Temperature trends in Switzerland and Europe: Implications for climate normals

This study discusses problems of the concept of normal period-based anomalies arising from climate variability and ongoing climate change. The widely used WMO 1961-1990 (61-90) standard normal period is compared to other consecutive 30-year normal periods in detail. Focus is given to the temperature distribution in Switzerland and on the European continent. In these regions, the temperature trend of the last decades led to an unusually high number of months with positive temperature anomalies relative to the WMO 61-90 standard normal period. Swiss anomalies based on the 61 -90 normal are up to 1.25 K higher than those based on the Latest 30-years Running Normal (LRN). The probability to observe a positive temperature anomaly with respect to the 61 -90 normal increased from 50% to near 80% for certain months of the year. Compared to the LRN, this change is statistically significant for 7 out of the 12 months on the 95% level. The strongest signal can be found for the summer months, whereas temperatures in fall do not show any trends. Similar results are found for more than 90% of the European continental area. For most regions, 2-5 are statistically inconsistent with the 61 -90 distribution. For southern France, parts of Spain and southern Scandinavia even 7-9 months are inconsistent. Copyright (C) 2005 Royal Meteorological Society.