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Correction for climate models : an Unexpected upheavals in the North Atlantic

the climate in Europe, the circulation of the Northern Atlantic plays a key role. Often the speech is from the "Gulf stream", but the term doesn't hit it. With him, only a wind is referred to driven flow that starts in the tropics and to the North-West ranges. What is crucial is the Atlantic meridional overturning circulation (Atlantic Meridional Overturning Circulation, AMOC) From the South, warm water comes to the surface. It cools, so that its density increases. Then this water sinks and flows at depth back in the direction of the tropics. For North-West Europe, this overturning circulation is important, because the cooling is that the transported heat is released to the atmosphere. This contributes greatly to the mild climate on the continent – Berlin is located approximately at the same geographical width as Battle Harbour on Labrador in Canada, where it is considerably colder. The intergovernmental panel on climate change considers it "very likely" that the AMOC is in the course of climate change is weaker, and is based primarily on model calculations. The basis of measurements. The Problem: so Far, there were just in high Latitudes, only a few. A large-scale measurement campaign has closed the gap a bit and calls into question some assumptions of the climate models.

the Railway to the sea base

From 2014 to 2016, researchers in the framework of the "Overturning in the Subpolar North Atlantic Program" (OSNAP) have observed the Umwälzzirkulation along a line of Canada on the southern tip of Greenland to Scotland, both on the surface and in depth. "As part of OSNAP, we have included in the Labrador sea off the canadian coast steel sunk, which were held of old railroad wheels on the ground hawsers and measuring devices for temperature, salinity and flow velocity at different Depths," says Johannes Karstensen from the Helmholtz-centre for ocean research Kiel (Geomar). Due to the roughness of the sea in the area of the Sections ranged only up to 50 meters below the surface – otherwise, the risk would have been too large, that the devices would be ripped from their moorings. "After about two years, departs with the ship, the route, and solves by the acoustic Signal, the bracket the bottom of the sea." The devices are on the rise then, and will be collected to be able to read the data.

the Labrador sea is researchers as a key region for the AMOC, and thus for the climate in the Northern hemisphere. Here, to the West of Greenland, sinking large amounts of water in the depth. "In climate models, a connection between the amount of newly formed deep, and found water and the AMOC," says Karstensen. The formation of the cold salt water, for example, by higher air temperatures or melt water from glaciers, could have a direct impact on the AMOC. "In the past few years, there was some evidence that that's not true, perhaps not quite," says the researcher. The new measurements have now shown clearly that The Atlantic overturning circulation is subject to significant fluctuations, over the months and seasons of the year. This was due not to fluctuations in the Labrador sea, but in the sea region but to the East of Greenland: In the Irminger and Iceland basin, the variability is approximately seven times greater than that in the Labrador sea, the Team of Susan Lozier, Duke University, Durham (US-state of North Carolina) in the journal "Science".

The Umwälzzirkulation and thus a huge heat transport takes place primarily in the East and not in the Labrador sea. "We have to digest the first time," says Karstensen. Now, it is important to find out where exactly the Bulk of the deep water is formed and how the process works in Detail. And the Modeler will need to refinish in order to capture the currents in the Atlantic ocean, and their interaction with the climate better.

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Better yet, a couple of years to measure

"the new results do not necessarily mean that the existing models produce incorrect results," writes Monika Rhein, University of Bremen, in an accompanying commentary that also appears in "Science". In them there is a connection between the Labrador sea and the AMOC will be made in periods of several years and decades. Loziers Team, however, only collected data over 21 months. In addition, there is evidence that the Changes in the Labrador sea are not caused exclusively produced locally by the formation of deep water. Conceivable is also, that these are influenced from the North Atlantic. But here, too, is the fact that this connection is discussed in the scale of decades to centuries, not to the short distance of the current measurements.

There is a Plethora of issues that make it, according to the researchers is absolutely necessary, the monitoring program OSNAP to continue over one or two decades. "We obtain data series that are long enough to decadal fluctuations in the climate system, i.e. over several decades, to track down," says Geomar researchers Karstensen. "And we could verify the climate models, the objectives of Yes the rooms for a long time, or to provide data in order to improve them."

In addition, continuous measurements are also helpful to determine whether the AMOC weakening, actually. Because that could, in particular. for Europe, although no film-cooling collapse, but significant interference to bring In his "a 1.5-degree report" from October 2018, the world writes the IPCC in its standard terms that it was "more likely than not" that the circulation had weakened in the past decades. Stefan Rahmstorf from the Potsdam Institute for climate impact research (PIK) says it was "very likely", and in his Blog "climate lounge" numerous studies, the evidence to supply.

Falters the ocean heating, or not?

An Argument is, for example, virtually all of the other regions of the world for more than 50 years, decreasing the temperature of the North Atlantic to the polar region, while warming. This pattern fit exactly to the predictions of the climate modeling, which involves a weakening of the AMOC. "Unfortunately, there is no long-term direct measurements on the 20. Century,“ says Rahmstorf. However, were made since 2004, measurements in the South on a line at around 25 degrees North, within the framework of the "Rapid"project. "You show a slowdown, but the period is 15 years is still too short to assess how much of that is natural variation and how much global warming contributes to."

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The Kiel researchers Karstensen is still significantly restrained. "I'm careful, the pattern in the surface temperature could also be due to other causes." For a reliable statement, you should really measure over a longer period of time, in order to detect short-term fluctuations as such. "So far, no clear Trend for the long-term decline in the measured data." But that could change in the next few decades – "if the Trend exists because".

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