The Gulf of Mexico is home to one of the world’s most energetic oceanographic phenomena – the Gulf of Mexico Loop Current. Reaching intensities of between 2 – 4 knots and measurable down to 1,000 m, the Loop Current System (LCS) also regularly sheds Loop Current Eddies (LCE).
An LCE is a highly energetic anticyclonic (clockwise) rotating ring of warm water, roughly 300 km across and 500 – 1,000 m deep, with current speeds of up to 4 knots. These break away from the extended Loop Current about every 8-9 months and slowly drift west-south westward towards Texas or Mexico at about 3-5 km per day.
When an LCE forms at the height of hurricane season, it has potential to fuel rapid intensification of hurricanes. This occurred in 2005, when an LCE separated in July, just before Hurricane Katrina passed over and “bombed” into a Category 5 hurricane.
In addition, there are deep eddies, which develop when upper current systems shift and meander. These deep eddies are offset from the upper currents and can propagate independently, with intensities of between 1-1.5 knots extending down to the sea floor.
But, despite 50 years of effort by the scientific community to understand the processes underlying the LCS, its behaviour remains unpredictable. To some extent, this is because of interactions with the deep eddies, which have been difficult to track from measurements near the sea surface. For this reason, a multi-year scientific study has been launched, led by the University of Rhode Island (URI). It includes a major deployment of Sonardyne’s Pressure Inverted Echo Sounders (PIES).
The LCS originates from the Yucatan Straits, from which a northward current flows into the Gulf of Mexico. Sometimes, the current bends eastwards, travelling just off the north coast of Cuba before exiting the Gulf through the Florida Straits. Episodically, though, at intervals ranging from six to 19 months, the LCS extends north, towards Louisiana, Mississippi and Alabama and along the Florida panhandle, before turning clockwise to flow south and then finally exiting east through the Florida Straits.
While in its ‘extended’ state, warm circulating eddies can break off the LCS into the western, northern and central Gulf. These eddies are so highly energetic that they regularly disrupt oil and gas operations. But, they’re also critical to the Gulf of Mexico’s oceanographic system, including its nutrient and food cycles and, most importantly, hurricane intensity.
Until now, most studies of the LCS have been limited to sea surface observations and satellite data, leading to a partial but incomplete understanding of how upper-ocean features, such as frontal eddies and meanders, are related to deep ocean flows. This has critically limited the ability to forecast behaviour of the LCS using numerical models. That is now changing. Following a recommendation by the US National Academies of Sciences, Engineering, and Medicine a long-term, US$ multi-million research program to plug the gaps in understanding and predicting the LCS is now underway. A core element of this scientific study is an array of seabed-mounted sensors, including Sonardyne’s PIES.
Pressure Inverted Echo Sounders
Sonardyne’s PIES were originally developed for the marine seismic industry to measure average sound velocity in the water column. They do this by transmitting a wideband acoustic pulse from their position on the seabed. This pulse is reflected off the sea surface and returns to the seabed where it is detected by the PIES. The resulting data enables two-way travel-time to be calculated. At the same time, an accurate measurement of depth (distance to the surface) is made using a highly accurate internal pressure sensor. Average water column velocity can then be calculated directly from the depth and travel time data.
Oceanographers, however, use PIES differently. Their goal is to derive important physical data, including the strength and direction of currents. This is based on the principle that there’s a strong correlation between two-way travel time (usually known as tau) and vertical profiles of temperature, salinity and density. As a consequence, where this profile has been derived from historical data, an empirical relationship can be derived, which enables the density profile to be inferred from tau. At a basic level, a laterally separated pair of PIES will, therefore, provide a vertical profile of velocity, and by deploying an array of PIES, local horizontal velocity and density fields can be mapped over the period of deployment.
URI has pioneered and refined the use of PIES for this purpose, including studies of some of the world’s most significant geostrophic currents, such as the Kuroshio Current off Japan.
