Cold-air outbreaks

My research is pretty much focussed on high-latitude weather, and things like polar lows [?] in particular. I’m currently writing a paper about marine cold-air outbreaks, i.e. large-scale departures of cold, polar air from over the sea ice into regions of relatively warm, open ocean. In these situations, the cold air sucks huge amounts of energy out of the water, much like what happens during tropical cyclones (hurricanes and typhoons).

My colleague Tom Bracegirdle and I have devised a fairly simple cold-air outbreak index. If you know the temperature of the sea surface and the air temperature at an altitude of some 2,000 metres, this index can be calculated at any time and place. I’ve done just that and picked out the most extreme cases in the last few years. This satellite image was taken over the North-East Atlantic around noon on 23 January, 2003 (click for a larger version):

The wind is coming from the north-west, down through the Fram Strait between Spitsbergen and Greenland and off the sea ice along the east coast of Greenland. After travelling a few hundred kilometres over the open ocean, it is heated enough for some no-nonsense clouds to develop. The white areas with the spirally shape are polar lows. The surface winds in this region were around 50 knots at the most intense. That’s not so far from hurricane force winds.

Another very cool feature is the von Kármán vortices [?] downstream of Jan Mayen near the middle left edge of the picture.

This kind of weather is important in many ways. The potential for accidents is only the most obvious issue. The worst accident in later years was when the British trawler FV Gaul went down in 1974, killing all 36 men on board [?]. Colleagues of mine here in Bergen have simulated the weather on this day using numerical weather prediction models, and they found that there was an “arctic front” (a phenomenon that is highly related to polar lows) in the region when the ship sank.

In Norwegian waters, there are numerous examples of ugly accidents at sea, especially in earlier times, when the forecasts were erratic or even non-existent.

Cold-air outbreaks are also important for the ocean. Alan Condron of Woods Hole, MIT is the lead author a new, important paper in the Journal of Geophysical Research. His group found that if the weather typically associated with cold-air outbreaks had been better represented in the widely used re-analysis data sets such as ERA-40 [?], it would have led to (in models)

enhanced surface latent and sensible heat fluxes and a dramatic increase in the cyclonic rotation of the Nordic Seas gyre by four times the average interannual variability. In response to these changes, Greenland Sea deep water formation generally increases by up to 20% in 1 month, indicating more active open ocean convection.

Condron’s study is a welcome addition to the literature, in that it couples the atmosphere and the ocean in an elegant manner. The disciplines of meteorology and oceanography, for all their similarities, are too often treated as separate entities. For instance, it was not until 1957 that one realized that the ocean phenomenon El Niño was intimately coupled with the atmospheric Southern Oscillation. Although one sometimes still refers to this system as simply El Niño, it is now known as the combination of the two: El Niño-Southern Oscillation, or ENSO [?].


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