Gulf Stream slowdown tied to changes in Southern Hemisphere



October 5, 2016

University of Washington
The ocean circulation that is responsible for England's mild climate appears to be slowing down. The shift is not sudden or dramatic, as in the 2004 sci-fi movie "The Day After Tomorrow," but it is a real effect that has consequences for the climates of eastern North America and Western Europe. Also unlike in that movie, and in theories of long-term climate change, these recent trends are not connected with the melting of the Arctic sea ice and buildup of freshwater near the North Pole. Instead, they seem to be connected to shifts at the southern end of the planet, according to a recent University of Washington study in the journal Geophysical Research Letters.
"It doesn't work like in the movie, of course," said Kathryn Kelly, an oceanographer at the UW's Applied Physics Laboratory. "The slowdown is actually happening very gradually, but it seems to be happening like predicted: It does seem to be spinning down."
The study looked at data from satellites and ocean sensors off Miami that have tracked what's known as the Atlantic overturning circulation for more than a decade. Together they show a definite slowdown since 2004, confirming a trend suspected before then from spottier data.
Looking at other observations to determine the cause, the researchers ruled out what had been the prime suspect until now: that massive melting and freshening in the North Atlantic could stop water from sinking and put the brakes on the overturning circulation, which moves warmer water north along the ocean's surface and sends cold water southward at depths.
"It appears that this 10-year slowdown is not related to salinity," Kelly said. In fact, despite more ice melt, surface water in the Arctic is getting saltier and therefore denser, she said, because of less precipitation. "That means the slowdown could not possibly be due to salinity -- it's just backwards. The North Atlantic has actually been getting saltier."
Instead, the authors saw a surprising connection with a current around the southern tip of South Africa. In what's known as the Agulhas Current, warm Indian Ocean water flows south along the African coast and around the continent's tip toward the Atlantic, but then makes a sharp turn back to join the stormy southern circumpolar current. Warm water that escapes into the Atlantic around the cape of South Africa is known as the Agulhas Leakage. The new research shows the amount of leakage changes with the quantity of heat transported northward by the overturning circulation.
"We've found that the two are connected, but I don't think we've found that one causes the other," Kelly said. "It's more likely that whatever changed the Agulhas changed the whole system."
She believes atmospheric changes may be affecting both currents simultaneously.
"Most people have thought this current should be driven by a salinity change, but maybe it's the [Southern Ocean] winds," Kelly said.
The finding could have implications for northern European and eastern U.S. climates, and for understanding how the world's oceans carry heat from the tropics toward the poles.
"I think it changes how we think about the whole Atlantic overturning circulation, of which the Gulf Stream is a part," said co-author LuAnne Thompson, a UW professor of oceanography. "It brings back the role of the atmosphere into what's controlling the climate in the high latitudes, that it's not all driven by what's happening in the oceans."
And while a slowdown of the Gulf Stream and broader overturning circulation, for whatever reason, would bring less warm water to eastern North America and Western Europe, any effects are overwhelmed by the overall warming due to global climate change.
"So that whole concept in the movie of New York harbor freezing doesn't make any sense," Kelly said. "If the Gulf Stream doesn't carry as much heat from the tropics, it just means that the North Atlantic is not going to warm up as fast as the rest of the ocean -- it's not going to cool down."

Story Source:
Materials provided by University of WashingtonNote: Content may be edited for style and length.

Journal Reference:
  1. Kathryn A. Kelly, Kyla Drushka, LuAnne Thompson, Dewi Le Bars, Elaine L. McDonagh. Impact of slowdown of Atlantic overturning circulation on heat and freshwater transportsGeophysical Research Letters, 2016; 43 (14): 7625 DOI: 10.1002/2016GL069789

Cite This Page:
University of Washington. "Gulf Stream slowdown tied to changes in Southern Hemisphere." ScienceDaily. ScienceDaily, 5 October 2016. .

Danish Scientist Targets Claim that Gulf Stream Is Slowing

REUTERS   British Antartic Survey/Handout

A Danish climatologist is pouring cold water over another misleading global warming claim.

