State vs. Action.
These two little words refer to describing the present condition of a system, its state, and how the system is changing, its action. In physical oceanography we often talk about the state of the ocean, much less about its action. There’s a reason for this.
Modern oceanography as we know it has its beginnings at the turn of the last century with the invention of the Nansen bottle, the reversing thermometers, and the Knudsen method to determine salinity through titration. The Winckler method enabled us to determine dissolved oxygen levels in seawater samples. Thanks to these developments physical oceanography made huge strides in mapping the distribution of heat, salt, and oxygen across the global ocean. Fortunately, for a first description of the state of the ocean, it didn’t matter that the observations might be years or decades apart.
In those early years we also learned how to estimate upper ocean currents and transports thanks to the astonishingly simple relationship between currents and pressure, the geostrophic balance. It only required an assumption that the deep ocean was at rest. While we knew this wasn’t the whole story, its ease of use gave it a seductive beauty. We relied on it completely until the 1970s; we had no choice for we did not have the tools to measure currents. I still find it amazing that by the 1960s we could shoot ICBMs from submerged submarines but we were still dependent upon Nansen bottles and reversing thermometers. A lot has happened since then.
Today we deploy current meters to measure transports in boundary currents and RAFOS floats to map out water movements. Thousands of Argo floats give us a continually updated view of the state of the ocean. Satellites map out surface currents across the entire global ocean. In short, we can today provide a regularly updated accurate global view of the state of the upper ocean – this is a tremendous development. But this knowledge does not project well into the water column, and it gets worse the deeper we look. Our ability to measure currents at depth is extremely limited. It has been suggested that we shouldn’t bother - perhaps reflecting concerns about cost; measuring currents at depth is indeed very costly with existing technologies. Or perhaps as modeling skills improve we won’t need to? Regardless, given all the technologies we can imagine today it is probably safe to say that we will never be able to construct accurate synoptic maps of currents at depth. The sad thing is that this perception has clouded our thinking about other possibilities. I’m thinking of course of how the merchant marine can help us.
Think about it, one single acoustic Doppler current profiler on a MM-vessel can scan currents along its path with ~10 m vertical and ~1 km horizontal resolution! You can’t get this level of detail about the ocean by any other means. And repeatedly depending upon a vessel’s schedule. So why is it that this methodology hasn’t found favor with the oceanographic community? True, it can’t map currents, but it can resolve the structure of currents and their transports over and over again with unmatched accuracy. I’m thinking particularly of the ADCP’s horizontal coverage since the dominant scale in the ocean regardless of timescale in measured in a few 10s of kilometers, what is known as the radius of deformation. You’d think that this ability to scan ocean activity from 1 to 1000 km scales with cm/s accuracy would have considerable appeal, but not. A combination of factors seems to contribute to this lack of initiative.
First, for many decades our research vessels were enough for the ocean studies we had in mind, and for many tasks they will continue to be absolutely essential. But today, as we become increasingly concerned about changes in ocean circulation on longer timescales, floats, current meters and research vessels become a poor and costly match to the task; this is where the MM could help.
Second, while we are aware of the MM and have worked with it as a limited extension to our present needs, we have never seen MM-vessels as an asset to partner with on a regular basis. Consider, they operate in all oceans. Many of them operate on tight schedules between specific ports, which is exactly what is needed for studies of gyres and their boundary currents. To enable this, we need to develop a framework, an institutional culture, for teaming up with the MM. We know that it is more than willing to help, but we need to take the initiative. The reason we haven’t may in part be driven by a lack of instrumental resources, and this leads me to my next point.
Third, with a few creative initiatives we could enhance the tools we have today to scan currents at greater depths and thus get a handle on thermocline and deep ocean circulation (see my Jan 15, 2024 post). Further, with the μ-chip technologies that are available today I’m absolutely convinced we could develop XCTDs to profile water column properties on a repeat and regular basis supplementing the Argo array by covering areas it can’t reach (see my Apr 22, 2024 post). Deployed on MM-vessels these new tools would give us an unprecedented ability to monitor the state and action of main thermocline and deep ocean currents in areas of special interest or concern.
A group called Science RoCS (https://sciencerocs.org) has been formed to explore how to further develop these ideas of partnering with the merchant marine to further marine science. If you’d like to know more about this activity please write to info@sciencerocs.org.