Isn’t it amazing, we spend tens of millions of dollars each year on the global Argo array, and yet we have a hard time maintaining a healthy XBT program on select routes. The former gives an accurate hydrographic picture of the global ocean at broad-brush scales whereas the latter can give us missing spatial detail, but only of temperature. Without salinity we are handicapped dynamically in estimated geostrophic velocities, but more importantly will be the continued lack of knowledge about freshwater inventory, transports, and distribution. I find it immensely curious if not shocking that there has been no effort to bridge the gap between the two. What do I mean by that?
As the title implies, we need to profile salinity as well. But here’s the thing, we don’t need these to profile to Argo depths (2000 m), nor is it crucial to profile upper ocean salinity to Argo accuracies. At depth, at temperatures below 10°C in the Gulf Stream, say, the high ratio of eddy/mean velocity ensures that the T/S properties vary only very gradually along isopycnal surfaces. The Argo array covers the deep ocean domain well. But in the upper ocean, T/S properties are set not only by the ocean but also the atmosphere. This is where a future XCTD can make a major contribution. The space-time variability is so huge there is no need for each measurement to be accurate to three decimals. Relaxing the accuracy requirement by an order of magnitude might allow for a much simpler design of a conductivity sensor. Further, the fact that an expendable probe only has to work once for O(3) minutes means no risk of biofouling. It may also mean that the electrode surfaces will remain stable enough to allow for a simple four-electrode DC circuit. The Neil Brown CTD that was developed in the late 1960s was a tiny tubular conductivity sensor through which ocean water flowed. (You can see a sketch of it in the description of Yvette in the Instruments section of the blog.) When it became operational the CTD was capable of delivering salinity to 3 decimal points. My point is that even though it was a tiny sensor it delivered extraordinary accuracy.
Using that knowledge as a starting point let’s suppose the electrodes for the conductivity sensor can be laid on a flat surface and span at most 1 cm. The lay-out accuracy of circuits on modern chips is O(10-7) m or better, which means that we can have a chip-to-chip reproducibility of O(10-5). This suggests two things. First, that sensors can be mass-produced at low cost, and second, having calibrated one, we have in effect calibrated them all because of the tight spatial control of the electrodes. This is an important point for it means calibration is ‘built-in’ at fabrication – they don’t have to be calibrated individually, what would add enormous cost to the probe. Besides, at maximum depths the salinity will be known independently from history or Argo. That can serve as a check. A shipboard thermosalinograph could potentially provide a second check. The analog to digital conversion circuitry should be built into the chip as well. The data would be formatted suitable for transmission up a standard XBT wire. There is no need to go deeper than the standard 900 m in use today – and even that is more than enough for salinity. It would be desirable to include a pressure sensor, but this has proven to be too costly. XBTs rely on fall rate to get depth – not perfect but acceptable. It is essential these probes be Fiats, not Lamborghinis. I have zero doubt that such a sensor can be developed and fit into the current XBT housing. Of course the development will cost, but in mass production the XCTD will augment the global Argo array enormously. The scientific return on investment will be huge because unlike the Argo floats XCTDs can be deployed from vessels in regular traffic along predesigned sampling patterns, or repeatedly in areas of particular interest, or from underway research vessels. A couple of figures will illustrate will show how it can help.
These plots from Line W (the Slope Sea and Gulf Stream at ~69°W) and Station S at Bermuda show an extremely variable upper ocean transitioning to a tight T/S relationship below 10°C and 18°C, respectively. The 10°C isotherm ranges between 300 and 900 m from the Slope to the Sargasso side of the Gulf Stream. The 18°C isotherm rarely dips below 400 m at Bermuda.
The main takeaway message is the striking transition from a highly variable ocean in the top few hundred meters with a sharp transition to a stable T/S pattern at depth. Mass-produced XCTDs can greatly improve our coverage of the upper ocean thereby extending and complementing the global Argo array’s coverage of the deeper waters.
NSF and/or NOAA should seek proposals to develop the XCTD sensor technology and fit it into the existing XBT probe. If you think this is worthwhile please spread the word, but if not please let me know why!