“People have thought, you get this cold blob, and it’s going to maybe affect Britain,” says Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, an expert on the Atlantic circulation phenomenon known by the technical name meridional overturning circulation, or AMOC. “But this is relatively new to me, that you get this pretty strong warming off the U.S. East Coast, and this could have serious extreme weather consequences for the United States.”
Rahmstorf recently wrote a post explaining his interpretation of these connections at the climate scientist blog RealClimate.org. “There is a strong case that the warm [sea surface temperatures] off the US coast and the cold blob in the subpolar gyre are linked, both being caused by an AMOC slowdown,” he argued there. A number of scientists have suggested that these warm temperatures could have enhanced Jonas.
We certainly can’t say that his argument is, at this point, a matter of scientific consensus or anything close to it. As Rahmstorf himself admits, it’s pretty novel. But the logic isn’t hard to follow, and is supported by other recent publications. So let’s examine Rahmstorf’s argument, while acknowledging outstanding uncertainties:
There’s evidence that the AMOC is weakening.
2015 was the year that the world learned of the North Atlantic cold ‘blob’ — a region of record cold temperatures southeast of Greenland that, oddly, accompanied record heat around the rest of the world.
This blob, some scientists think, may be the result of an ominous change in the AMOC — a long-forecast slowdown that was famously (or infamously, if you care about scientific accuracy) dramatized in the film The Day After Tomorrow.
The AMOC is a gigantic ocean system that’s driven by differences in temperature and the salinity of sea water. In the North Atlantic, warm and salty surface waters flow northward toward Greenland, a major source of heat transport to higher latitudes, where they play a key role in keeping these regions warmer than they might otherwise be.
In this region, cold and salty water is so dense that it sinks and travels back southward deep below the ocean’s surface. Meanwhile, more warm water travels north again. That’s what keeps the circulation going. Here’s a diagram of it, provided by Rahmstorf:
All of this is all dependent on cold, salty water sinking in the Labrador and Greenland-Norwegian Seas. But if the ocean freshens at high latitudes — due to, say, large infusions of meltwater from Greenland — there could be a problem. If far North Atlantic waters aren’t salty enough, they may not sink as much, and this could slow or even eventually shut down the circulation. The result would be a cooling in the North Atlantic region, as less ocean heat reaches the region — aka, the “blob.”
There is evidence that the circulation is indeed changing. Rahmstorf and his colleagues published a study last year contending as much — finding a decline in the strength of the circulation since around 1970. Other research also supports the idea of more recent changes to the circulation in the past decade or so.
Granted, other scientists remain skeptical, at least for now, that recently documented changes in the circulation exceed the range of variability that occurs naturally in this system. But Rahmstorf, for one, says that at this point, he’s quite confident that the cold blob in the North Atlantic is indeed tied to an ocean circulation slowdown.
“I haven’t seen anyone coming up with a viable alternative explanation,” he says.
The ‘blob’ seems to be paired with warming off the U.S. coast
But it’s not just the cold ‘blob’ — there’s reason to think that this phenomenon is being accompanied by a warming of the ocean off the U.S. coast, in a kind of oceanic yin and yang. Indeed, there is growing scientific evidence that seas are warming off the U.S. eastern seaboard (especially the Gulf of Maine), and doing so in a way that would seem to reflect an AMOC change.
Take, for instance, a study just out in the Journal of Geophysical Research: Oceans, by eleven researchers with the National Oceanic and Atmospheric Administration. They used a high resolution climate model to examine what will happen in the North Atlantic region as global warming proceeds, and found that for a doubling of atmospheric carbon dioxide above pre-industrial levels, the upper ocean waters in the northwest Atlantic continental shelf region would warm “nearly three times faster than the global average.”
The reason? The authors note a “robust relationship between a weakening Atlantic Meridional Overturning Circulation (AMOC) and an increase in the proportion of Warm-Temperate Slope Water entering the Northwest Atlantic Shelf.” In particular, they find that as the AMOC weakens, the Gulf Stream shifts northward, carrying warm and salty water toward our coast.
“We have pretty good certainty that the northwest Atlantic is going to be a hotspot for climate change,” says Vincent Saba, a fisheries biologist for the National Oceanic and Atmospheric Administration who was the lead author of the study.
Normally, the Gulf Stream tends to veer east out over the Atlantic around Cape Hatteras, says Saba. But instead, it has been shifting more toward the northwest, and entering the Gulf of Maine. Then, some of that warm water flows back southward along the coast toward New Jersey and Maryland, he said.
“There is lots of natural variability in this system, the Gulf Stream changes position naturally, there are these lower frequency swings,” Saba cautions. “But then you add climate change on top of that, and that’s kind of what we’re looking at,” he says.
It’s not just NOAA’s high resolution model showing this, but observations. For instance, recently published research has found that the Gulf of Maine warmed faster from 2004 through 2013 than virtually any other part of the world ocean, a change that researchers have partly attributed to the changing Atlantic circulation.
Extra warmth off the U.S. coast can have dramatic weather consequences
A warming of waters off the U.S. East Coast would a troubling development for many reasons. The warming of the Gulf of Maine has already had devastating consequences for fisheries, for instance.
But changes to weather should also occur. Whether it’s a blizzard or a hurricane, storms tracking across an unusually warm ocean region can be enhanced and strengthened.
When it comes to last year’s Boston blizzard or last weekend’s dramatic snow dump from Jonas, scientists have pointed the finger at warm ocean waters. Or as Michael Mann, a climate scientist at Penn State University, has put it to me previously, these storms “feed off warm seas both for their intensification, and for the amount of moisture that is available to produce snowfalls.” Mann confirmed for this story that this interpretation applies to Jonas.
Kevin Trenberth, a scientist at the National Center for Atmospheric Research, added this by email when it comes to Jonas:
In winter it is cold over the continent. But it is warm over the oceans and the contrast between the cold continent and the warm Gulf Stream and surrounding waters is increasing. At present sea surface temperatures are more [than] 3F above normal over huge expanses (1000 miles) off the NE coast and water vapor in the atmosphere is about 10 to 15% higher as a result. Up to half of this can be attributed to climate change.
Which is not to say that warm ocean temperatures are the only cause here — many other factors go into any weather event, including Jonas. But warm seas are, at least, an important contributing factor.
Again, it’s important to reiterate that while Rahmstorf is putting all of these pieces together, it doesn’t mean that he’s won a consensus to his way of thinking. Still, the connections he’s drawing make sense and draw on recent scientific research (as cited above). So now, we’ll have to see how other scientists react.
For Rahmstorf, the possible connection means that, ironically, the scientifically panned film The Day After Tomorrow may have accidentally gotten something kind of right when it had a shutdown of the AMOC followed by a big snowstorm hitting New York.
“They had these blizzards for completely the wrong reasons, but the end result is a big snow storm,” he says. “And that’s kind of ironic: Despite the wrong physics, there’s a grain of truth now in that movie.”
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