"The impact of polar mesoscale storms on northeast Atlantic Ocean circulation," by Alan Condron & Ian A. Renfrew, Nature Geosci., 6 (2012); doi: 10.1038/ngeo1661

Nature Geoscience, 6 (published online December 16, 2012) 34-37; doi: 10.1038/ngeo1661

The impact of polar mesoscale storms on northeast Atlantic Ocean circulation

• Alan Condron and
•  Ian A. Renfrew

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Abstract

Atmospheric processes regulate the formation of deep water in the subpolar North Atlantic Ocean and hence influence the large-scale ocean circulation¹. Every year thousands of mesoscale storms, termed polar lows, cross this climatically sensitive region of the ocean. These storms are often either too small or too short-lived to be captured in meteorological reanalyses or numerical models^2, 3, 4. Here we present simulations with a global, eddy-permitting ocean/sea-ice circulation model, run with and without a parameterization of polar lows. The parameterization reproduces the high wind speeds and heat fluxes observed in polar lows as well as their integrated effects, and leads to increases in the simulated depth, frequency and area of deep convection in the Nordic seas, which in turn leads to a larger northward transport of heat into the region, and southward transport of deep water through Denmark Strait. We conclude that polar lows are important for the large-scale ocean circulation and should be accounted for in short-term climate predictions. Recent studies^3, 4 predict a decrease in the number of polar lows over the northeast Atlantic in the twenty-first century that would imply a reduction in deep convection and a potential weakening of the Atlantic meridional overturning circulation.

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http://www.nature.com/ngeo/journal/v6/n1/abs/ngeo1661.html

Polar mesoscale storms missing from models

'Missing' polar weather systems could impact climate predictions

Intense but small-scale polar storms could make a big difference to climate predictions according to new research from the University of East Anglia and the University of Massachusetts.

Difficult-to-forecast polar mesoscale storms occur frequently over the polar seas, however they are missing in most climate models. 

Research published today in Nature Geoscience shows that their inclusion could paint a different picture of climate change in years to come. 

Polar mesoscale storms are capable of producing hurricane-strength winds which cool the ocean and lead to changes in its circulation. 

Prof Ian Renfrew, from UEA's School of Environmental Sciences, said: "These polar lows are typically under 500 km in diameter and over within 24-36 hours. They're difficult to predict, but we have shown they play an important role in driving large-scale ocean circulation. 

"There are hundreds of them a year in the North Atlantic, and dozens of strong ones. They create a lot of stormy weather, strong winds and snowfall – particularly over Norway, Iceland, and Canada, and occasionally over Britain, such as in 2003 when a massive dump of snow brought the M11 to a standstill for 24 hours. 

"We have shown that adding polar storms into computer-generated models of the ocean results in significant changes in ocean circulation -- including an increase in heat travelling north in the Atlantic Ocean and more overturning in the sub-polar seas. 

"At present, climate models don't have a high enough resolution to account for these small-scale polar lows. 

"As Arctic Sea ice continues to retreat, polar lows are likely to migrate further north, which could have consequences for the 'thermohaline' or northward ocean circulation – potentially leading to it weakening." 

Alan Condron from the University of Massachusetts said: "By simulating polar lows, we find that the area of the ocean that becomes denser and sinks each year increases and causes the amount of heat being transported towards Europe to intensify. 

"The fact that climate models are not simulating these storms is a real problem because these models will incorrectly predict how much heat is being moved northward towards the poles. This will make it very difficult to reliably predict how the climate of Europe and North America will change in the near-future." 

Prof Renfrew added: "Climate models are always improving, and there is a trade-off between the resolution of the model, the complexity of the model, and the number of simulations you can carry out. Our work suggests we should put some more effort into resolving such storms." 

'The impact of polar mesoscale storms on Northeast Atlantic ocean circulation' by Alan Condron from the University of Massachusetts (US) and Ian Renfrew from UEA (UK), is published in Nature Geoscience on December 16, 2012. 

Picture credit: NEODAAS / University of Dundee

http://www.uea.ac.uk/mac/comm/media/press/2012/December/polar-storms-climate-change

Arctic Storms: A Climate Danger Nobody’s Talking About

by Michael Lemonick, Climate Central, December 18, 2012

Summer and fall are hurricane season, but for the storms known as polar lows, prime time falls in the dead of winter, when frigid air blows off sea ice to collide with warmer, moister air in the North Atlantic. Polar lows are a lot smaller and weaker than hurricanes, they’re generally shorter-lived, and the only danger they generally pose is to shipping and oil rigs.

However, according to a new study in Nature Geoscience, the dozens of polar lows that roil the Greenland, Iceland and Norwegian seas every year may have an effect on the climate of North America and Europe. And if polar lows move northward with the changing climate, as some studies have predicted, winters in both places could become colder, even as the planet warms.

A polar low northeast of Scandinavia in the Barents Sea. Credit: Erik Kolstad/flickr.

As if that weren’t bad enough, a northward displacement of these Arctic storms could also raise sea level higher along America’s mid-Atlantic coast than the average increase of 3 feet or so projected for the world as a whole by 2100.

It all has to do with the Atlantic meridional overturning circulation (AMOC), a vast conveyer belt of sea water that includes the Gulf Stream. Warm equatorial water travels up along the U.S. East Coast, and then peels off toward the northeast to bathe England and Western Europe with relatively balmy water and air. Without this current’s moderating influence, Madrid, for example, which is as far north as Chicago, would be a lot colder.

When the water reaches the region between Greenland and Norway, it finally cools, becomes denser, and sinks to the bottom of the ocean, where it flows south again, eventually to rise, warm, and begin the journey all over again.

