slope, and the eddy typically tracks anticyclonic pathways during the collision/reflection process. Along the western slope, a strong ET collision produces a southeastward reflection, and the accompanying cyclone to the northeast strongly enhances the reflection. Near the southern GOM slope, eddy pathways tend to propagate alongslope with an onshore‐offshore oscillatory trajectory because of the competition between topographic and planetary β effects. In the southwestern GOM a bimodal pathway occurs, i.e., northward migration with an anticyclonic route for relatively stronger eddies and southward dissipative propagation along the shelf edge for weaker ones.

Intense currents having speeds between 10 and 40 cm.s−1from 1000 m below the surface to the bottom have been measured in the Loop Current (LC) region and in the Western Gulf of Mexico (WGM). The observations come from moored current meters sampling from the near‐surface to the bottom at 3300–3500 m and from August 2008 to August 2010. Ten intermittent intensified currents are documented. These events last 10 to 30 days and take place in the region just between the surface‐intensified anticyclonic‐cyclonic features, as deduced from altimetry and measured currents. In the WGM, they occur between the cores of anticyclone‐cyclone pairs. In the Eastern Gulf of Mexico (EGM), they occur in the Loop Current edge between its anticyclonic circulation and a neighboring cyclone. During each event, the analysis shows an increase in the barotropic contribution to the current. In the Loop Current region, these events are often associated with intensification of cyclonic vorticity in the deep layer.

The Loop Current penetrated deep into the Gulf of Mexico in early 1989. After several eddy formation and reattachment cycles, a southwestward propagating anticyclonic eddy was formed during the summer. The Loop Current and subsequent eddy produced strong currents over widespread areas on the Louisiana continental slope, prompting a series of current measurement programs. Because of the lack of satellite infrared coverage in the Gulf of Mexico during the summer, the trajectories of 53 ARGOS tracked drifting buoys deployed in the Gulf of Mexico in 1989 were assembled to determine the synoptic history of the Loop Current and anticyclonic eddy during this event. Ten of the most critical summertime buoy trajectories are discussed here. In addition, the trajectories of three of the buoys simultaneously deployed at different radii in the eddy were chosen for analysis with a kinematic feature model. The model assumes the looping buoy motion is generated by an elliptical orbit around a translating eddy center. The buoy trajectories were used to determine time series of the feature model parameters, including eddy center, shape and circulation characteristics. The time series results indicate that the eddy actually propagated in a series of short sprints separated by longer stalls. During the sprints, the eddy propagated as an elliptical but otherwise symmetric solid body. During the stalls, significant asymmetries developed. The asymmetries are consistent with attachment to the Loop Current during the first stall, the effects of topographic Rossby wave dispersion during the second, and the effects of planetary Rossby wave dispersion during the third. The results provide both a framework for the interpretation of in situ observations and a detailed evolutionary history for numerical modelers.

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

The years 2014-2017 marked a period of anomalous and extreme Loop Current (LC) activity that resulted in significant impact on offshore operations for the first 18 months, followed by a 4- to 6-month period of more typical behavior and then 15 months of uncharacteristic inactivity. The objectives of this study are to (i) provide a detailed description of the kinematic structure of the Loop Current Eddies (LCEs) observed during this time frame; (ii) determine the vertical extent of each eddy and the effect of eddy activity on near-bottom currents, and (iii) establish a relationship (if any) between eddy activity and intensity of inertial oscillations within the water column. We present data collected from ADCPs at several locations in the northern Gulf of Mexico along with an extensive network of drifting buoys in correlation with daily frontal analyses and a comparison with historical events. To estimate the intensity of inertial oscillation and its variability in time, we computed rotary current spectra for periods characterized by high and low eddy activity. While circulation patterns during early 2014 were fairly typical, an apparent sudden increase in energy marked the beginning of hyperactivity in late June. Eddies Lazarus, Michael, Nautilus, and Olympus spun off the LC, each eddy experiencing multiple reconnections with the LC before its final separation in addition to amplified intensities (over 3 knots, max 4.5+ knots). December 2015 marked the beginning of the second phase. Another eddy (Poseidon) separated from the LC in April 2016; however, the winding down process of the LC circulation was already in effect. Poseidon did not exhibit a significant northward migration and the LC itself remained far south. In the central Gulf, the passage of Poseidon was followed by the passage of a subsurface cyclonic eddy, without a significant surface expression, propagating westward. For the next 18 months, the LC remained extremely inactive. While currents in the near-bottom layer were not directly correlated with the flows in the upper part of the water column during the period of the high eddy activity, the later period was characterized by more energetic near-bottom flows, suggesting LCE’s may stimulate generation of topographic Rossby waves that cause strong near-bottom intensities. No significant correlation between the presence of LCE’s and intensity of inertial oscillations was found. Results of recent near-surface and water column current observations are presented in the paper.

