Combine this observation with recent research suggesting a slowing of the powerful ocean phenomenon known as the Atlantic meridional overturning circulation, or AMOC — which carries warm water northward and sends cold water back southward deep beneath the surface — and the cold patch starts looking pretty ominous. It could, perhaps, be an indicator that less warm water, and less heat overall, is making its way northward, a development long predicted as a result of climate change.
That is how it has been interpreted by some leading climate scientists. Stefan Rahmstorf, an ocean physicist with the Potsdam Institute for Climate Impact Research and author of the study mentioned above, has written that very cold temperatures in the subpolar North Atlantic Ocean in the winter of 2014-2015 “suggests the decline of the circulation has progressed even further now than we documented in the paper.”
But in a new study in Geophysical Research Letters reporting on deep ocean measurements from this region, two researchers present an alternative interpretation. They say that they found “exceptional” levels of deep ocean convection, or mixing of surface waters with deep waters of a sort that helps drive the overturning circulation, during in the winter of 2014-2015 — the height of the cold “blob.” And they attribute that temperature phenomenon to natural climate variability, driven by local weather and winds.
“We find that the observed temperature variability is explained without invoking a trend in the lateral heat transport that would be representative of an AMOC slowdown,” Femke de Jong and Laura de Steur of the Royal Netherlands Institute for Sea Research write in the paper. They therefore question whether the “cold blob” has anything much to do with a slowing Atlantic circulation or one key change that some think could be contributing to that — namely, growing volumes of freshwater melting from Greenland.
Or as a press release describing the research puts it: “This rejects a hypothesis that posed that increased meltwater from Greenland weakened deep water formation and caused the cold blob.”
To reach their conclusions, de Jong and de Steur studied data from an ocean mooring installed by the Royal Netherlands Institute for Sea Research in 2003, and analyzed it through July of last year. The research is part of the broader OSNAP project — which stands for Overturning in the Subpolar North Atlantic Program.
The mooring — whose cable extended over a mile deep into the ocean, allowing for vertical measurements of key attributes such as the temperature of the water and also its salinity at different depths — was named LOCO, or Long-term Ocean Circulation Observations (you have to think scientists have fun with these names). The study says the mooring was “located right in the area of interest, namely the center of the [sea surface temperature] anomaly.”
LOCO’s large vertical range of measurement is important because if the ocean is highly stratified — with its warmest layer at the top and getting progressively colder below — then such an instrument will clearly detect that. But if the surface layers are mixing downward because they are so cold and salty — and therefore so dense — in the process of deep convection, then the vertical column of ocean water as a whole will show more homogeneity. And it is ocean waters such as this that are believed to drive the overturning circulation, or AMOC.
The study found this character to the water, observing particularly vigorous deep convection during the winter of 2014-2015, when the surface of the ocean was so strikingly cold. It is, de Jong says, one of the first confirmations that an overturning circulation occurs, as has long been suspected, in the Irminger Sea as well as in the Labrador Sea on the western side of Greenland. This appears to be an important new finding about how the ocean works.
“We found that the convection in the Irminger Sea was deeper [than] anyone had seen before,” de Jong said in an email. “We’re exited about this because (1) this establishes this basin as another area that is potentially important for the overturning circulation and (2) shows that convection is still going quite strong.”
But the study attributes the particularly cold character of the water simply to atmospheric phenomena operating in the area that winter, which was longer than usual, extending cold temperatures into April.
“The ‘cold blob’ that is seen in the North Atlantic was mostly caused by the same strong winter that caused the convection,” de Jong continued. “Local cooling by the atmosphere is able to adjust temperatures much quicker than changes in the (much more sluggish) ocean heat transport can.”
The study, in other words, implies that this is business as usual for the subpolar North Atlantic Ocean — and not cause for alarm.
But Michael Mann and Stefan Rahmstorf of Penn State University and the Potsdam Institute for Climate Impact Research aren’t so convinced. The two were authors on a paper that suggested that an “unprecedented” Northern Hemisphere overturning circulation slowdown has occurred since 1975, apparently tied to climate change. They argued that there is a long-term cooling trend in this broad region, separate from any shorter time-frame atmospheric variability, that is because of oceanic changes. And they have also pointed to the “blob” as a possible indicator of this trend.
“They are looking at short-term variability while we are looking at climatic trends; the mechanisms behind those are very different,” said Rahmstorf by email, in commenting on the new study.
Rahmstorf further argued that measuring ocean convection in a localized way is not necessarily enough to determine what is happening with the larger Atlantic overturning circulation. “Convection is a highly stochastic, weather-driven process,” he continued. “The linkage between local convection and the AMOC is complex and long-term; the most simple way to phrase this is that the AMOC responds like a long-term integrator of the convection events of the previous decades.”
The debate reprises, in a sense, what happened only last week when another paper suggested that Greenland’s melting hasn’t been large enough, at least not yet, to slow down overturning circulation in another key region where it occurs, the Labrador Sea. That study similarly raised questions about whether scientists can say for sure that an ocean freshening trend is blocking the sinking of cold, fresh water that drives the circulation or AMOC.
So it is fair to say that although there are many intriguing (and troubling) ideas out there, scientists are not in full agreement about what is going on in the North Atlantic Ocean. The good news, though, is that with ever-increasing scientific interest in ocean occurrences on both sides of Greenland, we can expect more and more research to help sort all of this out.
Indeed, the OSNAP mission heads back out to sea next month to make more observations — and to try to find just what is happening to the critical circulation of the ocean in these remote, freezing waters.
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