Thermographic images released by the National Oceanic and Atmospheric Administration (NOAA) last year showed a mysterious blue blob of cold seawater off the coast of southern Greenland and Iceland. At the time, researchers said the cold blob was likely the result of melting from Greenland’s vast ice sheet, with cold water flowing into the nearby Labrador Sea.
This meltwater was presumed to have slowed the Atlantic Meridional Overturning Circulation (AMOC), or Gulf Stream, that sustains temperate weather in much of western Europe, our East Coast, and the UK.

One of the first researchers at the trough of misinformation was Prof. Michael Mann of Pennsylvania State (and “Hockey Stick” fame), who told the Washington Post that this North Atlantic cold blob happened to fit with his just-published study. Mann also suggested that a “dramatic melting” of Greenland’s ice sheet would slow or stop the AMOC, sending the world into an abrupt climate shift as seen in the climatastrophe flick The Day After Tomorrow.
But Tor Eldevik, a Norwegian climate researcher and professor, subsequently told the newspaper Aftenposten in March 2016 that he was “not convinced that the blue blob was caused by melting Greenlandic ice.” Or that it was slowing the Gulf Stream.
Eldevik said that affecting the AMOC would require vast amounts of freshwater coming into contact with more saline ocean water, which would dramatically change salinity levels and possibly slow the current down. That theory might make sense, Eldevik said, except the currents around Greenland simply aren’t strong enough, or large enough, to slow or stop the Gulf Stream.
Another climate researcher, Peter Langen, showed that this melting Greenlandic ice claim has nothing to do with global warming but rather the result of a particularly cold winter. Dr. Langen, a climatologist at the Danish Meteorological Institute (DMI), said that while Greenland’s coastal areas are melting a little, there simply hasn’t been enough freshwater released to affect the ocean’s circulation in any significant way.
In fact, a year-long study of Greenland’s interior ice sheet showed how very little precipitation is lost via evaporation or melting due to the island’s unique “thermal lid.”
The study, published in Science Advances, illustrates how this unique thermal lid prevents snow and ice from escaping the island, allowing the ice sheet to continuously accumulate in mass. And despite computer climate models claiming Greenland’s ice sheet would be one of the first fatalities in a warming world, this first-of-its-kind study offers convincing evidence that Greenland’s ice sheet remains robust and stable.
Langen also noted that ocean modeling from various studies shows that there simply isn’t enough freshwater being released to influence the Gulf Stream at any great distance from Greenland. After reviewing two new studies, Langen adds that the cold spot seen on NOAA’s map is most likely the result of a simple weather anomaly: an unusually cold winter.
Langen says if the cause of the cold blob was meltwater, it would get mixed and thinned out. The Labrador Sea, which is southwest of Greenland’s southern tip, helps to mix surface and deep ocean water. The saltier the water, the more mixing occurs. If the salinity decreases, so does the mixing. Put simply, the freshwater becomes diluted in such a large area that it is unable to slow or stop oceanic circulation.
Langen adds that the blue blob showed up on NOAA’s maps during the very cold winter of 2014-2015, and the frigid weather actually resulted in increased mixing of surface water with the cold, deep seawater. Langen also explained that researchers reconstructed the cooling of water in the area through complex energy computations, making the weather theory the strongest, and most likely, explanation for the North Atlantic cold blob.
It appears that the cold water observed in the North Atlantic was simply due to a cold winter. And yet, climate alarmists readily attempted to spin it into further proof of global warming. But an analysis of the data shows that, once again, such dramatic claims were overstated.
Thomas Richard is a freelance writer living outside of Boston, MA.

Gulf stream slowdown to spare Europe from worst of climate change


11 de julio de, 2016 James Hakner

The Thermohaline Circulation is a vast system of ocean currents that operates like a conveyor belt, transporting warm water from the tropics to Europe. Credit: istockphoto.com/aristoo

Read more at: https://phys.org/news/2016-07-gulf-stream-slowdown-europe-worst.html#jCp

Europe will be spared the worst economic impacts of climate change by a slowing down of the Gulf Stream, new research predicts.

