January 24, 2011

Bye-Bye Polar Lows?

Filed under: Climate Changes, Polar

We suspect everyone who reads World Climate Report has experienced a mid-latitude cyclone. These are the low pressure features that routinely cross the United States with warm, cold, and occluded fronts that bring us rain and snow. Some of these lows, like the occasional Nor-easters, can produce high winds, large amounts of precipitation (either rain or snow), and can be associated with considerable damage. Many of our readers have also experienced tropical cyclones – low pressure features that can grow into hurricanes. With lows in the tropics, and lows in the mid-latitudes, some alert school child might ask about lows in the polar regions.

So here we’ll take at how “polar lows” and how they may change character in a warmer climate.

Polar lows are small cyclones that form over open sea during the cold season within cold polar or arctic air masses. Typically 100 to 200 miles in diameter, they possess strong winds and tend to form beneath cold upper-level troughs or lows when frigid arctic air flows southward over a relatively warm body of water (jump in the relatively warm waters of the Arctic, and you will come to appreciate the word “relatively”). Polar lows last on average only a day or two. They can develop rapidly, reaching maximum strength within 12 to 24 hours of the time of formation. They often dissipate just as quickly, especially upon making landfall. In some instances several may exist in a region at the same time or develop in rapid succession.

Satellite imagery (see below) shows polar lows with their characteristic spiral or comma shaped patterns of deep clouds, sometimes with an inner “eye” similar to those seen in hurricanes. Convective cloud bands occupy the surroundings, and given their many similarities to tropical cyclones, some people refer to polar lows as “Arctic Hurricanes” (they form in the Antarctic as well). Polar lows are difficult to predict even with current high resolution and high performing operational numerical models, because they usually occur in remote oceanic regions where data are too sparse to define the model initial state on a sufficiently fine scale. While they may seem irrelevant in our everyday lives, they represent a significant hazard to high-latitude operations, such as shipping and gas and oil platforms. Watch a few episodes of Deadliest Catch, and you may watch Sig and the boys cope with a polar low.


A classic polar low off northern Norway

We focus on polar lows this week given an article that has appeared in Nature written by two scientists with Europe’s University of Reading and University of Hamburg. Zahn and von Storch begin their piece noting “Every winter, the high-latitude oceans are struck by severe storms that are considerably smaller than the weather-dominating synoptic depressions. Accompanied by strong winds and heavy precipitation, these often explosively developing mesoscale cyclones— termed polar lows—constitute a threat to offshore activities such as shipping or oil and gas exploitation. Yet owing to their small scale, polar lows are poorly represented in the observational and global reanalysis data often used for climatological investigations of atmospheric features and cannot be assessed in coarse-resolution global simulations of possible future climates”.

They fired up regional climate models and found they could reasonably accurately downscale the output and reproduce the basic climatology of the polar lows of the North Atlantic. Satisfied with those results, they then simulated what would happen to the polar lows under various future scenarios suggested by the IPCC. In their own words, they describe the results stating:

[I]n projections for the end of the twenty-first century, we found a significantly lower number of polar lows and a northward shift of their mean genesis region in response to elevated atmospheric greenhouse gas concentration. This change can be related to changes in the North Atlantic sea surface temperature and mid-troposphere temperature; the latter is found to rise faster than the former so that the resulting stability is increased, hindering the formation or intensification of polar lows. Our results provide a rare example of a climate change effect in which a type of extreme weather is likely to decrease, rather than increase.

We were caught a bit off guard by that final sentence in the abstract. While it is true that a decline in the number of polar lows is an example of a type of exteme event that is projected to decline under a warming climate, we were a bit suprised by Zahn and von Stroch’s assertion about how unusual it is to find any type of extreme weather phenomenon that is expected to decrease rather than increase. If you look through our back pages, you will find dozens of examples in which we show theoretical modeling studies and/or empirical studies predicting or showing no change or even a decline in everything from hurricanes to tornados to heavy rain events to blizzards. The most recent IPCC assessment states on page 308 “Observational evidence for changes in small-scale severe weather phenomena (such as tornadoes, hail and thunderstorms) is mostly local and too scattered to draw general conclusions”.

We conducted a web search on “Global Warming and Extreme Weather” and found over 600,000 sites almost universally insisting that global warming will increase extreme weather events all over the world. We would never deny that the scientific literature contains many articles that indeed support this assertion, and predicting an increase in extreme weather events is certain to attract a lot of media coverage. As we have shown repeatedly in our essay series, the literature is alive with articles predicting or showing no change in extreme weather events throughout the world. These articles are certainly more difficult to publish (predicting or showing no change is a tough sell to reviewers and/or editors), and they tend to receive no press coverage whatsoever. Like it or not, predicting no change in extreme weather is simply not going to sell newspapers anytime soon.

Anyway, if you are planning on spending a lot of time in the Arctic seas during winter in coming decades, be comforted to know that the buildup of greenhouse gases may have decreased the number of polar lows you will likely encounter in the future!

Reference:

Zahn, M., and H. von Storch. 2010. Decreased frequency of North Atlantic polar lows associated with future climate warming. Nature, 467, doi:10.1038/nature09388.




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