Mixing It Up

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.

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.

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.

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 bltrecipes.wordpress.com. 

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