Starting in June 2014, ocean currents in the Gulf of Mexico became uncharacteristically detrimental to offshore operations. An unprecedented event of extreme Loop Current activity, not only in spatial and temporal extent but also in current intensity and vertical structure, was observed and documented for a year and half. The unusual activity was attributed to either increased inflow from the Caribbean Sea through the Yucatan Channel or amplification caused by local frontal cyclonic eddies. Anticyclonic eddies that shed from the Loop Current during this ongoing event maintained relatively large structures following their initial detachments and exhibited reconnections with the Loop Current within days to weeks of separation. This process was repeated by several eddies more than twice. In situ measurements indicated frequently reoccurring ocean current intensities upwards of 3.0 knots sustained for several weeks at a time with peak observed amplitudes over 4.0 knots. In this study, we present data collected from ADCP measurements and an extensive network of drifting buoys in correlation with daily frontal analyses based on all available in situ data and satellite imagery – both public and proprietary. The Loop Current and each eddy shed during this period is discussed with emphasis on structure, intensity, and migration pattern. Investigation was made into a possible link to El Niño and/or other large-scale air-sea phenomena and teleconnections; however, the root cause(s) for the circulation hyperactivity in the Gulf, if any, remains unknown. The study concludes with lessons learned and a brief description of long-term forecast modeling efforts to mitigate risk and enhance safety and efficiency in offshore operations.
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 unique circulation characteristics of the Gulf of Mexico (GOM) pose a significant threat to the safety of offshore oil and gas operations pertaining to installation of new production systems, drilling, and maintenance of existing offshore infrastructure. Operators in the area rely on realistic estimates of the location of the sharp fronts (regions of high horizontal shear) characteristic of the warm-core Loop Current (LC) and Loop Current Eddies (LCEs) and smaller cold core cyclonic eddies (CEs) to estimate working windows. However, locating these features is not a trivial undertaking because it requires review and analysis of multiple observational and model data sources. In this paper, we describe the frontal analysis (FA) methodology used to define such features. This technique has been accepted by industry as the best representation of the continuous front that delineates the most distinct current gradients defining the sharp outside edge of the LC/LCEs. Definition of LC/LCE features is accomplished by defining the position and extent of the associated front, defined as the 1.5 knot current threshold. This involves performing an analysis of satellite imagery (snapshots and composites) and satellite-derived products (altimetry and geostrophic velocities), in-situ measurements (i.e., public and proprietary drifting buoys, rig-mounted ADCPs, vessel-mounted ADCP transects, etc.), and previous feature location/progression analyses, all weighted appropriately. The resulting front is then used to map these features and provide actionable information regarding their surface current velocities, migration speed and direction, angular rotation, and axis orientation. Systematic analysis of the behavior of the LC system since 1984 has resulted in a unique oceanographic dataset comprising the location and evolution of LCEs. By incorporating frequent deployment of aircraft-deployed, satellite-tracked, drogued drifting buoys and the analysis of their track data, the FA provides the most accurate and extensive near-real-time information available regarding the location and intensity of currents affecting offshore operations. WHG’s FA product is commonly accepted throughout the industry and within the scientific community as the closest to ground truth for the placement of the major oceanographic features in the region. Understanding the details of this methodology will provide the basis for comparison of observations with new numerical modeling efforts (under development as part of the NASEM UGOS program) to effectively assess the accuracy of nowcast results and will eventually lead to better model forecasts for the benefit of various stakeholders.
The Gulf of Mexico's unique circulation characteristics pose a particular threat to marine operations and play a significant role in driving the criteria used for design and life extension analyses of offshore infrastructure. Estimates from existing reanalysis datasets used by operators in GOM show less than ideal correlation with in situ measurements and have a limited resolution that disallows for the capture of ocean features of interest. In this paper, we introduce a new high-resolution long-term reanalysis dataset, Multi-resolution Advanced Current Reanalysis for the Ocean – Gulf of Mexico (MACRO-GOM), based on a state-of the-science hydrodynamic model configured specifically for ocean current forecasting and hindcasting services for the offshore industry that assimilates extensive non-conventional observational data. The underlying hydrodynamic model used is the Woods Hole Group – Tendral Ocean Prediction System (WHG-TOPS). MACRO-GOM is being developed at the native resolution of the TOPS-GOM domain, i.e. 1/32° (~3 km) hourly grid for the 1994-2019 time period (25 years). A 3-level downscaling methodology is used wherein observation based estimates are first dynamically interpolated using a 1/4° model before being downscaled to the 1/16° Inter-American Seas (IAS) domain, which in turn is used to generate time-consistent boundary conditions for the 1/32° reanalysis. A multiscale data assimilation technique is used to constrain the model at synoptic and longer time scales. For this paper, a shorter, 5-year reanalysis run was conducted for the 2015-2019 time period for verification against assimilated and unassimilated observations, WHG's proprietary frontal analyses, and other reanalyses. Both the frontal analyses and Notice to Lesses (NTL) rig mounted ADCP data was withheld from assimilation for comparison. Offshore operations in the GOM can benefit from an improved reanalysis dataset capable of assimilating existing non-conventional observational datasets. Existing hindcast and reanalysis model datasets are limited in their ability to comprehensively and reliably quantify the 3D circulation and kinematic properties of the main features partly because of limited assimilation of observational data. MACRO-GOM incorporates all the advantages of available HYCOM-based reanalyses and further enhances the resolution, accuracy, and reliability by the assimilation of over three decades of WHG's proprietary datasets and frontal analyses for continuous model correction and ground-truthing. The final 25-year high resolution dataset will provide highly reliable design and operational criteria for new and existing infrastructure in GOM.
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