BLT 2022 edition - we're off!

We’re headed to sea again!
This time with the BLT (Boundary Layer Turbulence) project.

We MOD members Matthew, Bethan, Nicole, Arnaud, Helen, Gunnar, Sara, Isabella, Charlotte and Andrea have spent the past few days getting ready to head out on the RRS Discovery. Departing Southampton our group is headed for the Rockall trough to collect data on turbulence in a submarine canyon, aiming to solve the mysteries of bottom boundary layers and mixing and how that drives the upwelling limb of the meridional overturning circulation.

The meridional overturning circulation is the name for the slow pole-pole flow of water in the global ocean. Simplified, surface waters at the north and south poles gets cooled by the chilling temperatures and becomes dense enough to sink to the bottom. It then travels towards the equator as North Atlantic Deep Water (coming from the Arctic) or Antarctic Bottom Water (coming from Antarctica). These cold dense waters must eventually be brought back up to the surface to complete what is known as the Global Overturning Circulation. While we understand the formation processes of deep water fairly well, how it rises back up is still an open question. Since this circulation of the ocean is crucial for its ability to sequester heat and carbon, a better understanding of the processes involved is important for understanding climate change.

The Meridional Overturning Circulation (MOC). Figure 14.11 from Professor Lynne Talley’s book “Descriptive Physical Oceanography”.

For a long time oceanographers believed that the upwelling that closes the global overturning circulation occurred throughout the ocean interior. However, recent observations and theories suggests that upwelling is actually concentrated along sloping bottom boundaries in the ocean, such as steep continental slopes or walls of a submarine canyon, and that highly localized turbulence within thin (typically tens of metres thick) layers near the seafloor, known collectively as the bottom boundary layer, is what actually drives the upwelling of waters from the abyss.

Now that we are on the third (!) BLT expedition, we are much more experienced (though there are always new problems to run into). We have been setting up in Southampton and getting everything on the ship ready for science at sea, where we will spend several weeks profiling. Our main instruments are the epsifish - a microstructure profiler we built in house - and the fast CTD (conductivity, temperature and depth) profiler that can can be winched up and down rapidly for high resolution. 

By Monday last week everyone in our group had arrived: Matthew, Bethan, Nicole, Arnaud, Helen, Gunnar, Sara, Isabella, Charlotte, and Andrea. We are a good mix of experience levels (from student to professor) and disciplines (engineering and science). We received our cabin assignments and went for a walk around Southampton. 

Isabella and Andrea working on the winch

Sara and Charlotte tightening things up

On Tuesday we began mobilizing the equipment. We opened the containers that were shipped here from San Diego and those that were left here after the previous BLT cruise. We started setting up the lab, and the epsifish/FCTD boom was constructed.

Andrea on the winch.

Testing the epsi-fish

By the end of Wednesday the boom had cables attached to it that connected it to power and ran all the way to the lab. We had cameras and lights up too. Back on the science side, there were many debates about where and when to deploy moorings during the next couple of weeks. 

On Thursday the lab was almost finished being set up, the electronics were placed and tidying up had started. The new grad students on the cruise (Charlotte and Andrea) were frantically downloading the software they need while the ship was still on the dock. 

Look at the lab! So neat. This is he “before” picture. (There will be an “after” one too for comparison)

Sunny in Southampton, the Friday shirts were on.

Now it’s Friday and we have strapped down the lab and are ready to sail tomorrow! The lab looks professional, many Matlab scripts have been passed around, and the epsi was put into the water off the boom for the first time.

Wish us luck on the seas!

P.S You can read more on the BLT project here and one of the previous cruises here.

Written by Andrea Rodriguez-Marin Freudmann

A new way of measuring microstructure

This whole project is called the Boundary Layer Turbulence experiment, a name which might provoke the question, “How do we actually measure turbulence in the ocean?” Generally, the tried-and-true way is to use tiny sensing elements called piezoelectric beams. These output a voltage when they are deflected, just like the needle on a record player. We deploy them on torpedo-shaped instruments that descend slowly through the water. When the vehicle descends through turbulent water, the beams get deflected by the flow moving back and forth.

(Most of) our 30 hour timeseries in the lower 400 m of the ocean at the location shown in the map. Colors show the quantity epsilon in logarithmic units. Epsilon is a measure of ocean turbulence - it quantifies the dissipation of turbulent kinetic energy. Black contours show temperature.

