Réf. Beniston 2005b - A

Référence bibliographique complète
BENISTON M. Mountain climates and climatic change: An overview of processes focusing on the European Alps. Pure and Applied Geophysics, 2005, Vol. 162, p. 1587-1606.

Abstract: This contribution provides an overview of the intricacies of mountain climates, particularly as they pertain to the European Alps. Examples will be given of issues that are related to climatic change as observed in the Alps during the course of the 20th century, and some of the physical mechanisms that may be responsible for those changes. The discussion will then focus on the problems related to assessing climatic change in regions of complex topography, the potential shifts in climate during the 21st century that the alpine region may be subjected to, and the associated climate-generated impacts on mountain environments.

Mots-clés
Climate, climatic change, modeling, North Atlantic Oscillation, snow, mountain regions.

Organismes / Contact
Department of Geosciences, University of Fribourg, Chemin du Musé 4, CH-1700, Fribourg, Switzerland. Martin.Beniston@unifr.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
Temperature, Precipitation,
NAO
     

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Europe/Alps   Focus on Säntis   2500 m asl for Säntis 20th century

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
The climate of the Alpine region is characterized by a high degree of complexity, due to the interactions between the mountains and the general circulation of the atmosphere, which result in features such as gravity wave breaking, blocking highs, and föhn winds. A further cause of complexity inherent to the Alps results from the competing influences of a number of different climatological regimes in the region, namely Mediterranean, Continental, Atlantic, and Polar.

1961-1990 climatological mean in the Swiss Alps (Altdorf, Basel, Bern, Davos, Lugano, Neuchâtel, Säntis and Zürich) compared to global temperature anomalies, for the period 1901-2000 (JONES and MOBERG, 2003) show that the interannual variability in the Alps is higher than on a global or hemispheric scale. The warming experienced since the early 1980s, while synchronous with global warming, is of far greater amplitude and exceeds 1.5°C for this ensemble average. This represents roughly a three-fold amplification of the global climate signal in the Alps (DIAZ and BRADLEY, 1997).

Closer investigation reveals that climatic change in the alpine region during the 20th century has been characterized by increases in minimum temperatures of up to 2°C, a more modest increase in maximum temperatures, little trend in the precipitation data, and a general decrease of sunshine duration through to about the mid-1980s (BENISTON, 2000). Several periods of warming can be observed during the instrumental record, with the 1940s exhibiting a particularly strong warming and then a cooling into the 1950s. The most intense warming occurs in the 1990s, however (JUNGO and BENISTON, 2001), which can be explained in part by the behavior of the North Atlantic Oscillation (NAO; BENISTON and JUNGO, 2002). It has been observed to strongly influence precipitation and temperature patterns on both the eastern third of North America and western half of Europe; the influence of the NAO is particularly conspicuous during winter months.

It was shown in recent years (BENISTON et al., 1994; HURREL, 1995; ROGERS, 1997; SERREZE et al., 1997) that a significant fraction of climatic anomalies observed on either side of the Atlantic are driven by the behavior of the NAO.

BENISTON (2000) has shown that temperature, moisture and pressure trends and anomalies at high elevations stand out more clearly than at lower levels, where boundary-layer processes, local site characteristics and urban effects combine to damp the large-scale climate signals. Climatic processes at high elevation sites can thus in many instances be considered to be the reflection of large-scale forcings, such as the NAO. These findings have been confirmed through numerical experimentation by GIORGI et al. (1997), who have underlined the altitudinal dependency of the regional atmospheric response to large-scale climatic forcings.

When computed for 1901-1999, 56% of the observed pressure variance in Switzerland can be explained by the behavior of the NAO. From 1961-1999, this figure rises to 83%, which is considerable bearing in mind the numerous factors that can also determine regional pressure fields. As for pressure trends, the synchronous behavior between temperature and the NAO is striking, particularly in the second half of the 20th Century.

A particular feature of the positive phase of the NAO index is that it is invariably coupled to anomalously low precipitation and milder than average temperatures, particularly from late fall to early spring, in southern and central Europe (including the Alps and the Carpathians), while the reverse is true for periods when the NAO index is negative.

At Säntis, for example, the extreme low tails of the minimum temperature distribution disappear during periods of high NAO index, in favor of much warmer temperatures. Temperatures below -15°C at Säntis, which account for roughly 30% of the winters where the NAO index is below the 10% level, occur only 15% of the time in winter months that experience high NAO values. This implies that the periods with extreme cold conditions are reduced by 50%.

Moisture and precipitation in the alpine region are also influenced by the behavior of the NAO. In the case of the negative index threshold, over 50% of the values recorded in winter exceed 90% relative humidity, while in the case of the positive threshold this level of relative humidity is exceeded only 35% during the winter months (results not shown here). There is thus a clear reduction in ambient moisture at high elevations.

While at low elevations, the NAO signal may be weak or absent in the Alps, higher elevation sites are on the contrary sensitive to changes in NAO patterns (BENISTON et al., 1994; BENISTON and REBETEZ, 1996; HURRELL, 1995; HURREL and VAN LOON, 1997; GIORGI et al., 1997).

Since the early 1970s, and until 1996, the wintertime NAO index has been increasingly positive, indicative of enhanced westerly flow over the North Atlantic. Over the Alpine region, positive NAO indices have resulted in surface pressure fields that have been higher than at any time this century. Investigations by BENISTON et al. (1994) concluded that close to 25% of the pressure episodes exceeding the 965 hPa threshold recorded this century in Zürich (approximately 1030 hPa reduced sea-level pressure) occured in the period from 1980-1992, with the four successive years from 1989-1992 accounting for 16% of this century's persistent high pressure in the region.

