CHAPTER ONE

Introduction

One of the most significant, energetic, yet not well understood oceanographic features in the Americas is the Gulf of Mexico LCS, which consists of the LC and the LCEs it sheds. The LC originates as the Yucatan Current moves northward into the Gulf of Mexico. It then bends to the east and exits through the Florida Straits. The current is typically described in two states (see Figure 1.1): (1) the “retracted state,” wherein the current turns rather abruptly to the east and travels just off the north coast of Cuba before directly exiting the Florida Straits, and (2) the “extended state,” wherein the LC continues north (as far as approximately 28° North Latitude) before turning anticyclonically (clockwise) to the east and then south along Florida’s west coast before turning east again (left) to exit the Florida Straits.

Current intensities of 2–4 knots have been observed within the LC, and LC structures can be sensed down to depths of about 1,000 meters. In the extended state, the LC will spin off warm-core, anticyclonic eddies, roughly 300 kilometers across and 500 to 1,000 meters deep, that can maintain current speeds of up to 4 knots within the circulations even after disconnecting from the LC. These energetic LCEs generally propagate to the west (see Figure 1.1, position 3) and often threaten oil and gas operations in the northern, central, and western Gulf. Smaller, more measurable, and significantly cyclonic cold-core eddies have been observed to spin from the edges of the LCE and the extended LC itself (Rudnick et al., 2015). Just like hurricanes, tropical cyclones, and strong winter storms, LCEs are named (e.g., the first three LCEs of 2016 were named Michael, Nautilus, and Olympus). The LCS sees an extended state with significant eddy shedding occurring at intervals ranging from a few weeks to as long as 19 months, with an average shedding period of 8 to 9 months (Hall and Leben, 2016). A more detailed description of LCS processes is included in Appendix A (Sturges and Lugo-Fernández, 2005).

Understanding the dynamics of the LCS is fundamental to understanding the Gulf of Mexico’s full oceanographic system and vice versa. Hurricane intensity, offshore safety, harmful algal blooms, oil spill response, the entire Gulf food chain, shallow-water nutrient supply, the fishing industry, tourism, and the Gulf Coast economy are all affected by the position, strength, and structure of the LC and associated eddies. Although the LC is the dominant physical oceanographic feature in the Gulf, the episodic northward intrusion into the Gulf of Mexico and the associated eddy shedding are not clearly understood (Sturges and Lugo-Fernández, 2005).

The scientific community has spent more than 50 years trying to understand LC dynamics and the variability of associated eddy shedding. Most of the scientific observations of the LCS have been limited to surface features in the Gulf such as sea surface height (SSH) and sea surface temperature (SST), largely from satellite observations. Routine observational data assimilated into numerical models of the LCS, therefore, have been largely limited to these surface data.

There have been a number of full water column “field studies” in discrete parts of the LCS. However, they are limited in geographic scope and are not long term in nature. While they have advanced the general description of the LCS and provided valuable insight on forcing mechanisms, there are still significant gaps in understanding the formation, variability, and structure of the LCS and its interaction with other dynamic processes in the Gulf or the Gulf’s varying bathymetry. As a result, numerical predic-

tions of the LCS spanning more than a few days have not improved enough to be used for operational or strategic planning and emergency response. Recent advances in observational technologies for measuring the Gulf’s subsurface waters over significantly larger spatial scales and/or over longer time periods will relieve some of that constraint.

The purpose of this consensus study is to recommend a strategy for addressing the key gaps in general understanding of LCS processes (summarized in Box 1.1), thereby leading to a significant improvement in predicting LC/LCE position, evolving structure, extent, and speed. This will increase overall understanding of Gulf of Mexico circulation and promote safe oil and gas operations and disaster response in the Gulf of Mexico. This strategy includes advice on how to design a long-term observational campaign and complementary data assimilation and numerical modeling efforts.

As reflected throughout the report, a combination of efforts needs to occur in conjunction to improve understanding and prediction of the LCS:

• Incorporation of existing federally, internationally, and privately funded observational and modeling efforts;
• Collaboration with U.S., Mexican, and Cuban government agencies and/or institutions as well as with private industry; and
• Investments in new observations, new technologies, and improved data assimilation and ocean–atmosphere modeling for the Gulf of Mexico.