Scientists have long suggested that global warming could lead to a slowdown – or even shutdown – of the vast system of ocean currents, including the Gulf Stream, that keeps Europe warm.
Known as the Thermohaline Circulation, this system operates like a conveyor belt, transporting warm water from the tropics to Europe, where evaporation decreases salinity and density so that the water sinks.
As the world warms, melting icecaps and increased rainfall are widely predicted to slow this process down by flooding oceans with cold freshwater.
Some experts even fear that the process could shut down altogether, plunging Europe into a new ice age.
However, a new study by the University of Sussex, Universidad Nacional Autónoma de México and the University of California, Berkeley finds that, rather than cooling Europe, a slowdown of the Thermohaline Circulation would mean the continent still warms, but less quickly than other parts of the world.
This would lead to a rise in welfare standards in Europe, concludes the research, which is published in the leading economics journal the American Economic Review.
Professor Tol, Professor of Economics in the School of Business, Management and Economics at the University of Sussex, said: "Cooling is probably a good bit more harmful than warming, particularly in Europe. People rightly fear that climate change would cause a new ice age.
"Fortunately, our study finds no cooling at all. Instead, we find slower warming: a boon for Europeans."
Of course, as  redistribute rather than create heat, slower warming for Europe means slightly accelerated warming elsewhere.
The study, therefore, adds to a growing body of evidence predicting a rich/poor divide in the  stakes. Developing countries will be less able to cope with rising sea levels, for example, and - as this research suggests - may warm faster than other, more developed parts of the world.
More information: David Anthoff et al. Shutting Down the Thermohaline Circulation, American Economic Review (2016). DOI: 10.1257/aer.p20161102 

Read more at: https://phys.org/news/2016-07-gulf-stream-slowdown-europe-worst.html#jCp

Gulf Stream is slowing down faster than ever, scientists say


The Gulf Stream that helps to keep Britain from freezing over in winter is slowing down faster now than at any time in the past millennium according to a study suggesting that major changes are taking place to the ocean currents of the North Atlantic.
Scientists believe that the huge volumes of freshwater flowing into the North Atlantic from the rapidly melting ice cap of Greenland have slowed down the ocean “engine” that drives the Gulf Stream from the Caribbean towards north-west Europe, bringing heat equivalent to the output of a million power stations.
Scientists believe that huge volumes of freshwater flowing into the North Atlantic from the rapidly melting ice cap of Greenland have slowed down the ocean “engine” that drives the Gulf Stream (Getty)

However, the researchers believe that Britain is still likely to become warmer due to climate change providing the Gulf Stream does not come to a complete halt – although they remain unsure how likely this is.
Calculations suggest that over the 20th century the North Atlantic meridional overturning circulation – the northward flow of warm surface water and the southward flow of deep, cold water – has slowed by between 15 and 20 per cent, said Professor Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany.
“There is more than a 99 per cent probability that this slowdown is unique over the period we looked at since 900 AD. We conclude that the slowdown many have described is in fact already underway and it is outside of any natural variation,” Professor Rahmstorf said.
The scientists calculated that some 8,000 cubic kilometres of freshwater has flowed from Greenland into the Atlantic between 1900 and 1970, and this rose significantly to 13,000 cubic kilometres between 1970 and 2000.
Freshwater is lighter than salty water which means that it tends to float on the surface of the ocean and in doing so disturbs the normal sinking of dense, cold saltwater to the ocean floor, which is the main driver of the Atlantic circulation.
Jason Box of the Geological Survey of Denmark and Greenland, who helped to calculate the amount of freshwater flowing into the Atlantic from melting ice caps, said that the slowdown can be linked to man-made climate change.
“Now freshwater coming off the Greenland ice sheet is likely disturbing the circulation. So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning, and this effect might increase if temperatures are allowed to rise further,” Dr Box said.
Michael Mann of Pennsylvania State University said: “Common climate models are underestimating the change we’re facing, wither because the Atlantic overturning is too stable in the models or because they don’t properly account for Greenland ice melt, or both.”