Polar storms spur ocean circulation

Dec 16, 2012
These polar storms can have hurricane-strength winds and are common over the polar North Atlantic, but are missing from climate prediction models due to their small size. Credit: NEODAAS Dundee Satellite Receiving Station

Though it seems like an oxymoron, Arctic hurricanes happen, complete with a central "eye," extreme low barometric pressure and towering 30-foot waves that can sink small ships and coat metal platforms with thick ice, threatening oil and gas exploration. Now climate scientists at the University of Massachusetts Amherst and in England report the first conclusive evidence that Arctic hurricanes, also known as polar lows, play a significant role in driving ocean water circulation and climate.

Results point to potentially cooler conditions in Europe and North America in the 21st century than other models predict.

Geoscientist Alan Condron at UMass Amherst and Ian Renfrew at the University of , U.K., write in the current issue of Nature Geoscience that every year thousands of these strong cyclones or polar lows occur over Arctic regions in the North Atlantic, but none are simulated by the latest prediction models, which makes it difficult to reliably forecast climate change in Europe and North America over the next couple of decades.

"Before polar lows were first seen by satellites, sailors frequently returned from the Arctic seas with stories of encounters with fierce storms that seemed to appear out of nowhere," says Condron, a physical oceanographer. "Because of their small size, these storms were often missing from their weather charts, but they are still capable of producing hurricane-force winds and waves over 11 meters high (36 feet)."

He and Renfrew say that despite the fact that literally thousands of polar lows occur over the Arctic region of the every year, none are simulated by even the most sophisticated . To understand the importance of these storms on climate, Condron and Renfrew therefore turned to a new, state-of-the-art climate model to simulate the high wind speeds associated with these "missing" storms.

"By using higher resolution modeling we can more accurately simulate the high wind speeds and influence of polar lows on the ocean," Condron says. "The lower-resolution models currently used to make climate predictions very much miss the level of detail required to accurately simulate these storms."

He and Renfrew find that by removing heat from the ocean, polar lows influence the sinking of the very dense cold water in the North Atlantic that drives the large-scale ocean circulation or "conveyer belt" that is known as the thermohaline circulation. It transports heat to Europe and North America.

"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," Condron points out.

"The fact that climate models are not simulating these storms is a real problem," he adds, "because these models will wrongly predict how much heat is being moving 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."

Condron also notes that other research groups have found that the number of polar lows might decrease in the next 20 to 50 years. "If this is true, we could expect to see an accompanying weakening of the thermohaline circulation that might be able to offset some of the warming predicted for Europe and North America in the near future."

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More information: '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. DOI: 10.1038/NGEO1661

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runrig
5 / 5 (6) Dec 16, 2012
Before anyone of a certain opinion comments may I just say that polar lows are "mesoscale" events and until relatively recently even weather ( as in timescales of a few days ) models were unable to model them due to the unfeasible computational time required. They are caused by "cold pools" - regions of cold air of small size but having anticlockwise circulation through a large slice of the vertical atmosphere. They therefore derive their energy from both the cyclonic curvature and marked instability when moving over relatively warm seas. Hence the allusion to "hurricanes" - however they do not have a warm, descending core and neither do they gain most of their energy from WV condensation as does the real thing.

http://www.ecmwf....rner.pdf
Urgelt
5 / 5 (2) Dec 17, 2012
Agreed, runrig.

In the tropics, hurricane formation zones are expanding as the seas warm, which means more tropical depressions may develop into hurricanes. Maybe. Shear is also a factor and hard to predict in a warming climate scenario. But in the arctic, the opposite phenomenon may be encountered: as seas warm, it may rob arctic lows of some of their strength, thus weakening the thermohaline circulation, which will (absent consideration of other influences) produce a cooling effect in Europe.

Whether weakening Atlantic thermohaline circulation produces a cooling effect in North America depends on what, exactly, will happen to the lower Atlantic segment of the Gulf Stream.

Here's my thought. If the Atlantic thermohaline circulation is weakened, less heat will be transported via ocean current from equator to pole. That implies warmer tropics and cooler pole (But it's only one factor). That heat wants to move North; if ocean currents aren't moving it, something else will.
Urgelt
5 / 5 (2) Dec 17, 2012
That 'something else' might be atmospheric. So if the Gulf Stream peters out, we might just see more energetic atmospheric phenomena taking its place. I'm not at all convinced that North America will be (on average) cooler, in that event.

But it's also possible that a weakened Gulf Stream will pass the East Coast of North America as it has been doing all along, but then bend back South before it reaches Iceland. In this scenario, the current can't deliver heat to the poles (though it's still a heat pump moving heat from the tropics to the temperate zone), so the current gains in temperature and heats up the East Coast rather more than today. Europe's loss might be North America's gain, heat-wise.

None of that is easy to model with confidence, so that's only my opinion, not fact. We really have no idea how warming is going to affect Atlantic thermohaline circulation, or when.

Including polar lows in the models will move us one step closer to better modeling, at least.
cantdrive85
1 / 5 (6) Dec 17, 2012
These storms as well as nearly all weather and ultimately the climate on this planet are the result of the electric currents that flow through the Earth's electric field. Water and air currents are the observable representation of the electric currents that flow through this field. The Van Allen belts are the space version of these currents, and as more data is assembled it will reveal the inter-connectivity of all these currents as well as the solar currents flowing throughout the Sun's heliosphere.