Cold extremes in North America vs. mild weather in Europe: the winter 2013/2014 in the context of a warming world

Post date: Jan 5, 2015 10:19:00 AM

We all know that temperatures vary on a range of time and spatial scales. Well known examples are the daily and seasonal cycles, associated with the amount of incoming solar radiation. When averaged over an entire season (for example the winter), variations are also observed. In a gradually warming world the winters get milder, but superposed on this "trend" a considerable amount of variability occurs: every winter is different. Why? One of the reasons is that the circulation is an important driver of temperature variability. In this way it can happen (and in fact it does happen) that the large-scale circulation patterns may lead to record-cold conditions in some regions, while leading to record-mild conditions in other places. We have analysed this for the winter 2013/2014, which for the Netherlands was characterized by a complete absence of Hellmann-days (days where mean daily temperature is below freezing), while on "the other side of the lake" it was seemingly very cold in the US. But how exceptional were the winters and does climate change play any role in this? This is where the paper is all about. And it is out now in early online press with BAMS (Bulletin of the American Meteorological Society). The abstract is given below as well as one of the figures. Click on the abstract to go the paper. E-mail me if you can not access it.

Caption: Fig. 2d from the paper. Data: climate explorer.

Title: Cold extremes in North America vs. mild weather in Europe: the winter 2013/2014 in the context of a warming world

Abstract: "The winter of 2013/2014 had unusual weather in many parts of the world. Here we analyse the cold extremes that were widely reported in North America and the lack of cold extremes in western Europe. We perform a statistical analysis of cold extremes at two representative stations in these areas: Chicago, Illinois and De Bilt, the Netherlands. This shows that the lowest minimum temperature of the winter was not very unusual in Chicago, even in the current warmer climate. Around 1950 it would have been completely normal. The same holds for multi-day cold periods. Only the whole winter temperature was unusual, with a return time larger than 25 years. In the Netherlands the opposite holds: the absence of any cold waves was highly unusual even now, and would have been extremely improbable half-way through the previous century. These results are representative of other stations in the regions. The difference is due to the skewness of the temperature distribution. In both locations, cold extremes are more likely than equally large warm extremes in winter. Severe cold outbreaks and cold winters, like the winter of 2013/2014 in the Great Lakes area, are therefore not evidence against global warming: they will keep on occurring, even if they become less frequent. The absence of cold weather as observed in the Netherlands is a strong signal of a warming trend, as this would have been statistically extremely improbable in the 1950s."