A novel underway profiler for high resolution ocean
Figure 1: RBR Research Scientist, Dr. Mathieu Dever, deploying the EcoCTD onboard the R/V Neil Armstrong(Photo credit: J. Doucette © Woods Hole Oceanographic Institute)
There has been a growing interest in observing the vertical dynamics of the ocean. Numerical modeling has uncovered important processes that could be responsible for vertical mixing, but they have only started to be observed in situ. With operationally challengingspatio-temporal scales of 0.1 – 10 km horizontally, 1 – 100 m vertically, and timescales of hours to days, these processes have been traditionally difficult to sample. In order to characterize the variability associated with submesoscale eddies and fronts, researchers are looking for ways to provide evidence of water exchanges between the upper ocean layer and ocean interior.
In 2018, this led RBR Research Scientist Dr. Mathieu Dever – who at the time was a post-doctoral researcher at Woods Hole Oceanographic Institution with Dr. Amala Mahadevan – to begin investigating submesoscale instabilities at ocean fronts. To identify these instabilities, they began looking at biophysical properties of water masses. At submesoscales, biological and physical properties occupy similar timescales. As a water mass is subducted it will maintain the same biological profile, meaning that biological properties – such as chlorophyll, backscatter, and oxygen – can be used as semi-conservative tracers to identify vertical mixing.
So, they knew what needed to be measured but were stuck on how. At sea, CTD rosettes take discrete samples of the water column, but stations are often far apart and cast times are slow, meaning that submesoscale features are often missed. A profiler that could take high resolution measurements at these difficult to sample spatio-temporal scales was the answer.
To meet the observational needs for submesoscale motions, Drs. Mahadevan and Dever determined that a profiler would need to be able to (1) profile the upper 500m of the ocean, (2) provide vertical profiles at sub-kilometer lateral resolutions within a few minutes, and (3) measure both bio-optical and physical properties simultaneously. With these requirements in mind, they developed the EcoCTD, a fast, biophysical underway profiler to observe submesoscale features.
The profiler was built to offer a lighter weight and cheaper alternative to similar profilers on the market. The objective was to take advantage of the typically underutilized steaming time between stations, thus creating a profiler that could be used underway with little to no impact on other science operations onboard.
What is the EcoCTD?
Figure 2: Design of the EcoCTD(Image credit: Dr. Mathieu Dever)
The EcoCTD is a lightweight profiler that connects a CTD with bio-optical sensors (Figure 2). Built around an RBRconcerto3 CTD, the EcoCTD accommodates one of two potential oxygen sensors, both featuring a fast time constant (~1 s): the RBRcoda T.ODO or the JFE RINKO. Additionally to the oxygen, a WetLabsECOPuck is used to measure chlorophyll, backscatter, and fluorescence.
The EcoCTD also includes two lead collars to increase the weight of the probe. While a lightweight sensor was an important design consideration, Dr. Dever also needed the sensor to be rapidly free-falling to flush the inductive conductivity cell, enabling high-quality measurements of the water column. With line drag impacting free-falling velocity, the profiler needed to be heavy enough to maintain its speed as it went deeper in the water column. With the lead collars, the EcoCTD weighs 13 kg in air, heavy enough to overcome line drag, but light enough to be handled by one person. However, these weights are also customizable to adjust fall rate to the sampling’s objectives.
The RBRconcerto3 CTD was an attractive base for a number of reasons. With falling rate a key consideration, its cylindrical shape was appealing. The fast thermistor and accuracy of the profiler were also important given the speed at which it falls through the water column – up to 3 to 4 m/s. The logger provides data from all sensors synchronously, thus streamlining post-processing of the data. Finally, the low power consumption of the CTD, which runs on 8 AA batteries, simplifies shipping, handling, and battery replacement.
How does it work?
The EcoCTD was designed to take full advantage of ship time. First, the profiler is attached to a lightweight winch and thrown over the side of the boat (Figure 3), free-falling to the desired depth as the ship is underway. It is then reeled back up and once the profilerreaches the surface it can be dropped again, beginning the next profile. While a typical profile to 500 m takes approximately 12 minutes, the speed of the ship can change the temporal and lateral resolution depending on a researcher’s needs.
Figure 3: Illustration of the EcoCTD's sampling method while underway(Image credit: Dr. Mathieu Dever)
One early requirement for Dr. Dever was to ensure that the movement of the ship, in particular heaving, was not impacting the downcast. Dr. Dever confirmed this separation onboard the R/VNeil Armstrong, attaching motion sensors to both the ship and the profiler to ensure that their respective motions did not correlate. No significant cross-variance was found, meaning that the EcoCTD was indeed decoupled from the ship’s motion and therefore free-falling.
In 2018 and 2019, Dr. Dever tested the EcoCTD as part of an ONR-funded project (CALYPSO), in an area of the Mediterranean Sea where the inflow of colder, fresher water from the Atlantic Ocean meets warmer, saltier water from the Mediterranean. These two different water masses were found to have distinct signatures through temperature and salinity data; however, considering these data alone makes identifying submesoscale mixing challenging. Oxygen, chlorophyll, and backscatter all illustrate mixing, with the backscatter data in particular highlighting a subduction event, with a “tongue” of water being brought down, ventilating the ocean interior. Using the additional bio-optical measurements provided by the EcoCTD, Dr. Dever was able to see this submesoscale mixing take place.
Figure 4: Results from the EcoCTD along the Almeria-Oran front, illustrating two distinct water masses through salinity and temperature data. Oxygen, chlorophyll, and backscatter data also provide evidence for subducted water and vertical, submesoscale mixing.(Image credit: Dr. Mathieu Dever)
Where do we go from here?
Future work includes modifying the profiler to allow for even more measurements; adding or removing sensors to sample different parameters or modifying the weight of the profiler – by removing its lead collars – to account for different water column depths or lateral resolutions.
The EcoCTD is currently an open-source project, meaning that you can make one yourself. Learn more about how you can build your own EcoCTD – and even tailor it to your specific needs – at the EcoCTD Open Project website.
M. Dever, M. Freilich, J. T. Farrar, B. Hodges, T. Lanagan, A. J. Baron, and A. Mahadevan. EcoCTD for Profiling Oceanic Physical-Biological Properties from an Underway Ship. Journal of Atmospheric and Oceanic Technology, 37(5):825-840, 2020. doi: 10.1175/JTECH-D-19-0145.1