A synoptic climatology of major avalanche winters in Norway
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A near continuous record of avalanche deaths in Norway is compiled for 130 years from 1855, and shows a distinct downward trend, but large variations from winter to winter, with a periodicity of 12–13 years. Twelve major avalanche winters are selected, and their synoptic climatology examined using Grosswetterlagen, an index of atmospheric circulation over southern Norway, and monthly sea level pressure anomaly maps. More than one climatological pattern is needed to explain these winters. Major avalanche winters tend to be cold in December and/or January, and have more meridional and less mixed weather types compared with minor avalanche winters. Some have enhanced north or east circulations associated with large anomalies in the semi-permanent pressure systems. Others display intense anticyclonic blocking near Britain that maintains sustained west to north-west airflows onto Norway. A switch in circulation pattern from these to a more southerly regime is also important. These aspects of synoptic climatology are related to the mechanisms for producing big avalanches.