Click here to view the article online

Just go to the directory where the geo_em*.nc files are found, and run with:

python plotdomains.py

It will create a KML file called domains.kml. Double-click on this to open in Google Earth.

Download the script [here]. Hope you like it.

Also shown are locations where the pressure was higher (H) and lower (L) than normal. The arrows show anomalous flow patterns (where there was an above-normal frequency of winds).

The coldest region was Russia, where the high over the Barents Sea region led to an increased frequency of northerly winds. In Europe, the westerlies were pretty much dead throughout the whole winter, with the storms heading into Southern Europe instead. The high near Iceland gave less frequent northerlies down through Baffin Bay, and even southerlies into The Labrador Sea at times. This gave very high temperatures in Greenland and the Canadian Arctic.

Needless to say, we’re trying to find out why this all happened.

This was the forecast for 19–20 November 2009. The numbers are in millimeters.

A new study represents a step towards better forecasting of severe weather in polar regions. Cold air outbreaks over the ocean can be linked to large-scale weather patterns, and this leads the way to using new tools to forecast such events.

Marine cold air outbreaks are the breeding grounds of severe weather in the maritime polar regions. An MCAO is a large-scale departure of cold air, typically from regions covered with sea ice, into regions with open ocean. The temperature contrast between the (relatively) warm ocean and the air can be more than 20 degrees Celsius, and this leads to rising air and the development of low-pressure systems. The best-known weather phenomenon associated with MCAOs is the polar low. Sometimes compared to hurricanes, although smaller in scale, these cyclones can produce hurricane-force winds and large amounts of snow. MCAOs therefore form a significant part of the marine hazard in populated coastal regions of Norway, Russia, Iceland, Japan and even the British isles. MCAOs are also important factors in determining the amount of heat that is lost from the ocean to the air during winter. Such heat loss is associated with deep-water formation and the strength of the oceanic thermohaline circulation, the Great Ocean Conveyor. In a new study, published in Climate Dynamics and led by Bjerknes researcher Erik Kolstad, the correspondence between large-scale weather patterns and MCAOs is explored. Over the Nordic Seas region, a pronounced high-pressure anomaly over Greenland, either acting alone or in concert with a strong low-pressure anomaly over northeastern Europe, were shown to be favourable for MCAOs to form. This is potentially important because such conditions are linked to negative phases of the North Atlantic Oscillation, the primary weather pattern in the North Atlantic region. This means that any progress in medium and long range forecasting of the NAO – a large research field – may also be translated to enhanced forecasting of MCAOs.

Complete reference: Kolstad, E. W., Bracegirdle, T. J. & Seierstad, I. A. (2009), Marine cold-air outbreaks in the North Atlantic: temporal distribution and associations with large-scale atmospheric circulation, Climate Dynamics, 33(2), 187-197.

A pre-print of the article can be downloaded here.

Previous studies have identified an association between near-surface temperature anomalies in the Northern Hemisphere and weak stratospheric polar westerlies. Large regions in northern Asia, Europe and North America have been found to cool in the mature and late stages of stratospheric weak vortex events. A substantial part of the temperature changes are associated with changes to the tropospheric Northern Annular Mode and North Atlantic Oscillation pressure patterns. The apparent coupling between the stratosphere and the troposphere may be of relevance for weather forecasting, but only if the temporal and spatial nature of the coupling is known. Here we show, using 51 winters of re-analysis data, that the tropospheric temperature development relative to stratospheric weak polar vortex events goes through a series of well-defined stages, including geographically distinct cold air outbreaks. At the inception of weak vortex events, a precursor signal in the form of a strong high-pressure anomaly is found over Northwest Europe. At the same time, long-lived and robust cold anomalies appear over Asia and Western Europe. A few weeks later, near the mature stage of weak vortex events, a shorter-lived cold anomaly emerges off the east coast of North America.The probability of cold air outbreaks in different phases of the weak vortex life cycle increases by 40–70 % in four key regions. This shows that the stratospheric polar vortex contains information that can be used to enhance forecasts of cold air outbreaks. 300-year pre-industrial control runs of 11 state-of-the-art coupled climate models corroborate our results.

Here’s a list of sites to run the story:

Scientific American | USA Today | Yahoo! Finance | CNBC | Forbes

The original press release (that I wrote) was printed more or less verbatim elsewhere:

Science Daily | Science Centric

The Canadian new service Canwest also wrote about this after interviewing my by e-mail:

Edmonton Journal | The Province | dose.ca

That night they rode through a region electric and wild where strange shapes of soft blue fire ran over the metal of the horses’ strappings and the wagonwheels rolled in hoops of fire and little shapes of pale blue light came to perch in the ears of the horses and in the beards of men. All night sheetlightning quaked sourceless to the west beyond the midnight thunderheads, making a bluish day of the distant desert, the mountains on the sudden skyline stark and black and livid like a land of some other order out there whose true geology was not stone but fear. The thunder moved up from the southwest and lightning lit the desert all about them, blue and barren, great clanging reaches ordered out of the absolute night like some demon kingdom summoned up or changeling land that come the day would leave them neither trace nor smoke nor ruin more than any troubling dream.

– Blood meridian

The polar low season started early this year. In this picture, which was taken on Tuesday 28 October, 2008, we see a pretty specimen centered roughly over Shetland. This polar low is part of the first proper cold-air outbreak this winter, yielding October snow in London for the first time since the 1930s.

The polar lows season in the North Atlantic usually starts in October/November and ends in March/April. The season that ended earlier this year was characterized by weak activity and few cold-air outbreaks.