A lot of talk this summer about the status of the Atlantic Meridional Overturning Circulation (the headlines erroneously referred to the “Gulf Stream”) – which seems to be weakening.
PBS’s excellent “Terra” program has a good piece on this, interviewing Stefan Rahmstorf, who knows as much about this as nearly anyone, as well as Jennifer Francis of Woods Hole Research Center, and others.
Terra does a good job explaining not just the effect of reduced heat flow to Northern Europe, but also some other knock-on effects, like accelerated sea level rise on the eastern shores of North America, and reduced carbon absorption by the North Atlantic, a critical carbon sink.
In addition, there’s another important caveat – see below.
Followers of this blog, of course, should be well versed on this topic, as I spoke to Michael Mann about it some years ago, in a piece that’s gotten a lot of views.
The most important new (to me) wrinkle here is that some recent research posits the existence of the “cold spot” without necessarily invoking a collapsing AMOC.
University of Miami researchers discover that a swath of cooling water in the subpolar region is unrelated to an ocean circulation slowdown.
The pattern of temperature change in the world’s oceans may not be a sign of an impending abrupt climate change event, as depicted in the film “The Day After Tomorrow,” according to a new study from researchers at the University of Miami’s (UM) Rosenstiel School of Marine, Atmospheric, and Earth Science.
In order to explore a pattern of temperature change in a region of water in the subpolar North Atlantic known as a warming hole that has been cooling over the last century, the UM Rosenstiel School researchers utilized a cutting-edge climate model. According to scientists, the Atlantic Meridional Overturning Circulation (AMOC), which carries energy to the North Atlantic, has been thought to be the cause of this cooling.
“However, our study shows the warming hole during the past century is unlikely due to a slowdown of the AMOC. Instead, the warming hole is actually a consequence of human-driven changes in the atmosphere” said Chengfei He, a postdoctoral researcher in the Department of Atmospheric Sciences at the Rosenstiel School. “Our findings suggest that this warming hole will not result in an abrupt climate change event lethal to humans as depicted in Hollywood movies.”
Geological records, like the Greenland ice core, have shown that the majority of sudden climate changes in Earth’s history were caused by a slowdown of the AMOC. “The warming hole is believed as a fingerprint of the AMOC in the present day. Its appearance suggests the AMOC may not be stable. Our results do not support this idea,” said Amy Clement, a professor in the Department of Atmospheric Sciences at the Rosenstiel School, and a co-author of the study.
The researchers utilized a climate model that is a digital Earth that can recreate previous climate changes and predict future climate change. He and his coauthors ran the model with a motionless ocean to see how the North Atlantic temperature reacts to changes in the atmosphere caused by greenhouse gas and aerosol emissions. Because the ocean has no circulation, every change in ocean surface temperature is determined by the atmospheric conditions above.
Under global warming, the atmospheric westerlies shift northward and enhance the local wind over the subpolar North Atlantic and result in the warming hole.
“This cooling trend is partially compensated by the warming due to the rise of greenhouse gases and the damping effect in sea surface temperatures,” according to the authors. This study advances our ability to attribute patterns of change in the ocean to different factors, and hence improves our ability to anticipate how the ocean will change in the future.
Abstract:
The sea surface temperature (SST) in the subpolar North Atlantic decreased during the past century, a remarkable feature known as the “warming hole (WH).” It is commonly claimed that the WH results from the slowdown of the Atlantic meridional overturning circulation. However, using an ensemble of the Community Earth System Model coupled to a slab ocean model simulation, we show the atmosphere alone can account for ∼50% of the observed cooling trend in the subpolar North Atlantic and ∼90% of the cooling relative to the temperature change of the global ocean. We find this cooling is caused by increased local westerlies in response to external forcing that enhance heat loss from the ocean through turbulent heat fluxes. This cooling is partly compensated by the damping in SST and the warming due to the rise of greenhouse gases. We further suggest that wind-driven ocean processes can enhance the cooling in the real world, but that nonetheless the ultimate driver resides in the atmosphere.
RealClimate 24 Jul 2023 (Stefan Rahmstorf) What is happening in the Atlantic Ocean to the AMOC ?
Worth a read if time permits . . .
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“Timing of the critical AMOC transition is still highly uncertain, but increasingly the evidence points to the risk being far greater than 10 % during this century – even rather worrying for the next few decades. ”
https://www.realclimate.org/index.php/archives/2023/07/what-is-happening-in-the-atlantic-ocean-to-the-amoc/
“As depicted in Hollywood movies”
I’m pretty sure nobody ever predicted anything even resembling what was predicted in The Day After Tomorrow, so that doesn’t feel like much of a “correction”…
I don’t see that the “motionless ocean” model reproducing the warming hole (“cold blob”) changes the result that the warming hole slows down the AMOC. AFAICT the MO model just says that a lot of colder water in the North Atlantic (from regional melt and westerlies and maybe precip changes) doesn’t need ocean currents to create the warming hole.