THREE CASES FOR ADVANCING THE UNDERSTANDING OF THE LOOP CURRENT SYSTEM

Deepwater Horizon

Improved predictive skill in forecasting the LCS could benefit those who respond to oil spills. For example, the immediate response decisions following the Deepwater Horizon oil spill event were hampered by a lack of real-time in situ observations in the deep ocean (below 1,000 meters) in the northern Gulf of Mexico at the time of the oil spill (Liu et al., 2011). Such observations are necessary to provide first responders with information on the transport and dispersion of the subsurface oil plume, as well as how and what fraction of the spilled hydrocarbon mixture reached the surface. The lack of deep water observations failed to inform data assimilation schemes and, hence, numerical circulation models. Deep water observations, if properly accounted for in models, could have improved model prediction skill and response decision makers could have had a better understanding about the interaction of the LCS with the oceanographic processes of the continental shelf in the northern and eastern Gulf of Mexico.

The impact of the Deepwater Horizon disaster could have been far more environmentally and economically damaging if the LCS had been in its extended position and hence present in the immediate vicinity of the spill. As shown in Figure 1.2, the surface oil trajectory did reach the upper extent of the LC on May 17, 2010, but immediately thereafter, the LC shed an eddy, effectively breaking a direct connection between the region of the spill and the West Florida Shelf and Florida Straits.

Hurricane Katrina

The northward instrusion of the LC (or lack thereof ) as well as the position of LCEs play an important role in hurricane intensification in the days prior to landfall. Hurricane Katrina’s maximum sustained winds increased to 173 mph on August 28, 2005, as the tropical storm moved over and along the LC and LCE vortex (see Figure 1.3), which were the areas with the highest upper ocean heat content in the Gulf of Mexico at that time (Scharroo et al., 2005). Near-real-time information from satellite altimeters was used to monitor the LC and LCEs. Their presence can be delineated by SSH greater than 17 cm (about 7 inches) in elevation, as shown by the black contour line in Figure 1.3. As Katrina moved into the Gulf, it intensified from a Category 3 storm to a Category 5 storm while it traversed the LC for just 1 day. Rapid intensification of tropical cyclones by the oceanic heat content in the LC and LCEs (Shay et al., 2000;

Zambon et al., 2014) poses a significant threat to offshore drilling and production in the Gulf (Kaiser and Yu, 2010), not to mention the impact on coastal communities, their inhabitants, their natural resources, their economy, and their survival.

Katrina is not an isolated example of tropical cyclone–LC interaction. Hurricane Ike is another good example. The impact of the LC on Hurricane Ike has been shown and predicted by a fully coupled atmosphere-wave-ocean model (see Figure 1.4). Ike was initially weakened by its first landfall over Cuba and re-intensified by going over the LC and an LCE in the western Gulf before making the second landfall near the Galveston

Bay on September 13, 2008. The fully coupled model showed the extreme wind, high waves, ocean currents, and storm impacts near landfall, which highlights the importance of the state of the ocean conditions and the atmosphere–ocean coupling process in predictions of hurricanes and their full impacts (Chen and Curcic, 2016).

Impact on Industry: Recent Highly Active Years

Prolonged periods with the LC in its extended state can be devastating for oil and gas operations, as not only do the strong currents cause additional fatigue on infrastructure, they present significant operational safety concerns for the operator, causing activities to be restrained or even shut down at significant cost. The 18-month period from June 2014 to December 2015 is of particular interest because of the uncharacteristically active LCS behavior and its correspondingly significant detrimental effects on offshore oil and gas operations. This hyperactivity event affected sites across the entire northern, central, and even some portions of the western Gulf of Mexico for prolonged periods with strong and highly variable current velocities. The precise cause(s) for

