San Nguyen

Mixing It Up

Oceanographers search for turbulent-driven upwelling at bottom of the ocean

The deep ocean is a mysterious and hard to reach place, and our understanding of what happens down there is very limited. The Multiscale Ocean Dynamics group at Scripps has set out, along with scientists and engineers from MIT and Woods Hole Oceanographic Institution, the University of Southampton, the University of Exeter, and France's IFREMER to try and observe turbulent processes right at the sea floor. We joined RRS Discovery in Southampton, England at the beginning of June ready for 40 days at sea in the Rockall Trough just west of Ireland. We are currently three weeks into the cruise, positioned above a steep-sided canyon, where we will be focusing our observations.

Gunnar Voet and Helen Dufel deploying the McLane Moored Profiler on the back deck of RRS Discovery along with the help of the crew and engineers from the National Marine Facility. Photo: San Nguyen.
Gunnar Voet and Helen Dufel deploying the McLane Moored Profiler on the back deck of RRS Discovery. Photo: San Nguyen

The coldest waters in the ocean are found down deep, but their journey begins at the surface in the polar regions. As sea ice forms, the water gets cold and dense, sinking to the bottom. From there, it travels towards the equator as North Atlantic Deep Water or Antarctic Bottom Water. It must then be brought back up to the surface to complete what is known as the Global Overturning Circulation. While we understand the formation process of deep water, how it rises is still an open question.

Starting with Walter Munk's seminal 1966 work, “Abyssal Recipes," it was believed that the upwelling occurred throughout the ocean interior. However, with more direct observations of mixing in the ocean and a combination of theoretical and numerical studies, another theory has emerged: that upwelling is concentrated along sloping bottom boundaries in the ocean. It is this theory that we are here in the Rockall Trough to test.

As a nod to Munk’s original paper, this experiment is called Boundary Layer Turbulence and Abyssal Recipes, or BLT Recipes for short. To measure the processes in the bottom boundary layer (BBL) we have come equipped with specially adapted instruments and techniques.

The Scripps team on the back deck of the RSS Discovery as it heads out from Southampton toward Rockall Trough. Left to right: Gunnar Voet, Matthew Alford, Helen Dufel, Sara Goheen, San Nguyen, Mike Goldin, Nicole Couto, Bethan Wynne-Cattanach. Photo: Marie-Jose Messias.
The Scripps team on the back deck of RSS Discovery. Left to right: Gunnar Voet, Matthew Alford, Helen Dufel, Sara Goheen, San Nguyen, Mike Goldin, Nicole Couto, Bethan Wynne-Cattanach. Photo: Marie-Jose Messias.

Two major components of this cruise are a dye release and tracer release, led by Marie-Jose Messias from Exeter. Together, these will allow us to track the water on timescales of days to months. We have already done the dye release, with initial results suggesting that there is movement of water along the sides of the canyon.

To confirm that the water is moving upwards across layers of constant density (diapycnal upwelling), and not just being stirred around within these layers, we need to measure the turbulence itself. We have with us a total of 5 different turbulence profilers, two of which were designed and made in-house in our lab at Scripps. These are the fastCTD and the epsi-fish. The fastCTD can travel at speeds of up to 5m/s, giving us highly resolved profiles of temperature fluctuations as well as conductivity, temperature, and pressure. The epsi-fish is our microstructure profiler which measures both temperature and velocity fluctuations simultaneously at very small scales. For this experiment, we really want to measure as close to the bottom as possible, so altimeters have been attached to both profilers to monitor our distance above the seafloor.

The epsi-fish ready to be deployed.
The epsi-fish ready to be deployed. The instrument plummets to around 400 meters off the bottom, at which point a chute is deployed, slowing the instrument down to 0.5 meters per second for sampling near the bottom. Photo: Matthew Alford

Finally, we will be deploying a series of moorings that will provide us with long term records of the velocity, temperature, and turbulent parameters in the canyon. The combination of instruments on these moorings will allow us to estimate the water mass transformation happening in the BBL in several different ways.

This is a very rewarding cruise to be part of, with turbulence measurements unlike any before this and potential implications for our understanding of the ocean’s role in the climate. But ground-breaking science doesn’t come without its fair share of breakages. We’ve had a few nose dives into the sea floor, some escaped dye and many other complications that are just part of doing work at sea. Luckily, our fabulous engineering team and the wonderful crew of the RRS Discovery have been on hand to fix any problems and get our experiment back on the road again.

We're only halfway through the cruise, so there is sure to be much more excitement over the coming weeks. You can follow along with what we find and learn more about the instruments we’re using on our blog

Wynne-cattanach is a PhD student in physical oceanography and Couto is a project scientist in the Multiscale Ocean Dynamics group at Scripps Oceanography

About Scripps Oceanography

Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.

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