Turbulence in the deep ocean evolves incredibly rapidly.  Here is a figure of turbulence and temperature data we collected over a 30 hour time period. As the tide causes flow up the canyon beneath us (at hour 23:00), you can see the temperature layers (black contours) rising. Colder water from the deeper part of the canyon is moving along the canyon floor into shallower depths and displacing the water above it.  As time goes on, the displaced water reaches a point just like a breaking wave on the beach, where it breaks.  A 200-m wave (20 stories high) is breaking, 2 km beneath the sea!  At that point, very strong turbulence starts (orange/red colors). With the next swoosh of the tide a similar thing happens with temperature surfaces rising and breaking, but this time it looks a little different - we sampled this flow for 2-1/2 tidal cycle and you can see how complex things are!

To resolve the rapidly changing turbulent environment of the bottom boundary layer, we want to sample it as fast as humanly possible. When we profile between the seafloor and 400 m off the bottom every 13 minutes as we have been, we can see the turbulent flows evolving. This allows us to make a pretty picture as shown but more importantly it gives us insight into the structures that arise from turbulence and their effects on the ocean circulation - which is our main goal.  We learned from the dye and our profiling mooring (previous posts) that water is moving up the canyon - and this turbulence is what allows it to happen. We seek to understand the details, and that requires densely sampled, precise measurements.

Our new “epsi-fish” profiler has several new innovations that make such rapid sampling of the deepest layers of the ocean possible.  First, we deploy the instrument from a long boom off the aft quarter of the ship, on a 3000-m slack data cable rolled up on our winch.  By keeping the instrument well away from the ship’s propellers, we can sample while keeping the ship in one place. This may seem trivial, but being able to dynamically position the ship to stay in the same place while profiling is a big deal. Without our long boom, the ship would have to steam away slowly while paying out cable, so the instrument stays safely behind the ship.  This would require more cable, and profiles would take longer because the cable would need to be hauled back in.  Plus, the ship would need to periodically turn to stay in the same place.  Staying in one place saves lots of time and keeps our profiles closely spaced in time.

MOD engineer Sara Goheen recovering Epsifish after a deployment. Here you can see the chute is in its "popped" position. Photo credit: San Nguyen

The second ingredient for fast profiling is a “pop the chute” mechanism.  Most profilers fall slowly all the way down, which is great for measuring turbulence all the way down, but in this experiment we mostly care about the bottom few hundred meters. We don’t want to wait the long time, about 45 minutes, that it would take to slowly go from the surface to the bottom and back.  Instead, we “skydive” down quickly with our drag screens clamped at the instrument’s sides, then deploy them with a command from the surface that triggers a servo actuator, releasing the chute and allowing us then to repeatedly profile the bottom 400 m every 13 minutes.

A third key enabling technology is a precise encoder on our sheave or pulley at the end of the boom which pulls the line steadily off the winch drum.  (The block in itself is a thing of great beauty, which is counterbalanced and has enough degrees of freedom to move with the ship and the seas without chafing the cable.  And for that matter, the completely electric direct-drive winch is another innovation that allows thousands of profiles to be taken at high speed with little service owing to its small number of moving parts relative to a hydraulic or geared winch). By carefully counting turns of the sheave, we know exactly how much line is out and can match that to the measured depth of the profiler.  Knowing how much line we have out is important for two reasons: paying out too much cable can create loops that can get tangled, possibly resulting in knots that if pulled through the sheave can cut the line and lose the instrument.  Second, the instrument detects the seafloor with an altimeter, allowing us to approach within 10 m of the bottom.  However, too much line out can cause the vehicle to hit the seafloor since all the slack must be hauled in before the vehicle begins to rise.

Our final bit of kit, humble as it may sound, is a “crash guard” for protecting the probes in the event of a bottom impact (the author was driving the winch just tonight when said crash guard was put to the test).  In essence a ring slightly proud of the probes, computational fluid dynamics (CFD) modeling was done to ensure that turbulence from the crash guard would not impact the sensitive readings from the shear probes.

Steve Woodward hooking Epsifish to bring it safely back on board. 

All of these pieces together took a huge amount of work by our engineers to develop in the last two years, but have made for a wonderfully successful operation during our long time at sea this summer. We are excited to use these tools in many experiments to come.

Boundary Layer Turbulence - the experiment begins!

To prepare for our exciting Boundary Layer Turbulence Experiment (follow along with the cruise blog) our team has been working around the clock to prepare three different tools for the experiment:

1. Moorings that, together with instruments from Kurt Polzin at Woods Hole, will measure the turbulence and mixing of cooler water with the warmer water above. These required the usual attention to detail and care in packing and planning each element. Beyond that, we are adding a new element for this experiment: one of our profiling moorings will have an epsi to make turbulence measurements.

