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The reason is that in the past several years,
amid record warm global temperatures (2014, 2015 and 2016 seem likely to be the
third-hottest, second-hottest and hottest years on record, respectively), ocean
temperatures to the southeast of Greenland have often been quite cold.
Sometimes, according to temperature maps provided by the National Oceanic and
Atmospheric Administration, they have even shown record-cold temperature
anomalies in and around a region known as the Irminger Sea.
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. |
