In 2019 Léon Chafik and I wrote a paper that broke new ground on getting direct estimates of volume, heat, and freshwater flux divergences in the subpolar North Atlantic. It was made possible by two separate programs measuring currents between Scotland and Greenland, both made graciously possible by the operators of vessels in regular traffic. The first vessel, the Nuka Arctica, made 3-week round-trips between Nuuk in Greenland and Denmark It started in 1997 and ended in 2013. The second operation, on the high-seas ferry Norröna, started effectively around 2009 continued for a decade measured currents in the Faroe-Shetland Channel and between the Faroes and Iceland. These two lines encompassed 1.2 million square km of the subpolar North Atlantic, an area comparable to that of France, Spain and Portugal combined.
These subpolar waters lose tremendous amounts of heat to the atmosphere. The same is true of the Nordic Seas, farther north. It is in these seas that the waters in upper branch of the Atlantic meridional overturning circulation (AMOC) sink into the lower south-flowing branch. The question we wanted to explore is how well can we estimate the heat losses, and how well do they compare with direct air-sea heat flux estimates over the same area? The novel aspect is that we had accurate estimates of the mean velocity field along both lines. Our approach was straightforward. The hydrographic properties of these waters are well-known from the many oceanographic studies throughout the 20-century. And having now established the mean flow quite accurately from these two MM-vessel operations, we could integrate the product of velocity and property (temperature or salinity) to get direct estimates of these property fluxes across the two lines. The difference would constitute a divergence. The integration was from Scotland to Greenland and from the surface to sigma = 27.55 kg/m3, the density surface separating the upper and lower branches of the AMOC.
From the volume transport divergence calculations, we found that AMOC lost half its volume in the subpolar North Atlantic and the other half in the Nordic Seas. The heat flux divergences showed a far more striking difference with more than twice as much heat lost in the Nordic Seas than in the subpolar North Atlantic. This is consistent with the fact that the Nordic Sea deep water is densest of all AMOC water with temperatures around 0°C or less. The leads us to note (what is generally known but not appreciated) that the AMOC produces two return flows, a shallow branch from essentially the Irminger Sea, and a deeper one from the Nordic Seas. While variations in subpolar SST will shape regional weather and climate, conditions in the Nordic Seas determine whether Europe and Scandinavia will experience a warm interglacial, or cold glacial climate. The difference between these two states is set by whether warm water from the North Atlantic, cool there, and return as deep, dense water, or not. We make this point because so much of the AMOC discussion appears to pivot around the subpolar North Atlantic (and the warming hole) instead of the Nordic Seas. I think much of this is mis-directed. The far more serious question is what physical processes control the strength of overturning in Nordic Seas.
In this paper we also examined various maps of estimated heat loss across the subpolar North Atlantic. It came as quite a shock to see how much they differed, clearly an indication of the difficulties in estimating the radiative, direct, and indirect components of heat transfer. This must be quite a challenge given the extensive cloud coverage, and the strong and spatially variable winds around the Irminger Sea leading to a paucity of useful measurements on which to build the flux estimates.
The reason for bringing this up is that we may have another opportunity to conduct a similar study, this time in an area closer to home with far better meteorological coverage, namely the Bermuda Triangle. As many of you know, we have been scanning ocean currents from the Oleander, a freighter operating weekly between Bermuda and New Jersey, since the early 1990s. It appears possible that a second line, between Bermuda and Jacksonville, FL, might become available along which we could scan currents. Imagine that, two perfect lines from the continent to Bermuda encompassing 780,000 km2! With these two operations I think one could build an accurate picture of heat transport divergences that could serve as framework for detailed studies of ais-sea fluxes in an area that is far better surveyed thanks to its nearby location to many oceanographic laboratories along the east coast.
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. Geophysical Research Letters. https://doi.org/10.1029/2019GL082110