AMOC slowdown

[Alphabet Hotel]

Change happening too fast could be enough to push us over a tipping point? That is the most frightening paper I've seen in a long time.

[vox_mundi]

Record-High Arctic Freshwater Will Flow to Labrador Sea, Affecting Local and Global Oceans
https://phys.org/news/2021-02-record-high-arctic-freshwater-labrador-sea.html



Freshwater is accumulating in the Arctic Ocean.

The Beaufort Sea, which is the largest Arctic Ocean freshwater reservoir, has increased its freshwater content by 40% over the past two decades. How and where this water will flow into the Atlantic Ocean is important for local and global ocean conditions.

A study from the University of Washington, Los Alamos National Laboratory and the National Oceanic and Atmospheric Administration shows that this freshwater travels through the Canadian Archipelago to reach the Labrador Sea, rather than through the wider marine passageways that connect to seas in Northern Europe. The open-access study was published Feb. 23 in Nature Communications.

"The Canadian Archipelago is a major conduit between the Arctic and the North Atlantic," said lead author Jiaxu Zhang, a UW postdoctoral researcher at the Cooperative Institute for Climate, Ocean and Ecosystem Studies. "In the future, if the winds get weaker and the freshwater gets released, there is a potential for this high amount of water to have a big influence in the Labrador Sea region."

The finding has implications for the Labrador Sea marine environment, since Arctic water tends to be fresher but also rich in nutrients. This pathway also affects larger oceanic currents, namely a conveyor-belt circulation in the Atlantic Ocean in which colder, heavier water sinks in the North Atlantic and comes back along the surface as the Gulf Stream. Fresher, lighter water entering the Labrador Sea could slow that overturning circulation.

Fresher, lighter water floats at the top, and clockwise winds in the Beaufort Sea push that lighter water together to create a dome.

When those winds relax, the dome will flatten and the freshwater gets released into the North Atlantic.



... The volume of freshwater now in the Beaufort Sea is about twice the size of the 1983-1995 case studied, at more than 23,300 cubic kilometers, or more than 5,500 cubic miles. This volume of freshwater released into the North Atlantic could have significant effects. ... "A freshwater release of this size into the subpolar North Atlantic could impact a critical circulation pattern, called the Atlantic Meridional Overturning Circulation, which has a significant influence on Northern Hemisphere climate."

Jiaxu Zhang et al. Labrador Sea freshening linked to Beaufort Gyre freshwater release, Nature Communications (2021)
https://www.nature.com/articles/s41467-021-21470-3

[vox_mundi]

Possible Irreversible Changes to Sub-Systems Prior to Reaching Climate Change Tipping Points
https://phys.org/news/2021-02-irreversible-sub-systems-prior-climate.html

Recently a pair of researchers with the University of Copenhagen published a paper in the Proceedings of the National Academy of Sciences describing their work looking into the possibility of changes to the Atlantic Meridional Overturning Circulation (AMOC) and the circumstances that could lead to such changes. In their paper, Johannes Lohmann and Peter Ditlevsen noted that climate models show that irreversible changes to sub-systems such as the AMOC, one of Earth's global sub-systems, can occur prior to a tipping point if changes occur at a fast pace.

They further noted that fresh water pouring into the Atlantic Ocean due to melting ice could result in just such an irreversible change. More recently, an international team of climate scientists has published a Perspectives piece, also in PNAS, outlining the harm that could result if irreversible changes occur prior to tipping points, and also what models suggest will happen if the Earth does start to reach certain tipping points.

... Climate models suggest that under such scenarios, there is no turning back. Once a tipping point is reached, we cannot solve the problem by stopping carbon emissions. More recent research has suggested that there may be some pre-tipping points that could trigger climate changes on a smaller scale but which would still be irreversible. Fresh, cold water entering the North Atlantic, for example, could result in permanent changes to the AMOC. Notably, the AMOC is responsible for the mild temperatures in Europe.

In their paper, the authors of the Perspectives piece describe likely outcomes of different scenarios that could lead to pre-tipping point sub-system changes. They note, for example, that the world's oceans comprise sub-systems. In addition to rising water levels, the world's oceans are experiencing acidification due to carbon dioxide absorbed from the atmosphere. Acidification harms sea life. Additionally, prior research has shown that the upper part of the ocean absorbs heat from the atmosphere at a much higher rate than lower regions. The lower regions are only now beginning to feel the effects of warming, a long-term event that could change the entire underwater ecosystem—a change that would be irreversible.



