Beniston 2005a

Référence bibliographique complète

Abstract: Investigations conducted for several Swiss mountain climatological sites, and in particular the Saentis high mountain site at 2,500 m above sea level, show that positive temperature anomalies during the winter season currently exceed those of all other seasons. These ‘‘heat waves’’ exhibit daily maximum temperature anomalies sometimes in excess of 16°C, and are observed to have increased substantially since the late 1960s. These events are related to the North Atlantic Oscillation (NAO) that exerts significant controls on snow cover and surface-atmosphere temperature feedbacks in the alpine region. A glimpse to the future is provided for the period 2071–2100, based on regional climate model simulations which suggest that warm winter spells may increase by 30%. The impacts of such events, particularly in terms of snow and water availability and the mountain economies that depend on these resources, need to be incorporated into future strategic resource and economic planning in the Alps.

Organismes / Contacts

University of Fribourg, Department of Geosciences. Martin.Beniston@unige.ch

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

Massif / Secteur

Site(s) d'étude

Période(s) d'observation

Saentis site compared with 10 other high elevation sites in the Swiss Alps

(1) - Modifications des paramètres atmosphériques

On a single-day basis, exceedances over the 30-year average daily mean reached 9.3°C in Basel and 11.5°C at the high elevation site of Saentis (during the 2003 summer).

While such summer heat waves are indeed characterized by large departures of maximum temperatures from their mean values, these anomalies are smaller than those observed in winter at high elevations in the Alps in the last quarter of the 20th century. Daily maximum temperature anomalies (hereafter referred to as Tmax) during winter have been observed to exceed 15°C at sites such as Saentis (altitude: 2500m), Grand-Saint-Bernard (altitude: 2479 m) or Jungfraujoch (altitude: 3572 m), for example, with exceedances of 10°C and more commonly occurring in the last 2–3 decades.

The winter exceedances are larger than in the three other seasons since the 1970s, and more obviously for the +10°C above the mean. Winter Tmax for the 10°C threshold increase sharply from 0 or 1 days per decade that are observed prior to the 1970s, to over 20 days in the 1990s. In no other season is there such a remarkable increase in the number of strong temperature anomalies.

Temperature at the Säntis and the ten other high elevation sites have risen by over 1.5°C since 1900, as large as the rate observed at low elevation sites; the rate of warming in the Swiss Alps observed during the 20th century is more than double the global-average figure. Winters in the Alps clearly warmed much more rapidly than any of the other season, with a consequen increase in the potential for more frequent, intense, and persistent warm winter spells.

As for the 20th century, winter stands out as the season with the highest frequency of exceedance, with a 30% increase projected to occur for both the 5°C and 10°C thresholds. In addition, maximum wintertime Tmax anomalies are simulated to exceed 18.5°C (as compared to 16.2°C in 1998, for example), in a climate that is projected to warm on average by 4°C in winter in Switzerland.

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(2) - Impacts du changement climatique sur le milieu naturel

Warm winter spells have significant impacts on many environmental and socio-economic systems, particularly if temperatures exceed the freezing point for any length of time, since this results in snow-melt, sharply reduced seasonal snow cover, enhanced early runoff into river basins that originate in the mountains, perturbations to the growing period of alpine vegetation.

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(3) - Impacts du changement climatique sur l'aléa

These warm winter spells may have impacts, especially if the temperatures exceed the freezing point, inducing a melting of snow enhanced avalanche risk for the exposed slopes.

Paramètre de l'aléa

Sensibilité du paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(5) - Syntèses et préconisations

Need for a carefull assesment of the manner in wich snow and hydrology will be affected in the future by such kind of events during the winter.

(1) - Modifications des paramètres atmosphériques
Reconstitutions
Observations	On a single-day basis, exceedances over the 30-year average daily mean reached 9.3°C in Basel and 11.5°C at the high elevation site of Saentis (during the 2003 summer). While such summer heat waves are indeed characterized by large departures of maximum temperatures from their mean values, these anomalies are smaller than those observed in winter at high elevations in the Alps in the last quarter of the 20th century. Daily maximum temperature anomalies (hereafter referred to as Tmax) during winter have been observed to exceed 15°C at sites such as Saentis (altitude: 2500m), Grand-Saint-Bernard (altitude: 2479 m) or Jungfraujoch (altitude: 3572 m), for example, with exceedances of 10°C and more commonly occurring in the last 2–3 decades. The winter exceedances are larger than in the three other seasons since the 1970s, and more obviously for the +10°C above the mean. Winter Tmax for the 10°C threshold increase sharply from 0 or 1 days per decade that are observed prior to the 1970s, to over 20 days in the 1990s. In no other season is there such a remarkable increase in the number of strong temperature anomalies. Temperature at the Säntis and the ten other high elevation sites have risen by over 1.5°C since 1900, as large as the rate observed at low elevation sites; the rate of warming in the Swiss Alps observed during the 20th century is more than double the global-average figure. Winters in the Alps clearly warmed much more rapidly than any of the other season, with a consequen increase in the potential for more frequent, intense, and persistent warm winter spells.
Modélisations	As for the 20th century, winter stands out as the season with the highest frequency of exceedance, with a 30% increase projected to occur for both the 5°C and 10°C thresholds. In addition, maximum wintertime Tmax anomalies are simulated to exceed 18.5°C (as compared to 16.2°C in 1998, for example), in a climate that is projected to warm on average by 4°C in winter in Switzerland.
Hypothèses

(2) - Impacts du changement climatique sur le milieu naturel
Reconstitutions
Observations	The amount and duration of snow cover on the alpine foreland have significantly reduced in the last quarter of the 20th century compared to previous decades (reduction of the ground snow cover from 43 to 22 days per winter in Bern, 15 to 3 days per winter in Lugano betwen the 1960s and the 1990s).
Modélisations
Hypothèses	Warm winter spells have significant impacts on many environmental and socio-economic systems, particularly if temperatures exceed the freezing point for any length of time, since this results in snow-melt, sharply reduced seasonal snow cover, enhanced early runoff into river basins that originate in the mountains, perturbations to the growing period of alpine vegetation.

(3) - Impacts du changement climatique sur l'aléa
Reconstitutions
Observations
Modélisations
Hypothèses	These warm winter spells may have impacts, especially if the temperatures exceed the freezing point, inducing a melting of snow enhanced avalanche risk for the exposed slopes.

Pays / Zone	Massif / Secteur	Site(s) d'étude	Exposition	Altitude	Période(s) d'observation
Switzerland	Swiss Alps	Saentis site compared with 10 other high elevation sites in the Swiss Alps		2500m asl and between 910 m and 3572 m asl	1961-1990 / 1901-2000