Shifts in the distributions of pressure, temperature and moisture and changes in the typical weather patterns in the Alpine region in response to the behavior of the North Atlantic Oscillation

Summary An investigation has been undertaken to assess the manner in which the North Atlantic Oscillation (NAO) influences average, climatic conditions, and also extremes of dynamic and thermodynamic variables. By choosing representative sites in the Swiss Alps, the present study shows that there is a high sensitivity of the extremes of the probability density functions of temperature, moisture and pressure to periods when the NAO index is either strongly positive or strongly negative. When the NAO index is strongly positive, temperature and pressure shift towards positive anomalies and there is a general reduction in atmospheric moisture at high elevation. Furthermore, a change in typical alpine winter weather patterns can be detected during strongly positive NAO anomaly phases. The winters of the last decade of the 20th Century (1989–99) are characterized by a substantial decrease in cold advective high pressure situations and simultaneously an important increase in warm convective high pressure systems. These patterns differ significantly from the weather types which have been recorded for earlier periods of the 20th Century. As a result of the highly-positive nature of the NAO index in the latter part of the 20th Century it is speculated here that a significant part of the observed warming in the Alps results from the shifts in temperature extremes induced by the behavior of the NAO. These changes are capable of having profound impacts on snow, hydrology, and mountain vegetation. Received January 11, 2001 Accepted Revised May 24, 2001