While URI has a long history of developing its own PIES instruments, it decided to use Sonardyne’s PIES, as well as its own, following a comparison study off the coast of Oregon*. This was primarily because the study indicated that the Sonardyne PIES could generate similar accuracy data efficiently, which potentially enables longer deployments – and because of their telemetry capability. Sonardyne’s integrated high-speed (up to 9,000 bps) acoustic telemetry capability also enables remote reconfiguration of the instruments and retrieval of their data wirelessly to surface vessels, without interrupting the bottom pressure record. These capabilities are based on Sonardyne’s extensive expertise in underwater acoustics, signal processing, hardware design and custom engineering, which URI recognises have the potential to reinforce future PIES development.
This expertise was central to reconfiguring a standard PIES as a CPIES (Current PIES) which was needed for this project to allow for near-seabed current data to be harvested alongside the PIES pressure and tau measurements. It also delivers important data on deep eddy currents above the seabed/water interface. The reconfiguration involved connecting an Aanderaa Doppler current sensor to the PIES, which then serve as a battery pack and data logger for the current sensor, deployed 50 m above the PIES on a float. Combining the deep current observations with the deep pressure observations enable data from the array to be referred to a common reference surface. This uses an assumption that near-bottom currents and bottom pressure have a similar relationship to pressure and winds over the land.
The initial two-year project comprises an array of 15 URI CPIES, five Sonardyne CPIES and five Bureau of Ocean Management PIES. These are in an array, spaced 60 km apart, at depths down to 3,500 m in the area of the extended LCS. Initially deployed in June 2018, for a nominal two- year study, the units are fitted with batteries that can keep them powered for up to 36 months. This will allow for data gathering continuity in the event of a subsequent expansion of the program.
Mapping meanders and deep eddies
The project’s main aim is to define what conditions control the shedding of an LCE from the parent Loop Current. Specifically, the data collected by the array will be used to test a hypothesis that, as the Loop Current flows northward off the Campeche Bank, small meanders interact with deep eddies, which jointly strengthen. When the Loop Current then crosses a seabed feature called the Mississippi Fan (a seabed extension of the river’s sediment delta), squashing and re-stretching occurs, which results in interaction between the deep eddies and meanders in the near surface of the Loop Current. It is these extended vertical interactions that are thought to provide the trigger mechanism for instability in the water column causing shedding of an LCE from the Loop Current.
To do this, the array has been configured to provide deep data in previously unsampled regions, including Mexican waters, as well as filling observational gaps in critical regions where the Loop Current interacts with topography. The latter will provide critical insight into the coupling process between the deep eddies and the upper meanders. Together these will enable production of daily 3D maps of the circulation of the Loop Current and its eddy field at a scientifically meaningful resolution.
An interim data retrieval campaign, using acoustic telemetry, was successfully completed in September. While the principal purpose of this was to recover an initial three-month-long data set, one notable feature noted in the data was echoes, thought to be from fish, shrimp or squid. This has been seen in other studies carried out by URI and we believe is related to transport of nutrients by deep currents crossing from the deeper to shallower thermocline side around the periphery of the Loop Current or a passing LCE.
Looking forward, the present array will inform planning for a longer-term, 10-year campaign, which could see a substantially expanded array of PIES deployed into Cuban, as well as Mexican and US waters. The aim of this larger array would be to provide near real-time data as input into LCS forecasting models. Such forecasts have the potential to benefit a wide range of users, from oil and gas operations and hurricane forecasters to fishing and tourism. Furthermore, improving ocean modelling in the Gulf of Mexico has the potential to provide a standard for improving prediction efforts in other ocean basins.
About the Author
Randy Watts is Professor of Oceanography at the University of Rhode Island, one of the world’s leading academic institutions for oceanography. Prof. Watts’ research has focused on understanding mesoscale dynamics of major ocean currents using moored instrumentation including observations made by inverted echo sounders, pressure gauges, current meters and hydrography.