Given the assumed role of fresh water in a potential slow-down of the AMOC we will need to know where and how fresh water out of the Arctic via the east Greenland Current mixes with saline water from the south to reduce its density. We already know that contact between these water masses is limited since the North Atlantic Current water reaches the Nordic Seas relatively unaffected. While the great salinity anomalies do freshen the North Atlantic Current some, they appear to have had no measurable impact on the rate of overflow through the Denmark St and Faroe Bank Channel, the source waters for the deep AMOC. The reason for this must lie in the multidecadal buffering of the Norwegian and Greenland Seas, the principal overturning basins in the Nordic Seas. This is good news. On the other hand, the fresh water may have a more immediate impact on the subpolar gyre overturning, the shallow AMOC. Here’s a way we could monitor this flux, both efficiently and cost-effectively.
We already have quite a bit of experience measuring currents accurately in the subpolar domain. With ADCPs installed on two ships in regular traffic, we have monitored upper ocean transports between Greenland and Scotland (the Nuka Arctica) and along the Shetland-Faroe-Iceland Ridge (the Norröna). The data from these routes were the basis for several flux and flux divergence studies Léon Chafik and I wrote. We learned a lot from those studies.
Looking ahead, the Greenland Royal Arctic Line and the Iceland Eimskip companies operate several vessels across the subpolar gyre between Cape Farewell, Reykjavik and Europe. Eimskip also has service between Iceland and North America. I think there is a transit in one direction or the other weekly. Imagine equipping these with ADCPs that reach to km depths. Today we know much more about how to improve data returns in rough seas. Imagine further equipping these vessels with a thermosalinograph to record surface properties along the ship’s track and an AXIS to take XBTs and XCTDs in boundary currents (especially the east Greenland Current) where there are large water property gradients that cannot be monitored with Argo floats. But more importantly, if you want to determine freshwater fluxes, best to measure velocity and salinity together and then integrate. What better way to do this than from ships in regular traffic?
The beauty of this approach is that captures in detail the velocity field from the surface down. From the repeat sampling it provides the basis for mapping out the mean field and eddy variability along its route. While coverage is only along the ship track, it provides a tight constraint (at the least ground truth) to remote sensing of the upper ocean (dynamic height, temperature, salinity, ocean color). Sustained over years this methodology allows one to determine the seasonal cycle and interannual variations. Weekly transits won’t resolve fast time scales but given the stochastic nature of the variability, it seems unlikely a lack of rapid sampling will significantly alias low-frequency developments. In any event, this can be tested and/or alleviated with remote sensing data. A further appeal of working from ships in regular traffic, besides the enormous cost benefits, is that all data can be made available in near real-time.
While I remain skeptical of any significant slow-down of the subpolar AMOC in the coming decades, we have every reason to want to map out and understand the transformation processes that allow closure of the AMOC. So besides instrumenting a modest number of vessels that transit the subpolar gyre with ADCPs, TSGs, and AXISs we should use RAFOS floats to track upper ocean circulation to better understand how the subpolar circulation increases residence times - there is a wonderful plot of a surface drifter that circles the Irminger/Labrador Seas twice. Perhaps that increased residence time/trapping is the reason for the famous warming hole south of Greenland?!
Ideally, we should learn how to work more closely with the merchant marine for ocean studies. Ideally, the agencies would challenge the oceanographic community to develop sensor systems that are optimized for these platforms. That question has never been asked.
Chafik, L. and T. Rossby (2019). Volume, heat and freshwater divergences in the subpolar North Atlantic suggest the Nordic Seas as key to the state of the meridional overturning circulation. Geophys. Res. Lett.,46, https://doi.org/10.1029/2019GL082110