The future of intense winter storms

When Winter Storm Xynthia powered ashore over Europe last weekend, it brought hurricane-force wind gusts, flooding rains, and a 1-meter storm surge topped by 8-meter high battering waves that overwhelmed sea walls in France, killing scores of people. Today, AIR Worldwide estimated the insured damage from the storm at $1.5 - $3 billion. Intense extratropical cyclones like Xynthia, with central pressures below 970 mb, make up less than 20% of all wintertime cyclones in the Northern Hemisphere, but cause the vast majority of the devastation and loss of life. The ten deadliest winter storms to hit Europe over the past 60 years all had minimum pressures lower than 970 mb. The situation is similar for North America, though the storms generally do not get as intense as their European counterparts (the four major Nor'easters this winter have had central pressures of 968, 969, 978, and 972 mb). It is important, then, to ask if these strongest of the strong storms are changing in frequency, and whether a future warmer world will have more or less of these storms.


Figure 1. Winter Storm Xynthia, as captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite. Image was acquired in two separate overpasses on February 27, 2010. MODIS captured the eastern half of the image around 10:50 UTC, and the western half about 12:30 UTC. Forming a giant comma shape, clouds stretch from the Atlantic Ocean to northern Italy. Xynthia peaked in intensity at 18 UTC February 27, with a central pressure of 966 mb. Image credit: NASA.

Have intense Northern Hemisphere winter storms increased in number?
Most of the material for this post comes from three sources: the 2007 IPCC report, a 2009 review titled, Extra-tropical cyclones in the present and future climate: a review, and Weather and Climate Extremes in a Changing Climate, a 2009 report from the U.S. Global Change Research Program (USGCRP). An increasing number of intense winter storms in some regions of the Northern Hemisphere over the last few decades of the 20th century was a common theme of many of the studies reviewed. However, the studies used different measures as to what constitutes an "intense" storm, and have some disagreement on which areas of the globe are seeing more intense storms. A 1996 study by Canadian researcher Steven Lambert (Figure 3) found a marked increase in intense wintertime cyclones (central pressure less than 970 mb) in the latter part of the 20th century. Most of this increase occurred in the Pacific Ocean. Other studies (Geng and Sugi, 2001, and Paciorek et al., 2002) found an increase in intense winter storms over both the North Atlantic and North Pacific in the latter part of the 20th century. Benestad and Chen(2006) found an increase in the number of intense storms over the Nordic countries over the period 1955-1994, but no trend in the western parts of the North Atlantic. Gulev et al. (2001) found a small increase in the number of intense North Pacific storms (core pressure below 980 mb), a large increase in the Arctic, but a small decrease in the Atlantic. McCabe et al. 2001 found an increase at both mid-latitudes and high latitudes, particularly in the Arctic. Hirsch et al. (2001) found that the number of intense Nor'easters along the U.S. East Coast (storms with winds > 52 mph) stayed roughly constant at three storms per year over the period 1951 - 1997. Over the period 1900 to 1990, the number of strong cyclones (less than 992 mb) in November and December more than doubled over the Great Lakes of North America (Angel and Isard, 1998). With regards to Europe, Lionello et al. conclude, "the bulk of evidence from recent studies mostly supports, or at least does not contradict, the finding of an attenuation of cyclones over the Mediterranean and an intensification over Northern Europe during the second part of the twentieth century".


Figure 2. Trends in strong extratropical cyclones with central pressures less than 980 mb, for the period 1989 - 2009, as estimated using thirteen different methods, M02 - M22, defined in Neu et al., 2012. The error-bars represent the 95% confidence range of the trend estimate. A trend is significant at 5% level if the error-bar does not include zero. Four of the thirteen methods showed a slightly significant downward trend in both summertime and wintertime Northern Hemisphere strong extratropical cyclones during the period. None of the methods showed a statistically significant trend in Southern Hemisphere strong extratropical cyclones during either summer or winter. Image credit: U. Neu, M.G. Akperov, N. Bellenbaum, R. Benestad, R. Blender, R. Caballero, A. Cocozza, H.F. Dacre, Y. Feng, K. Fraedrich, J. Grieger, S. Gulev, J. Hanley, T. Hewson, M. Inatsu, K. Keay, S.F. Kew, I. Kindem, G.C. Leckebusch, M.L.R. Liberato, P. Lionello, I.I. Mokhov, J.G. Pinto, C.C. Raible, M. Reale, I. Rudeva, M. Schuster, I. Simmonds, M. Sinclair, M. Sprenger, N.D. Tilinina, I.F. Trigo, S. Ulbrich, U. Ulbrich, X.L. Wang, and H. Wernli, "IMILAST – a community effort to intercompare extratropical cyclone detection and tracking algorithms: assessing method-related uncertainties", Bulletin of the American Meteorological Society, pp. 120919072158001, 2012. http://dx.doi.org/10.1175/BAMS-D-11-00154.1

