We have long known that the overturning of the oceans is highly asymmetric. In 1962 Stommel wrote a paper on the ‘Smallness of the sinking regions in the oceans’. A year later (summer 1963) he tasked me to show this experimentally. The reason for the asymmetry was obvious, sinking waters are buoyancy-driven, leading to an advection of cold water into the abyss. Getting those waters back toward the surface requires downward diffusion of heat to make them buoyant, a far less efficient process requiring vast areas where this can take place – hence the enormous asymmetry. But how do we measure or quantify this overturning? Especially now that we know that the sinking at high latitudes and the slow rising elsewhere are far more complex processes than originally envisioned. This is where the beauty of western boundary currents comes in.
The Atlantic meridional overturning circulation (AMOC) is not the simple overturning you might be led to think from looking at the modeling community’s 2-dimensional description of it. But thanks to western boundary currents there are a few spots in the Gulf Stream where we can keep easy tabs on the AMOC, both of which capture all water flowing poleward. This is a huge advantage because we have come to learn that the deep return flow is more widely distributed both vertically and horizontally, making it far more difficult to fully capture its transport. One site is the Florida St current, it has been monitored for many decades. Similarly, we have considerable knowledge of Gulf Stream transport between the US northeast and Bermuda. The Florida St is very well defined, but it doesn’t fully capture all water, some flows north to the east of the Bahamas, but that has been monitored for some time now. The US-Bermuda section captures all water flowing north, both the wind-driven circulation and the AMOC. Direct measurement of transport (since 1992) shows no evident change in upper ocean transport whereas analysis of hydrographic data indicates a ~2 Sv decrease in Gulf Stream transport since the 1930s, most of which appears to be due to a reduction in the wind-driven circulation with perhaps an 0.4±0.6 Sv reduction in the AMOC over time. This is the most robust estimate available since it is based on a combination of direct velocity measurement and high-quality hydrography. The paper below spells out how this was done.
We also know that the AMOC has two basic overturning sites, one in the subpolar North Atlantic and the other in the Nordic Seas. The latter overturning has been monitored rather accurately in terms of out (over-)flow and has been quite stable for several decades. We also have strong hydrographic and direct velocity evidence that flow into the Nordic Seas has been stable for the last 70-90 years. By subtracting out the Nordic Seas contribution from the total - the Gulf Stream section - any reduction in the subpolar AMOC will be less than 1 Sv. This is a far more robust result than any estimate based on subpolar SST or other proxies. The point is that it is far easier to quantify AMOC heading north than the deep AMOC return flow. Thus, if we anticipate change, then integral estimates of poleward flow at these Gulf Stream sections will be good ones to watch.
Rossby, T., J. Palter, and K. Donohue (2022). What can hydrography between the New England Slope, Bermuda and Africa tell us about the strength of the AMOC over the last 90 years? Geophys. Res. Lett., 49, e2022GL099173. https://doi.org/10.1029/2022GL099173.