Winter storms in Switzerland North of the Alps 1864/1865?1993/1994

Abstract

In the framework of the Swiss National Research Program 31 “Climate changes and natural disasters” the question was brought up whether a global warming of the atmosphere would have an influence on the frequency and/or intensity of the extratropical storms of the Swiss winter season. In order to investigate a possible trend, time series of days with a minimum wind speed estimate or measurement equivalent to Beaufort 7, 8 and 9 were established. The longest being a record of the mesonet station Zurich from the period 1864 until 1993 (130 years). Slightly shorter time series for three additional stations in Northern Switzerland were compiled to control the behavior of the longest record and to verify the observed temporal trend. From the location of the four investigated wind records the observational domain was restricted to the part of Switzerland north of the Alps whereas for further meteorological considerations the whole North-Atlantic-European area was included. A negative regional trend in the number of storm days has been observed during the last century. In particular, the period before 1940 has to be interpreted as windier than the following decades. The duration of a storm event also decreased on average. In spite of an increase in cyclonic westwind situations since about 1960 over Europe, Switzerland was hit by fewer storms during the same period. One explanation could be that the whole westwind belt has moved slightly further north where a deepening of the cyclones was observed in recent times. Switzerland is usually situated at the southernmost edge of the particular storm fields and is therefore less influenced by strong gales. Rare exceptions are the cases when a secondary depression directly hits Central Europe, e.g. as happened in February 1990 (storm “Vivian”).