Christoph Heinze et al. The quiet crossing of ocean tipping points, Proceedings of the National Academy of Sciences (2021)
https://www.pnas.org/content/118/9/e2008478118


Candidates for high-probability high-impact marine tipping elements that concern warming, deoxygenation, and ocean acidification as well as their impacts.

[vox_mundi]

Gulf Stream System At Its Weakest In Over a Millennium
https://phys.org/news/2021-02-gulf-stream-weakest-millennium.html

In more than 1,000 years, the Atlantic Meridional Overturning Circulation (AMOC), also known as Gulf Stream System, has not been as weak as in recent decades. This is the result of a new study by scientists from Ireland, Britain and Germany. The researchers compiled so-called proxy data, taken mainly from natural archives like ocean sediments or ice cores, reaching back many hundreds of years to reconstruct the flow history of the AMOC. They found consistent evidence that its slowdown in the 20th century is unprecedented in the past millennium; it is likely linked to human-caused climate change. The giant ocean circulation system is relevant for weather patterns in Europe and regional sea levels in the U.S.; its slowdown is also associated with an observed cold blob in the northern Atlantic.

 ... Previous studies by Rahmstorf and colleagues showed a slowdown of the ocean current of about 15% since the mid-20th century, linking it to human-caused global warming, but a robust picture about its long-term development has up to now been lacking: This is what the researchers provide with their review of results of proxy data studies.

"For the first time, we have combined a range of previous studies and found they provide a consistent picture of the AMOC evolution over the past 1600 years," says Rahmstorf. "The study results suggest that it has been relatively stable until the late 19th century. With the end of the little ice age in about 1850, the ocean currents began to decline, with a second, more drastic decline following since the mid-20th century." Already the 2019 special report on the oceans of the Intergovernmental Panel on Climate Change (IPCC) concluded with medium confidence "that the Atlantic Meridional Overturning Circulation (AMOC) has weakened relative to 1850-1900."

... "If we continue to drive global warming, the Gulf Stream System will weaken further—by 34 to 45% by 2100, according to the latest generation of climate models," concludes Rahmstorf. This could bring us dangerously close to the tipping point at which the flow becomes unstable.

In 20 to 30 years, further weakening of the Atlantic Meridional Overturning Circulation (AMOC) could result in more storms battering the UK, and more intense winters and an increase in damaging heatwaves and droughts across Europe.

A weakened Gulf Stream would also raise sea levels on the Atlantic coast of the US, with potentially disastrous consequences

L. Caesar, G. D. McCarthy, D. J. R. Thornalley, N. Cahill, S. Rahmstorf : Current Atlantic Meridional Overturning Circulation weakest in last millennium. Nature Geoscience. (2020)
http://dx.doi.org/10.1038/s41561-021-00699-z (embargoed)

https://amp.theguardian.com/environment/2021/feb/25/atlantic-ocean-circulation-at-weakest-in-a-millennium-say-scientists

[Alphabet Hotel]

Sci-hub does not have that latest AMOC slowdown paper. Any idea how to get a copy? Write the researchers?

[vox_mundi]

It will be off embargo sometime today.

L. Caesar, G. D. McCarthy, D. J. R. Thornalley, N. Cahill, S. Rahmstorf : Current Atlantic Meridional Overturning Circulation weakest in last millennium. Nature Geoscience. (2021)
https://www.nature.com/articles/s41561-021-00699-z

[Sciguy]

Another study found no decline in the AMOC.

https://os.copernicus.org/articles/17/285/2021/?mc_cid=cbeda0c2d5&mc_eid=b0f93db32f

Worthington, E. L., Moat, B. I., Smeed, D. A., Mecking, J. V., Marsh, R., and McCarthy, G. D.: A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline, Ocean Sci., 17, 285–299, https://doi.org/10.5194/os-17-285-2021, 2021.

A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline

Abstract

A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this; however direct observations are sparse prior to 2004, giving only “snapshots” of the overturning circulation. Previous studies have used linear models based on upper-layer temperature anomalies to extend AMOC estimates back in time; however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher-fidelity empirical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26∘ N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations and shows that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline as indicated by other proxies and high-resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate-change-related AMOC decline.