Negative NAO and cold Eurasian winters: how exceptional was the winter of 1962/1963?

Abstract

Since the mid 1980s frequent positive phases of the North Atlantic Oscillation (NAO) linked to a higher-than-normal pressure difference between the Azores high and Icelandic low pressure centres and stronger westerlies (Hurrell, 1995; Thompson and Wallace, 1998; Osborn, 2006) have led to higher winter temperatures over most of western Europe. Harsher winters were more frequent between the 1950s and the 1980s (see Figure 9 in Graham et al., 2006). Even though mild winters similar to the recent ones did occur they alternated with colder winter seasons. The best remembered is probably the winter of 1962/1963 when temperatures were well below zero across most of Europe. Figure 1 illustrates the unusual amounts of snow that occurred at lower elevations in Switzerland. With the exception of western Ireland, most of the British Isles was covered by a blanket of snow for most of the winter (Met Office, 2006a). On an anecdotal note, the harsh conditions prevailing during that winter led a then relatively unknown Bob Dylan, who was staying in a London flat at that time, to burn furniture in order to keep warm (Williamson, 2004). On mainland Europe it was the only time in the twentieth century that large lakes such as Lake Constance or Lake Zurich were completely ice-covered and it was only in March that the ice finally broke up (Pfister, 1999). Cold winters such as the one in 1962/1963 are generally associated with a negative NAO index characterized by weak westerlies (lower-than-normal pressure difference between Icelandic low and Azores high) and a reduced Atlantic influence in western Europe. The NAO index, and the late spring seasurface temperature pattern in the North Atlantic, can be used as indicators to forecast the conditions during the following winter (Graham et al., 2006; Met Office, 2006b). However, prediction of how much colder or warmer the winter season is going to be and in what regions the anomalies are likely to be most pronounced is subject to large uncertainties. A negative NAO index during the winter months does not always coincide with very cold winter conditions over the same regions. A recent example is the winter of 2005/2006. Despite the clearly negative NAO index the winter temperatures over most of Britain remained close to average values and were clearly higher than those experienced in 1962/1963. However, the winter 2005/2006 was the coldest winter since 1986 in parts of central, eastern and southern Europe. This illustrates that even if the NAO index gives an indication of whether a winter is warmer or colder than normal, the spatial pattern of the largest temperature anomalies can vary. In this study, we use reanalysis data from the National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR, Kalnay, et al., 1996) for surface temperature and sealevel pressure to compare the winter of 1962/1963 with other winters associated with a negative NAO index and very low temperatures over other regions of Eurasia. The purpose is to highlight how exceptional the winter of 1962/1963 was compared to other harsh winters in terms of amplitude and spatial extent of the cold temperature anomaly. The fact that the winter of 1962/1963 is so well remembered is partly due to the character of an average winter over most areas of Britain and western Europe; generally temperatures are above freezing and long spells of snow are rare. However, a shift in temperature by a few degC can transform the traditional green British winter into a snowy (white) one, thus making the difference even more obvious. In comparison, other areas of Europe/ Eurasia are characterised by average winters where snow and ice are common. Therefore, negative temperature anomalies are more likely to go unnoticed; a winter may be colder than normal but it does not change its colour.