October 24, 2011

Natural Variability Still Plays Large Role in Winter Climate

Filed under: Climate Forcings, Solar

The last couple of winters across the central and eastern United States as well as much of Europe were on the cold and snowy-side of things, to say the least. And of course, anytime there is some type of weather misery, a particular segment of the population likes to trot out global warming as the culprit. Cold, snowy, winters are no exception (despite your apparent (mis)conception as to what global “warming” would entail). Ironically, a subset of this same segment of folks was fingering global warming as the reason for the string of warmer-than-normal winters immediately preceding our past two shiverers (Figure1).

Now comes word that things other than global warming can lead to winter weather extremes. While that may come as shocker to some, it should be a snoozer to the vast majority.


Figure 1. Winter temperature history of the U.S, 1895-2011 (source: National Climatic Data Center).

A research team led by Sarah Ineson and primarily made up of scientists from the U.K.’s Hadley Centre Met Office have identified a fairly strong solar signal in Northern Hemisphere winter circulation patterns which are manifest over Europe and the eastern United States. According to their modeling studies, the difference in the amount of incoming solar radiation, in this case, primarily in the ultraviolet (UV) wavelengths, during the minima and maxima of the 11-yr solar cycle are large enough to produce a characteristic change in the winter circulation pattern of the atmosphere over North America. The patterns of circulation change induced by the sun are very similar to the positive and negative phases of the atmospheric circulation indices known as North Atlantic Oscillation (NAO) and the closely related Arctic Oscillation (AO). The NAO and AO are typically linked to winter weather on both sides of the Atlantic and sometimes with global warming.

When the NAO is in its negative phase, more cold air can seep south from the Arctic and impact the lower latitudes of Europe and the eastern U.S., which helps spin up winter storm systems. For instance, during the “snowmageddon” winter of 2009/2010, the NAO was at a near record low value (Figure 2).


Figure 2. Long-term history of the North Atlantic Oscillation (NAO) index during the winter season—the time of year when the NAO has its greatest influence on the weather. (source: http://www.cgd.ucar.edu/cas/jhurrell/nao.stat.winter.html).

It is interesting to note the (temporary) upwards trend in the NAO index from the 1960s through the mid-1990s (Figure 2). In the midst of this, folks were quick to point this trend out and to suggest that this was “consistent” with what we should expect from global warming (see here for example). As this (temporary) upwards trends peaked out and the NAO index began to return to the mean in recent years, this kind of talk petered out.

Here is what the Ineson team has to say about the role of the sun in all of this:

The average of recent winters (2008/9, 2009/10 and 2010/11) shows cold conditions over northern Europe and the United States and mild conditions over Canada and the Mediterranean associated with anomalously low and even record low values of the NAO. …Given our modelling result, these cold winters were probably exacerbated by the recent prolonged and anomalously low solar minimum. On decadal timescales the increase in the NAO from the 1960s to 1990s…may also be partly explained by the upwards trend in solar activity evident in the open solar-flux record….

The solar effect presented here contributes a substantial fraction of typical year-to-year variations in near-surface circulation, with shifts of up to 50% of the interannual variability. This represents a substantial shift in the probability distribution for regional winter climate and a potentially useful source of predictability. Solar variability is therefore an important factor in determining the likelihood of similar winters in future. However, mid-latitude climate variability depends on many factors, not least internal variability, and forecast models that simulate all the relevant drivers are needed to estimate the range of possible winter conditions.

In other words, what was once supposedly caused by global warming has now been linked to changes in solar output. And further, the solar influence on the NOA/AO seems substantial enough to impact the probability of particular characteristics of winter.

Certainly the Ineson study is only a small part of the overall investigation into the forces which shape the weather and the climate and there are questions that remain about their results. For example, their results are based upon a little observational data and a lot of modeling, and long-term observations don’t seem to indicate a strong historical relationship. However, the Ineson work does form part of a renewed interest in investigating the impact of the sun on winter weather variability (see here for example).

Which all goes to show not everything has a link to global warming, some things just happen naturally—a good fact to keep in mind next time you are reading some blustery commentary about why some extreme weather event came to pass.

References:
Ineson, S., et al., 2011. Solar forcing of winter climate variability in the Northern Hemisphere. Nature Geosciences, doi:10.1038/NGEO1281.

Lockwood, M., Harrison, R. G., Woollings, T. & Solanki, S., 2010. Are cold winters in Europe associated with low solar activity? Environmental Research Letters, 5, 024001.




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