The Loop Current Eddy (LCE) separation cycle energizes deep circulation in the eastern Gulf of Mexico, transferring energy from the surface intensified Loop Current (LC) to the typically quiescent lower layers. To document the generation and radiation of deep energy during this cycle, an array of 24 current and pressure recording inverted echo sounders (CPIES) is deployed in the region 89°W to 86°W, 25°N to 27.5°N with the intent to capture circulation near bathymetric features thought to be important for current-topographic interactions: Campeche Bank, Mississippi Fan, and West Florida Shelf. During the nearly two-year deployment, June 2019 to May 2021, three LCE separation events are observed, during which energy injected into the deep Gulf organizes into two distinct frequency bands (1/100 – 1/20 days–1 and 1/20 – 1/10 days–1). High-frequency variability dominates the array’s northwest corner in the vicinity of the Mississippi Fan. Wave properties are consistent with topographic Rossby Waves (TRWs) with wavelengths of 150 – 300 km. Their generation coincides with each LCE separation and is attributed to an upper-lower layer resonant coupling between surface meanders and the sloping topography of the Mississippi Fan. TRWs captured by the CPIES array will likely intensify as wavelengths shorten in steeper topography along propagation pathways towards the Sigsbee Escarpment, generating hazardous currents with the potential to disrupt oil and gas operations in the region.

A historical assessment of Karenia brevis in the western Gulf of Mexico

The Bay of Campeche, located in the southern Gulf of Mexico (GoM), is characterized by a semi-permanent cyclonic circulation commonly referred to as the Campeche Gyre (CG). Several studies documenting its upper layer structure have suggested a possible relationship between its seasonal variability and the wind stress, and that non-seasonal variability arises mainly from the interaction of the gyre with Loop Current Eddies (LCEs) that arrive in the region. Nevertheless, a partition of the contributions of these forcings to the circulation of the CG in a statistically consistent manner is still needed. This study examines the wind- and eddy-driven circulation with long-term numerical simulations of the GoM using the HYbrid Coordinate Ocean Model. Our results show that, in the absence of LCEs, the wind can sustain a seasonal-modulated circulation in the CG, confined within the upper 600 m. When considering LCEs, high fluctuations on the flow at intraseasonal time scales are imposed. We found that the LCEs influence the western Bay of Campeche circulation through two main mechanisms: (a) by decelerating and inhibiting the CG through a positive vorticity flux out of the bay, leading to reversals in the flow if LCE southward penetration is large, or (b) by strengthening the CG when a big cyclone, accompanying the LCE, enters the region. It is proposed that the second mechanism is responsible for inducing a net weak cyclonic circulation in the Bay in the absence of wind. Furthermore, past studies have shown that the CG behaves as an equivalent-barotropic flow, with topography acting to confine the CG to the west of the bay. In our modeling results, the role of topography manifests similarly among the different numerical experiments, resulting in closed geostrophic contours to the west of the bay that confine an upper-layer, nearly-symmetric, equivalent-barotropic CG.

Greatly increased concentrations of particulate iron were found within 1000 m of the bottom in the northwest Atlantic and eastern Gulf of Mexico (6 times and 3 to 4 times the average of shallower water, respectively) while only slightly increased concentrations were found within 1000 m of the bottom in the Caribbean and western Gulf of Mexico (2 times the average concentration in shallower water). These distributions agree with published light-scattering studies of deep ocean water, and are hemical evidence of a near-bottom nepheloid layer. It is concluded that the increase in the near-bottom concentrations of particulate iron is not a water mass effect, but arises from interaction of water with the bottom.

In August 2014, in situ measurements revealed an intense salinity drop impacting South Florida coral reefs, between Pulley Ridge (Southwest Florida Shelf) and the Florida Keys. The low salinity waters had a surface signal of 32 (down from 35.2) and extended over a 15–20-m deep lens. Satellite observations showed that this abrupt drop in salinity was due to a southeastward export of Mississippi River waters from the Northern Gulf of Mexico (GoM), revealing strong interaction between coastal and oceanic flows. Unlike previous events of marked long-distance Mississippi water export, this episode is not associated with Mississippi flooding conditions, which makes it a unique study case. We have developed a high-resolution (~2 km) comprehensive hydrodynamic numerical model of the GoM to study the conditions that controlled the 2014 Mississippi River water export episode. It is based on the Hybrid Coordinate Ocean Model (HYCOM) and assimilates remotely sensed altimetry and sea surface temperature observations, to ensure that the simulated upper-ocean is realistic. This regional model has a detailed representation of coastal physics (especially river plume dynamics) and employs high-frequency river discharge and atmospheric forcing. The combined use of the simulation and observations reveals a unique pathway that brought Mississippi waters first eastward along the Northern GoM continental shelf, under prevailing winds and the presence of an anticyclonic Loop Current eddy, then southward along the edge of the West Florida Shelf, before reaching the deep GoM. Unlike usually observed, the offshore advection of Mississippi River waters thus took place far from the Delta area, which is another specificity of the 2014 episode. Finally, in the Florida Straits, Mississippi waters were advected from the deep ocean to the continental shelf under the influence of both deep sea (particularly a cyclonic Loop Current frontal eddy) and shelf flows (wind-induced Ekman transport). The simulation, in tandem with data, thus helped analyze processes that are likely to affect the connectivity between reefs in the southern Florida region (Florida Keys, Dry Tortugas, Pulley Ridge) and remote areas (Mississippi Delta), as well as the local connectivity between neighboring reefs.