this level of activity remains unclear. The LC extended well north of 27°00’N and occasionally crossed 28°00’N into the vicinity of the Mississippi River Delta as it evolved. Anticyclonic eddies that shed from the LC during this event—Eddies Lazarus, Michael, Nautilus, and Olympus—maintained relatively large structures following their initial detachments, and exhibited reconnections, in some cases repeated, with the LC within days to weeks of separation. Both Nautilus and Olympus also exhibited split circulations, resulting in the formations of Eddies Nautilus II and Olympus II, respectively. At the same time, in situ surface drifter measurements taken by oil and gas service providers indicated frequently recurring ocean current intensities upward of 3.0 knots that were sustained for several weeks at a time, with peak observed amplitudes over 4.0 knots throughout the 18-month time period (Sharma et al., 2016). During this period, the Gulf of Mexico Research Initiative (GoMRI) was supporting a number of field studies that further added to the unusual wealth of observations (e.g., CARTHE [Consortium for Advanced Research on Transport of Hydrocarbon in the Environment], DEEP-C [Deep Sea to Coast Connectivity in the Eastern Gulf of Mexico], C-IMAGE [The Center for the Integrated Modeling and Analysis of the Gulf Ecosystem]).

Most operators in the Mississippi Canyon, Atwater Valley, Green Canyon, and Walker Ridge lease areas (see Figure 1.5) observed significant delays and downtime due to the adverse impact of elevated currents on critical current-sensitive operations, including, but not limited to, platform installation, hull wet tows, spar upending, drift-ins, riser installation, suction pile installation, unlatching the rig, subsea tree installation, pipe laying, remotely operated vehicle (ROV) deployments, and dynamic positioning (Sharma et al., 2016). Chevron’s Big Foot Tension Leg Platform site was one of the many affected by ocean currents. Nine out of the rig’s 16 tendons that anchor it to the ocean floor parted. As a result, the project, valued at $4 billion, was delayed (at the time) indefinitely.¹

This 18-month period of heightened LC activity (and thus additional monitoring), along with relatively rich data sets collected by GoMRI and industry partners during the same period, present a unique opportunity for analyses of models, and will be further discussed in Chapter 3.

___________________

¹ Information source: https://www.offshoreenergytoday.com/chevron-no-big-foot-oil-till-2018.

CHARGE TO THE COMMITTEE

This report was prepared by the Committee on Advancing Understanding of Gulf of Mexico Loop Current Dynamics in response to a request by the GRP of the National Academies of Sciences, Engineering, and Medicine (the National Academies) to design a suite of activities needed to better understand, model, and forecast the LCS (see Box 1.2). The committee’s goal was to provide the GRP with a set of actionable recommendations on a long-term observational set of campaigns, with complementary efforts in data-assimilative numerical circulation modeling necessary to improve the general understanding of LCS processes, thereby leading to significant improvements in both short-term and long-range predictability of LC/LCE position, evolving structure, extent, and speed. We seek to aid the GRP in investing research funds that will ultimately allow modelers to

• Improve predictive skill in forecasting the LC and/or LCE current speed, vertical structure, and duration out to a forecast period of a few days to 1 week
• Improve predictive skill in forecasting the extension of the LC (location and duration) and LCE propagation out to a forecast period of approximately 1 month
• Improve predictive skill in forecasting an eddy shedding event from an extended LC out to a forecast period of approximately 3 months

STUDY APPROACH AND METHODOLOGY

The Committee on Advancing Understanding of Gulf of Mexico Loop Current Dynamics was formed in late 2016 and completed its work over the course of 9 months. It held three meetings, two of which were used to gather input from numerous agencies and institutions on the history and current state of LCS research, observations, modeling efforts, emerging technology, and international efforts. Report recommendations were developed and agreed on at the third meeting. Subcommittees of the committee responsible for specific topics within the study consulted regularly during the study period by phone and by email to exchange information.

Report Organization

This report begins with a brief primer on the LCS. In Chapter 2, it summarizes the current understanding of LCS dynamics, the state of observational data and technology availability, and numerical modeling and data assimilation limitations. In Chapter 3, the report then identifies gaps and makes recommendations to fill those gaps in the areas

of observations, observational technology, data assimilation and modeling, and scientific analyses. Finally, in Chapter 4, the committee recommends features and boundaries to the GRP regarding a solicitation to undertake an LCS observation and model improvement campaign during the ensuing decade.