In summary, the best science we have shows that there has not been a statistically significant increase in the number of intense wintertime extratropical storms globally in the past two decades, but there has been and increase in the North Pacific and Arctic. Increased wave heights have been observed along the coasts of Oregon and Washington during this period, adding confidence to the finding of increased intense storm activity. The evidence for an observed increase in intense wintertime cyclones in the North Atlantic is uncertain. In particular, intense Nor'easters affecting the Northeast U.S. showed no increase in number over the latter part of the 20th century. This analysis is supported by the fact that wintertime wave heights recorded since the mid-1970s by the three buoys along the central U.S. Atlantic coast have shown little change (Komar and Allan, 2007a,b, 2008). However, even though Nor'easters have not been getting stronger, they have been dropping more precipitation, in the form of both rain and snow. Wintertime top 5% heavy precipitation events (both rain and snow) have increased over the Northeast U.S. in recent decades (Groisman et al., 2004), so Nor'easters have been more of a threat to cause flooding problems and heavy snow events. In all portions of the globe, tracks of extratropical storms have shifted poleward in recent decades, in accordance with global warming theory. Note that the historical data base for strong winter storms is in better shape than the data base we are using to try to detect long-term changes in hurricanes. The Ulbrich et al. (2009) review article states:

The IPCC AR4 (cf. Trenberth et al. 2007, p. 312) states that the detection of long-term changes in cyclone measures is hampered by incomplete and changing observing systems. Recent studies found, however, a general reliability of results for cyclones in the Northern Hemisphere. There are no sudden shifts in intensities that would indicate inhomogeneities, and also a comparison with cyclone activity estimated from regional surface and radiosonde data (Wang et al. 2006b; Harnik and Chang 2003) confirmed the general reliability of the data".

However, the data is not as good in the Southern Hemisphere, so the finding that intense winter storms are also increasing in that hemisphere must be viewed with caution.


Figure 3. Number of intense winter cyclones with central pressure less than 970 mb in the Northern Hemisphere, North Pacific, and North Atlantic between 1899 - 1991. Image credit: Lambert, S.J., 1996: Intense extratropical Northern Hemisphere winter cyclone events: 1899-1991. J. Geophys. Res., 101D, 2131921325.