Mean wintertime values for minimum and maximum temperatures, relative humidity and precipitation at Säntis were analised for four distinct periods of the 20th century (1901-1999, 1950-1999, 1975-1999, and 1989-1999). The bias, which NAO index exceeding the 90-percentile has imposed on temperature and moisture variables, is relatively small when considering the entire 20th century (1901-1999), but then increases as one approaches the end of the 20th century. In the last decade, from 1989-1999, the bias for minimum temperatures exceeds 1°C. Had there not been such a strong positive NAO forcing in the latter years of the 20th century, minimum temperatures would not have risen by almost 1.5°C (decadal mean for the 1990s minus century mean from 1901-1999) but by less than 0.5°C. The bias imposed by strongly-positive NAO thresholds on maximum temperatures follows the same trends, but is not as high as for minimum temperatures; even in the absence of the NAO forcing, maximum temperatures would have risen substantially in the latter part of the 20th century.

In terms of moisture, relative humidity has decreased in winter, with a bias of close to 10% in the period 1989-1999, resulting from the NAO forcing; mean DJF relative humidity would have otherwise remained relatively constant throughout the century. Precipitation is also seen to be considerably marked by the NAO forcing in the last decade of the 20th century, with a substantial drop of 20% of winter precipitation linked to the high and persistent NAO index recorded during this period.

Removal of the biases imposed by high NAO episodes would have resulted in relatively modest increases in minimum temperatures and reduced rates of maximum temperature warming, thus leading to Alpine-scale warming comparable to global average warming (JONES and MOBERG, 2003).
Modélisations
According to the HadCM3 GCM and the HIRHAM RCM, used in the context of an EU 5th Framework Program project (CHRISTENSEN et al., 2002), alpine climate in the latter part of the 21st century will be characterized by warmer and more humid conditions in winter, and much warmer and drier conditions in the summer. Although the RCM grid is a relatively coarse 50 km, the results confirm earlier studies by MARINUCCI et al. (1995) and ROTACH et al. (1997).
Hypothèses
 

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

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
   

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

Références citées :

BENISTON, M., REBETEZ, M., GIORGI, F, and MARINUCCI, M.R. (1994), An Analysis of Regional Climate Change in Switzerland, Theor. Appl. Clim. 49, 135159.

BENISTON, M. and REBETEZ, M. (1996), Regional Behavior of Minimum Temperatures in Switzerland for the period 19791993, Theor. Appl. Clim. 53, 231243.

BENISTON, M. (2000), Environmental Change in Mountains and Uplands. (Arnold/Hodder Publishers, London, UK, and Oxford University Press, New York, USA), 172 pp.

BENISTON,M. and JUNGO, P. (2002), Shifts in the Distributions of Pressure, Temperature and Moisture in the Alpine Region in Response to the Behavior of the North Atlantic Oscillation, Theor. Appl. Clim. 71, 2942.

CHRISTENSEN, J.H., CARTER, T.R., and GIORGI, F. (2002), PRUDENCE Employs New Methods to Assess European Climate Change, EOS, Trans. Am. Geophy. Union 83, 147.

DIAZ, H. F. and BRADLEY, R. S. (1997), Temperature Variations during the Last Century at High Elevation Sites, Climatic Change 36, 253279.

GIORGI, F., HURRELL, J., MARINUCCI, M., and BENISTON, M. (1997), Height Dependency of the North Atlantic Oscillation Index. Observational and Model Studies, J. Clim. 10, 288296.

HURRELL, J. W. (1995), Decadal Trends in the North Atlantic Oscillation Regional Temperatures and Precipitation, Science 269, 676679.

HURRELL, J. W., and VAN LOON, H. (1997), Decadal Variations in Climate Associated with the North Atlantic Oscillation, Climatic Change 36, 301326.

JONES, P.D. and MOBERG, A. (2003), Hemispheric and Large-scale Surface Air Temperature Variations: An Extensive Revision and an Update to 2001, J. Climate 16, 206223.

JUNGO, P., and BENISTON, M. (2001), Changes in 20th Century Extreme Temperature Anomalies at Swiss Climatological Stations Located at Different Latitudes and Altitudes, Theor. Appl. Clim. 69, 112.

MARINUCCI, M. R., GIORGI, F., BENISTON, M., WILD, M., TSCHUCK, P., and BERNASCONI, A. (1995), High Resolution Simulations of January and July Climate over the Western Alpine Region with a Nested Regional Modeling System, Theor. Appl. Clim. 51, 119138.

ROGERS, J. C. (1997), North Atlantic Storm Track Variability and its Association to the North Atlantic Oscillation and Climate Variability of Northern Europe, J. Climate 10, 16351647.

ROTACH, M., WILD, M., TSCHUCK, P., BENISTON, M., and MARINUCCI, M. R. (1997), A Double CO2 Experiment over the Alpine Region with a Nested GCM-LAM Modeling Approach, Theor.Appl. Clim. 57, 209227.

SERREZE, M. C., CARSE, F., BARRY, R. G., and ROGERS, J. C. (1997), Icelandic Low Cyclone Activity: Climatological Features, Linkages with the North Atlantic Oscillation, and Relationships with Recent Changes in the Northern Hemisphere Circulation, J. Climate 10, 453464.