by Judith Curry

The Washington Post has this dramatic headline: Global warming is now slowing down the circulation of the ocean with potentially dire consequences.

A new paper has been published that has the media talking about The Day After Tomorrow [link]. What is this new paper?

Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation

Stefan Rahmstorf, Jason E. Box, Georg Feulner, Michael E. Mann, Alexander Robinson, Scott Rutherford & Erik J. Schaffernicht

Abstract. Possible changes in Atlantic meridional overturning circulation (AMOC) provide a key source of uncertainty regarding future climate change. Maps of temperature trends over the twentieth century show a conspicuous region of cooling in the northern Atlantic. Here we present multiple lines of evidence suggesting that this cooling may be due to a reduction in the AMOC over the twentieth century and particularly after 1970. Since 1990 the AMOC seems to have partly recovered. This time evolution is consistently suggested by an AMOC index based on sea surface temperatures, by the hemispheric temperature difference, by coral-based proxies and by oceanic measurements. We discuss a possible contribution of the melting of the Greenland Ice Sheet to the slowdown. Using a multi-proxy temperature reconstruction for the AMOC index suggests that the AMOC weakness after 1975 is an unprecedented event in the past millennium (p > 0.99). Further melting of Greenland in the coming decades could contribute to further weakening of the AMOC.

Published in Nature Climate Change [link]; full manuscript available [here].

Stefan Rahmstorf has a post at RealClimate Whats going on in the North Atlantic? Excerpt:

The North Atlantic between Newfoundland and Ireland is practically the only region of the world that has defied global warming and even cooled. Last winter there even was the coldest on record – while globally it was the hottest on record. Our recent study (Rahmstorf et al. 2015) attributes this to a weakening of the Gulf Stream System, which is apparently unique in the last thousand years.

Climate models have long predicted such a slowdown – both the current 5th and the previous 4th IPCC report call a slowdown in this century “very likely”, which means at least 90% probability. When emissions continue unabated (RCP8.5 scenario), the IPCC expects 12% to 54% decline by 2100. But the actual past evolution of the flow is difficult to reconstruct owing to the scarcity of direct measurements.

What is new is that we have used proxy reconstructions of large-scale surface temperature (Mann et al, 2009) previously published by one of us that extend back to 900 AD to estimate the circulation (AMOC) intensity over the entire last 1100 years. This shows that despite the substantial uncertainties in the proxy reconstruction, the weakness of the flow after 1975 is unique in more than a thousand years, with at least 99 per cent probability. This strongly suggests that the weak overturning is not due to natural variability but rather a result of global warming.

Observations

Well, if there is anything I distrust more than climate model simulations of decadal to millennial scale ocean circulations and internal variability, it is Mannian proxy analysis of same. It seems like strip bark bristlecones and Tiljander sediments can tell us about Gulf Stream flow rates, as well as global temperatures. Remarkable.

So what do the actual ocean observations have to say? Anthony Watts has an extensive critique of the paper, pointing to a 2014 paper by oceanographer Thomas Rossby: On the long-term stability of Gulf Stream transport based on 20 years of direct measurements. The title pretty much speaks for itself, but Rossby had this to say in an interview:

“The ADCP measures currents at very high accuracy, and so through the repeat measurements we take year after year, we have a very powerful tool by which to monitor the strength of the current,” said Rossby. “There are variations of the current over time that are natural — and yes, we need to understand these better — but we find absolutely no evidence that suggests that the Gulf Stream is slowing down.”

In 2010, NASA issued a press release NASA Study Finds Atlantic Conveyor Belt Not Slowing, citing a paper published by Josh Willis of JPL using measurements from ocean-observing satellites and profiling floats. Punchline:

For now, however, there are no signs of a slowdown in the circulation. “The changes we’re seeing in overturning strength are probably part of a natural cycle,” said Willis. “The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling.”

Pierre Gosselin cites additional critiques from German scientists. Notably:

Climate scientist Martin Visbeck of the GEOMAR Helmholtz Centre for Ocean Research in Kiel sees Rahmstorf’s assertion of the results critically: ‘The study’s focus on the sub-polar part of the Atlantic and the spectral analysis are interesting,’ he says. But there are other AMOC assessments that point to a completely other development. The paper does not offer any strong indication of the development of the AMOC during the past fifty years.”

So, who you gonna believe? Climate models and Mannian proxies, or direct and satellite observations of ocean circulation?

Atlantic Multidecadal Oscillation

What is going on the high latitudes of the North Atlantic can’t be understood without the context of the Atlantic Multidecadal Oscillation. If you are unfamiliar with the Atlantic Multidecadal Oscillation (AMO), see this summary. For reference, the AMO figures prominently in the stadium wave. While the method to define the AMO is still debated, here I show the canonical analyses from NOAA.

The monthly values of the AMO are shown below.

A blow up of the more recent data (through Dec 2014) is shown below:

While the AMO index shows substantial variability, there are multi-decadal periods when the index is predominantly positive (warm) and negative (cool). To the extend that past behavior is any guide to the future, the current warm phase is expected to transition to the cool phase sometime in the 2020’s. While the the 1995 transition was sharp, the transition to the next cool phase might be sharp, or it might ‘flicker’ for a few years or even a decade. We’ll have to see how this plays out.

There is some evidence evidence that the warm phase of the AMO has peaked circa 2007, see the upper ocean heat content data shown below.

JC summary

What we are seeing in the high latitudes of the North Atlantic is natural variability, predominantly associated with the Atlantic Multidecadal Oscillation. Based open observational analyses, there is no sign of a slowdown in the Gulf Stream or the Atlantic Meridional Overturning Circulation.

Now, I am very interested in the AMO, since it strongly influences Atlantic hurricanes, Arctic sea ice, and Greenland climate. We are already seeing a recovery of the Atlantic sector of the Arctic sea ice, and some hints of cooling in Greenland.

With regards to Atlantic hurricanes, Bill Gray and Phil Klotzbach are looking a new definition of the AMO that they feel relates better to Atlantic hurricane activity [link], and they are seeing a transition to negative values.

And finally, the AMO does not act in isolation (e.g. the stadium wave); there are very interesting things going on in the Pacific also – perhaps a topic for a future post.