Atlantic on my mind

In recent times there has been a lot of talk about ocean circulation in relation to a changing climate, specifically the AMOC, that is, the Atlantic Meridional Overturning Circulation and its best-known part flowing on the surface, the Gulf Stream.

This is a topic that comes back to the fore every now and then, whether it is research (good and right) or media speculation (much less interesting), as in the case of the blockbuster The Day After Tomorrow, of which, among other things, on these pages you will find several er… reviews :-).

For this Sunday now almost over, I therefore recommend a few readings on the subject, to be done, however, taking into account a couple of non-trivial elements of knowledge that are hardly remembered:

1. The thermoaline circulation, that is, the complex system of surface and depth ocean currents, depends, as is clear from the name, on both temperature and salinity, but is set in motion in the same way as atmospheric circulation by the astronomical motions of the planet that condition the redistribution of heat on Earth. Among them, the rotation around the axis and the consequent coriolis force. Changes in temperature and salinity therefore certainly affect these motions, but to stop them it would also be necessary to stop the planet.
2. The above does not prevent these motions from changing, indeed their change is the basis of climate evolutions, precisely as a direct effect of astronomical inputs. Specifically, a possible slowdown in the Gulf Stream, due to changes in salinity induced by the spillage of fresh water into the North Atlantic from the melting of the earth’s glacial masses (non-marine!), would lead to changes in atmospheric circulation that would favor a cooling of a substantial part of the land of the northern hemisphere. This is a mechanism that is thought to be at the origin of millennial trends that then lead to the incipit of glacial phases.

Against this background, here are the readings for this Sunday.

First, a recently published paper in which, using proximity data and model simulations, the authors come to the conclusion that AMOC would now be at its millennial lows. The acceleration of the slowdown (excuse the pun), however, would have come in the last decades:

Current Atlantic Meridional Overturning Circulation weakest in last millennium – Nature Geoscience

The study is explained in an exhaustive TD on Twitter:

Just out: our new paper affirming the unprecedented slowdown of the Gulf Stream System (aka Atlantic meridional overturning circulation, ) in Nature Geoscience! @NatureGeosci A thread. 1/11 pic.twitter.com/2GovKz5evk

— Stefan Rahmstorf (@rahmstorf) February 25, 2021

Then another paper, also very recent, which says instead that despite the fact that decelerations have been detected, the trend of the last thirty years of AMOC would be not significant, therefore quite in contrast to what was claimed in the other study. The periods under review are significantly different, but the last one coincides:

A 30-year reconstruction of the Atlantic south overturning circulation shows no decline – EGU Ocean Science

And then another very recent study that looked at the freshwater flow of the Beaufort Gyre (BG), the largest low-salinity water reservoir in the North Atlantic as well as part of AMOC. According to this study, low salinity water has increased significantly in recent decades:

Labrador Sea freshening linked to Beaufort Gyre freshwater release – Nature Communication

Before closing and wishing you a good read, a couple more considerations:

1. The ability to observe these phenomena has increased enormously in recent times, so connecting what we “see” now with what has happened in the past and that we know thanks to proxi data is extremely difficult and not free from wide margins of uncertainty.
2. The presence of dynamics that would in fact lead to cooling is a confirmation of the existence of mechanisms that prevent the so-called runaway greenhouse effect, that is, a tendency to uncontrolled heating triggered to reach points of no return that have no scientific reason to exist but instead go very fashionable.

Enjoy.