We’ve come a long way. Fifty years ago we were just entering the era of modern physical oceanography – meaning that was when we for the first time were able to accurately and systematically see what the ocean was doing, i.e. measure ocean currents. Arrays of current meters could map out currents in defined areas and clusters of floats could reveal patterns of fluid motion and how water spreads. Two decades later NASA and partners started orbiting satellites that could map out surface currents around the entire globe. Argo floats, envisioned in their original form as a tool for mapping ocean currents at depth, have evolved to become an incredible global hydrographic resource. But sustained study of ocean currents at depth remains a major challenge. Mapping currents everywhere is out of reach, but with acoustic Doppler current profilers (ADCP) we can reach 600-1200 m depth. By putting ADCPs on merchant marine vessels we can study oceanic variability along routes of interest. Thanks to these operations we have learned much about Gulf Stream variability, and currents across the subpolar North Atlantic, and in the Antarctic. But profiling is limited to the upper ocean, the deeper ocean is still out of reach. I am convinced we can do better. Why and how might we go about this?
Deep ocean currents live a very different life from those of the upper ocean. Even though at depth, they must be driven by processes originating at the surface since there is no source of potential or kinetic energy in the abyss. Winds and upper ocean instabilities can set up pressure imbalances that can induce motions at depth. Deep convection and overflows can result in flows along bathymetric slopes that may extend over great distances. The weak stratification of the deep waters also means that their movements may be constrained by the shape of the ocean bottom. One can imagine that given enough degrees of freedom the deep mean field might in some sense be rather complex reflecting the topography of the ocean bottom. The only or at least an effective way to resolve the horizontal structure of deep currents is to scan their structure at high horizontal resolution. How might we do this?
There are three techniques that might be able to help, a deep reaching acoustic Doppler current profiler (ADCP), the acoustic correlation current profiler (ACCP), and the electric towed transport meter (TTM). The 38 kHz ADCP can under favorable conditions reach to 1200-1500 m depending upon the availability of backscatterers. These depths reach beyond the zero-crossing of the first baroclinic mode but not much more making it difficult to extrapolate the profiles to greater depths. But this might be worth further study. Although the 38 kHz ADCP has been around for a long time it has seen only limited use on UNOLS vessels, and even less so on merchant marine vessels. There is much to be learned here. Due to the high attenuation of sound at 38 kHz it is not feasible to increase acoustic power to reach to greater depths. Going to a lower frequency might be possible, but that would make for a very large cumbersome package. This will never happen if research vessels are the only potential customers since the development and production costs would be far too high for such a small market. There might be a way to get around these limitations but that will have to be a discussion for another day.
The other acoustic technique, the ACCP, should be reexamined. It was developed in the early 1990s by RDI (Bradley and Heines, 1991). It’s a rather small package, comparable in size to 75 kHz ADCP, but is structured entirely differently. It has a pinger and a set of receiver elements mounted on a flat plate. If I understand the concept right, the instrument transmits two brief signals that spread out spherically under the vessel. Backscatterers in the water column echo back the signals to the receiver elements. The receiver software then compares the two signals from the receiver elements to determine the best lateral shift of the second signal to match the pattern of the first signal. I’m told that it requires (or did at the time) extensive processing to determine the best match between the two. That shift is the relative velocity at that echo range. The spherical spreading means that the vertical resolution is limited, but for deep water profiling that is less of an issue and may even be an advantage since it would employ a larger volume of backscatterers. While experiments with the ACCP in the late 1990s did not fare so well (Griffiths et al, 1999), I have a feeling the time might be ripe for a fresh look at the ACCP concept. The hardware costs may be substantially less than for the ADCP since it has the potential to trade resolution for range. Just imagine MM vessels equipped with these to regularly and systematically scan the deep ocean velocity field – this is why I like to think of them as satellites orbiting earth at sea level.
The TTM takes an entirely different approach. It measures the vertically-averaged cross-track velocity of the water column along a ship’s track by measuring the minute electric potentials induced in the water column by its motion through the earth’s magnetic field. Thus, the electric potential in the direction of vessel motion is proportional to the vertical component of the magnetic field times the difference in cross-track velocity of the ship and vertically-averaged velocity of the ocean, with the former known from the GPS. In its simplest form the TTM measures only the cross-track component, which is the important one from a oceanic transport point of view. With additional engineering it one can also get the vertically averaged velocity in the direction of travel. The beauty of the TTM is that with a simple davit and winch system it can be deployed and retrieved in much the same way merchant marine vessels deploy continuous plankton recorders. Sadly, NSF did not fund this initiative when it was proposed some 10 years ago.
These are all potentially powerful tools for scoping out the deep ocean velocity field. While they may have only limited appeal for the occasional project on UNOLS vessels they could revolutionize our ability to probe the deep ocean when deployed on merchant marine vessels in regular traffic. The MM has an unmatched presence on the high seas. Why not develop a partnership with it, why not make working with the MM an integral part of our science, our culture if you will. Importantly, all three techniques lend themselves to a high level of automation making them cost-effective tools for sustained operation. So equipped, merchant marine vessels would enable us to probe the deep ocean on a systematic basis, see what it is doing, on what space and time scales, how action at depth might correlate with the upper ocean, and of course the role of bottom topography in shaping circulation at depth. There should be an X-Prize to encourage fresh thinking on technologies optimized for MM-based science!
Bradley, S. E. and K. L. Deines, 1991. Acoustic Correlation Current Profiler. IEEE J. Oceanic Engineering, 16(4), 408-414
Griffiths, G., S. E. Bradley, and G. Murdock, 1999. Factors affecting the performance of a shipboard acoustic correlation current profiler. IEEE 6th working conference on current measurement.
https://www.comm-tec.com/Library/Technical_Papers/RDI/ACCP_noise_paper.pdf