2. We’ll be needing to sample the dye cloud we release near the sea floor as fast as possible. So we have added a fluorometer to our fast CTD. Because the fast CTD rises and falls so fast and samples on both up and down casts, we’ll be able to sample the dye cloud up to 5 times faster than we would by using the standard ship’s CTD rosette.

3. We’ve made huge changes and improvements to the electronics, software and body of our beloved epsi profiler. Many of the electronics and software changes are invisible, but they greatly increase the reliability and usability of the system. The mechanical changes are more visible and more crucial for the BLT experiment, which only focuses on the bottom few hundred meters of a 2000-m-deep ocean. So we have: i) added a longer, 3000-m cable, to allow us to sample deeper; ii) designed a cool new facility for pressure testing all of our sensors to ensure they don’t have odd effects at depth; iii) completely rebuilt the instrument to be heavier and longer which will make it more sensitive and better at reaching the great depths; and iv) contrary to most microstructure profilers which simply fall slowly the entire way to the bottom like parachuters, epsi now has the ability to “skydive” wherein it keeps its drag screens retracted until receiving a command from above, at which time it “pops the chute” and falls slowly in the lower part of the water column which we care about. With an altimeter, We’ve tested and retested all of these features for months and months in the lab, on the R/V Beyster, and in the 10-m pool at our lab, and think we are ready to go. Indeed, our initial tests yesterday looked great.

Wish us luck - we’re doing new things with new tools and are excited.

The latest and greatest epsi profiler getting assembled in the lab.

Drawing of epsi diving quickly to depth and then popping its chute to take measurements close to the ocean floor.

Ready.....set.......

Though there have been some (substantial) fieldwork efforts going on here and there during this last pandemic year, like most of the world most of us have been stuck closer to home. With things stabilizing a bit (at least in the US) we are starting up again with our normal level of crazy :) Heading out in June are two fun and hopefully exciting projects in two very different parts of the world, looking at quite different things.

• The Boundary Layer Turbulence project will take place in the far North Atlantic. The MOD team and colleagues from several other universities will be delving into the deep dark ocean with some new tools, to see what processes turbulently mix water at the very bottom of the sea, where the ocean rubs on the seafloor. Spoiler alert - those ethereal lurking mysteries may hold a clue for how the whole ocean overturns. Stay tuned for more from them as they set sail soon.

• Half a world away, the SUNRISE project will take place in the sweltering Gulf of Mexico. This one is looking at surface processes, specifically how strong fronts associated with Mississippi and other river outflows interact with wind-driven oscillations in the surface ocean, and how they conspire to move heat, salt and nutrients around the coastal ocean.

Stay tuned for dueling updates!!

BLT Test Moorings Recovered

Earlier this year in September we deployed two moorings in our backyard in the San Diego Trough. The goal of the mooring deployments was twofold: First, the BLT team wanted to practice deploying and recovering a mooring system we are borrowing from our colleague Hans van Haren from NIOZ in the Netherlands. The NIOZ mooring sports a large number of in-house-built high-precision temperature sensors with their clocks synched via an inductive pulse. Their measurements provide information on ocean stratification at high frequencies and high vertical resolution and can be used to study turbulence. Second, we wanted to test a new design for a MAVS mooring. MAVS are acoustic travel time current meters that, paired with high precision thermistors, can be used to directly measure buoyancy fluxes. The test deployment of the MAVS will tell us whether the mooring is designed stable enough to allow for the high precision measurements needed to directly observe buoyancy fluxes. Eventually, both of these mooring types will be deployed during the main experimental phase of BLT in the Rockall Trough in summer 2020.

Today, we successfully recovered both moorings and brought all instruments safely back on board. The weather conditions offshore were perfect for smooth mooring recoveries from the R/V Sproul, one of the smaller ships of the research fleet based in San Diego. Data analysis in the upcoming days will tell us how the moorings performed and whether adjustments are needed before the moorings will be deployed in the Rockall Trough. An exciting byproduct of the test deployment will be information on near bottom flow conditions, stratification, turbulence and buoyancy fluxes in the San Diego Trough, a region so close to Scripps Institution of Oceanography and yet not very well explored..

Spencer and Jeremiah getting the CTD ready.

Spencer pinging on one of the moorings.

Bethan on the TSE winch.

Bethan and Brian winding the thermistor chain onto the TSE winch.

The WHOI team inspecting the recovered MAVS instruments.

Jay preparing dinner.