The cooling and sinking would happen in any case, but polar lows help it along by churning the ocean surface, forcing the water to give up its heat faster than it otherwise would; it’s like stirring a cup of tea to make it cool off.

“We think that perhaps 4 or 5 percent of the cooling is due to polar lows,” said lead author Alan Condron, of the University of Massachusetts, in an interview.

As the planet warms, sea ice is likely to diminish in the Arctic — especially in summer, but also, as you go further south, in winter as well. “A couple of previous studies have shown that if the ice migrates northward, these storms could migrate northward as well,” said co-author Ian Renfrew, of the University of East Anglia, in the U.K.

If that happens, their ocean-roiling, water-cooling effect on the AMOC would diminish and the current would presumably slow. That would mean less warm water and warm air for the U.S. East Coast and for Europe — and because water would back up like cars on a congested freeway, the New York-New Jersey region could experience greater sea-level rise by 2100 than the projected worldwide average.

This will be especially true if another effect kicks in. As melting ice caps and glaciers inundate the seas between Greenland and Norway with fresh water, the saltiness of those seas will diminish — and since fresh water is less dense than salt water, it will be even more difficult for surface water to sink for the return southward.

Dilution and the loss of polar storms could therefore combine to increase both the cooling of air and the backup of water — although when it comes to the storms, Condron and Renfrew aren’t making any firm predictions about how much. “That will have to wait for more research,” Renfrew said.

Once the scientists have a better idea of what changes in polar lows might do to ocean temperatures, that information still needs to be incorporated into climate models.

But since the storms are so small and localized, it will take more powerful models than any currently in existence to come up with an overall projection for how the changes would do to climate overall. “It will probably take 10 or even 20 years before we have the raw computing power to do that,” Renfrew said. 

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http://www.climatecentral.org/news/arctic-storms-the-climate-danger-nobodys-talking-about-15363

Kolstad & Bracegirdle, More Extreme Weather in the Arctic Regions

More Extreme Weather in the Arctic Regions

ScienceDaily (Feb. 5, 2009) — A new study published in Climate Dynamics by Erik Kolstad and Thomas J. Bracegirdle reveals that one of the most visible signs of climate change is the dramatically reduced ice cover in the Arctic. The retreat of the sea ice leads to rapid changes in the weather conditions in these areas.

The study reveals that regions that have been covered by sea ice until now will be exposed to new kinds of severe weather. This may have dire consequences for human activities in the Northern regions.

The study was led by a member of the International Polar Year project IPY-THORPEX (THe Observing system Research and Predictability EXperiment). The main focus of the project is to study extreme weather phenomena from the inside, with the purpose of acquiring new knowledge in order to improve weather forecasts.

Increased activity – and more extreme weather

Large increases in the potential for extreme weather events were found along the entire southern rim of the Arctic Ocean, including the Barents, Bering and Beaufort Seas. While these areas are sparsely populated, an increasing commercial marine activity is predicted there, paradoxically because the sea ice is set to retreat.

"One consequence of climate change is that new areas are uncovered, opening for commercial activities," said Dr. Erik Kolstad, at the Bjerknes Centre for Climate Research, who led the study.

At the same time, commercial activities in the North (e.g., fisheries, oil industry and shipping) will become increasingly vulnerable to extreme weather as the activities in these areas increase.

"It is important that we get better at forecasting these weather phenomena, in order to prevent the loss of human lives and environmental disasters in the future," Kolstad said.

In adddition, the limited existing infrastructure for responding to maritime accidents in the Arctic must be strengthened. As Arctic sea ice declines and commercial activities in the region increases, this becomes incresingly important, according to a new report recently released by the University of New Hampshire and the National Oceanic and Atmospheric Administration in the U.S.

Cold air is the “fuel”

Arctic weather has many faces. While the conditions over the Arctic Ocean ice sheet are frequently calm and cloudy, the warm regions with open ocean surrounding it are host to severe weather, such as explosive mid-latitude storms, polar lows and arctic fronts.

A common feature of these weather types is that they form when cold air masses wander out from over the ice sheets over the warm ocean to be heated from below. In the North Atlantic, such conditions arise frequently along the Gulf Stream and its northern branches. The North-East Atlantic (the Greenland, Iceland, Norwegian and Barents Seas) is particularly prone to marine cold-air outbreaks (MCAOs), as they are referred to in the paper.

Some good news

As the sea ice in the Northern Hemisphere retreats rapidly, the regions with the highest frequency of MCAOs today are "pulled" towards the north. Thus, a projected decrease in the strength of MCAOs along the most densely populated coastlines was also found. This may prove to be good news for people along the coastlines of Norway, Iceland, the British Isles and Northern Europe in general.

Flew into extreme weather

The IPY-THORPEX research team spent three weeks in the North of Norway and repeatedly flew into and over extreme weather conditions, using a high-tech aircraft to perform accurate measurements. The campaign yielded enormous amounts of new data.

The highlight of the field campaign was an unprecedented documentation of a "polar low," the arctic cousin of the tropical hurricanes, from beginning to end. The researchers have now started to scrutinize the data for hitherto unknown details about the many weather phenomena that were put on record during the campaign.


Kolstad and Bracegirdle. Marine cold-air outbreaks in the future: an assessment of IPCC AR4 model results for the Northern Hemisphere. Climate Dynamics, 2008; 30 (7-8): 871 DOI: 10.1007/s00382-007-0331-0

Adapted from materials provided by University of Bergen, via AlphaGalileo.

Link to article: http://www.sciencedaily.com/releases/2009/02/090205083526.htm