E. W. Kolstad & T. J. Bracegirdle, Marine cold-air outbreaks in the future: An assessment of IPCC AR4 model results for the Northern Hemisphere

Climate Dynamics, 30 (6-7) 871-885, 2008.

Marine cold-air outbreaks in the future: An assessment of IPCC AR4 model results for the Northern Hemisphere

Erik W. Kolstad* (Bjerknes Centre for Climate Research, Allégaten 70, 5007 Bergen, Norway; e-mail: gbsek@uib.no) and Thomas J. Bracegirdle (British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK)

(Received: 30 January 2007 Accepted: 16 October 2007 Published online: 6 November 2007)

Abstract

For many locations around the globe some of the most severe weather is associated with outbreaks of cold air over relatively warm oceans, referred to here as marine cold-air outbreaks (MCAOs). Drawing on empirical evidence, an MCAO indicator is defined here as the difference between the skin potential temperature, which over open ocean is the sea surface potential temperature, and the potential temperature at 700 hPa. Rare MCAOs are defined as the 95th percentile of this indicator. Climate model data that have been provided as part of the Intergovernmental Panel on Climate Change (IPCC) Assessment Report Four (AR4) were used to assess the models’ projections for the twenty-first century and their ability to represent the observed climatology of MCAOs. The ensemble average of the models broadly captures the observed spatial distribution of the strength of MCAOs. However, there are some significant differences between the models and observations, which are mainly associated with simulated biases of the underlying sea ice, such as excessive sea-ice extent over the Barents Sea in most of the models. The future changes of the strength of MCAOs vary significantly across the Northern Hemisphere. The largest projected weakening of MCAOs is over the Labrador Sea. Over the Nordic seas the main region of strong MCAOs will move north and weaken slightly as it moves away from the warm tongue of the Gulf Stream in the Norwegian Sea. Over the Sea of Japan there is projected to be only a small weakening of MCAOs. The implications of the results for mesoscale weather systems that are associated with MCAOs, namely polar lows and arctic fronts, are discussed.

Link to abstract: http://www.springerlink.com/content/q1w7wm57nk60v516/

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