Intense winter storms may increase in number
General Circulation Models (GCMs) like the ones used in the 2007 IPCC Assessment Report do a very good job simulating how winter storms behave in the current climate, and we can run simulations of the atmosphere with extra greenhouse gases to see how winter storms will behave in the future. The results are very interesting. Global warming is expected to warm the poles more than the equatorial regions. This reduces the difference in temperature between the pole and Equator. Since winter storms form in response to the atmosphere's need to transport heat from the Equator to the poles, this reduced temperature difference reduces the need for winter storms, and thus the models predict fewer storms will form. However, since a warmer world increases the amount of evaporation from the surface and puts more moisture in the air, these future storms drop more precipitation. During the process of creating that precipitation, the water vapor in the storm must condense into liquid or frozen water, liberating "latent heat"--the extra heat that was originally added to the water vapor to evaporate it in the first place. This latent heat intensifies the winter storm, lowering the central pressure and making the winds increase. So, the modeling studies predict a future with fewer total winter storms, but a greater number of intense storms. These intense storms will have more lift, and will thus tend to drop more precipitation--including snow, when we get areas of strong lift in the -15°C preferred snowflake formation region. For completeness' sake, some of the studies that show more intense winter cyclones in a warmer world are Lambert (1995), Boer et al. (1992), Dai et al. (2001), Geng and Sugi (2003), Fyfe (2003), Lambert (2004), Leckebusch and Ulbrich (2004), Lambert and Fyfe (2006), Pinto et al. (2007), and Lionello et al. (2008). A review article be Ulbrich et al. provides a nice summary. However, two studies--Pinto et al. (2007) and Bengtsson et al. 2006--suggest that the more intense winter cyclones will affect only certain preferred regions, namely northwestern Europe and Alaska's Aleutian Islands. At least three other studies also find that northwestern Europe--including the British Isles, the Netherlands, northern France, northern Germany, Denmark and Norway--can expect a significant increase in intense wintertime cyclones in a future warmer world (Lionello et al., 2008; Leckebusch and Ulbrich 2004; and Leckebusch et al., 2006). None of these studies showed a significant increase in the number of intense Nor'easters affecting the Northeast U.S. One interesting new study (O'Gorman, 2010) found that wintertime extratropical cyclones in the northern hemisphere would increase in intensity by 2100 primarily because the surface would heat up more than the upper air, making the atmosphere more unstable. In summer, the models predict a decrease in extratropical cyclones in the Northern Hemisphere. In the Southern Hemisphere, these storms were predicted in increase in intensity year-round. The models studied were the 2007 IPCC suite of climate models.

What the IPCC models say
The Lambert and Fyfe (2006) study, titled, "Changes in winter cyclone frequencies and strengths simulated in enhanced greenhouse warming experiments: results from the models participating in the IPCC diagnostic exercise", looked at thirteen models used to formulate the 2007 IPCC Climate Change report. Of these models, eleven simulated an increase in the number and intensity of the most intense cyclones (<970 mb pressure) in the climate expected by 2100. Two of the models did not, so it is fair to say that there is some uncertainty in these results. Nevertheless, the model results are compelling enough that the U.S. Global Change Research Program (USGCRP), a scientific advisory board created by the President and Congress, concluded this in their 2009 U.S. Climate Impacts Report: "Cold-season storm tracks are shifting northward and the strongest storms are likely to become stronger and more frequent". The USGRP concluded that an increase of between four and twelve intense wintertime extratropical storms per year could be expected over the Northern Hemisphere by 2100, depending upon the amount of greenhouse gases put into the air (Figure 3). If we assume that the current climate is producing the same number of intense winter storms as it did over the period 1961-2000--about 53--this represents an increase of between 8% and 23% in intense wintertime extratropical storms.


Figure 4. The projected change in intense wintertime extratropical storms with central pressures < 970 mb for the Northern Hemisphere under various emission scenarios. Storms counted occur poleward of 30°N during the 120-day season beginning November 15. A future with relatively low emissions of greenhouse gases (B1 scenario, blue line) is expected to result in an additional four intense extratropical storms per year, while up to twelve additional intense storms per year can be expected in a future with high emissions (red and black lines). Humanity is currently on a high emissions track. Figure was adapted from Lambert and Fyfe (2006), and was taken from Weather and Climate Extremes in a Changing Climate, a 2009 report from the the U.S. Global Change Research Program (USGCRP). The USGRP began as a presidential initiative in 1989 and was mandated by Congress in the Global Change Research Act of 1990, which called for "a comprehensive and integrated United States research program which will assist the Nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change".

Conclusion
The best science we have suggests that there has not been an increase in intense wintertime extratropical cyclones globally in recent decades, though there has been an increase in the Pacific and Arctic. Research by Barredo (2010) suggests that Europe has not yet seen a significant increase in damaging winter storms, since normalized damages from severe winter storms did not increase between 1970 - 2008. The 2013 IPCC report sums it up this way:

"Confidence in large scale changes in the intensity of extreme extratropical cyclones since 1900 is low. There is also low confidence for a clear trend in storminess proxies over the last century due to inconsistencies between studies or lack of long-term data in some parts of the world (particularly in the SH). Likewise, confidence in trends in extreme winds is low, due to quality and consistency issues with analyzed data."

The report says that extratropical cyclones are expected to shift poleward in a warming climate, but does not have any conclusions on how the most intense storms may change, other than to dump more precipitation.

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