Réf. Beniston & al. 2011 - A

Référence bibliographique complète

BENISTON, M., UHLMANN, B., GOYETTE, S., LOPEZ-MORENO, J.I. 2011. Will snow-abundant winters still exist in the Swiss Alps in an enhanced greenhouse climate? International Journal of Climatology, Vol. 31, 1257–1263. [Etude en ligne]

Abstract: Snow cover and duration are very variable components of the alpine environment and are often poorly reproduced in climate models. Using joint probability temperature/precipitation distributions to categorize cold/dry, cold/moist, warm/dry (WD) and warm/moist situations in winter, this study demonstrates that one particular mode (WD) exerts the strongest influence on snow. When the number of WD days is low, snow in the Swiss Alps is abundant, and vice versa. Since the 1950s, there has been an increase in the WD events and a subsequent reduction in snow cover. Snow-abundant winters have nevertheless occurred in recent years, when WD days are low, despite winter temperatures that are more than 1 °C higher than those in the mid-1900s. The WD mode thus represents a form of proxy to snow amount and duration; its evolution in an enhanced greenhouse climate can help identify whether snow-abundant winters may still occur in a much warmer world.

Mots-clés

Climatic change - Snow - Alps - Quantiles

 

Organismes / Contact

• Institute for Environmental Sciences, University of Geneva, Geneva, Switzerland
Instituto Pirenaico de Ecolog´ıa, CSIC, Zaragoza, Spain

This work was conducted in part in the context of the EU/FP7 ACQWA Project (www.acqwa.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

 Snow abundance

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 Swiss Alps

 

 

 

 

 

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

In terms of snow sparseness or snow abundance, the present analysis shows that the determining factor is the frequency of occurrence of the WD mode. If the frequency of this mode is low, the chances of seeing a snow-abundant winter are high, because of the absence of extended warm conditions with little precipitation that are detrimental to snow. In parallel, snow abundance will also be reflected in the CW mode, i.e. when sufficiently cold conditions with precipitation in the form of snow occur. […]

Synoptic situations characteristic of a snow-sparse winter, reflected in the frequency of occurrence of the WD mode, are generally related to the establishment of a persistent high-pressure ridge extending to the Alps that deflects storm systems well to the north. Snow-abundant winters, on the contrary, are associated with southerly incursions of moist and cold air. The North Atlantic Oscillation seems to have at least a partial bearing on sharply contrasting winters, as experienced in the Alps in the past two to three decades (Beniston, 2005).

It is thus of interest to assess the manner in which the four modes have evolved since the middle of the last century, especially the WD mode, as it is very closely linked to snow-sparse or snow-abundant winters. Since 1951, there have been changes in the behaviour of the four joint quantile modes at the four selected stations. Although there is some variability in the curves that reflect regional climatic differences and local site characteristics, the curves exhibit very similar, in-phase behaviour. This same figure also shows that there are decreases within the CD mode from approximately 30 days per winter in the 1950s to less than 15 days currently, and increases in the WD mode from an average of around 30 days per winter in the 1960s to 50 days currently. The CW and WW modes are less frequent than the dry ones, but exhibit a decrease and an increase since the 1950s, respectively. Thus there is clearly a long-term change in the frequency of occurrence of each of these four modes, as already discussed by Beniston (2009), partly associated with the rise in mean winter temperatures that the Alpine region has experienced over many decades.

The controls exerted by the WD mode on snow amount and duration help to explain much of the change towards reduced snow amount and duration today than was the case 40 or 50 years ago. […] Closer scrutiny of the data suggests that temperature exerts a strong influence on the frequency of occurrence of the ‘dry’ modes (roughly 40% for the CD mode and 60% for the WD mode, based on the ratio of cold/warm days with respect to cold/dry and warm/dry days, respectively), whereas for the ‘wet’ modes, the temperature influence is much smaller (about 15–20%).

Modélisations

The average shifts in the four temperature–precipitation modes (percentages with respect to the total frequency of occurrence) were calculated, averaged as previously for the four sites considered in the study, between the control and the future greenhouse climates. As could be intuitively expected, the frequency of the cold modes is substantially reduced for the future climate in 2071–2100; CD days diminish from about 26 days per winter in the control climate to 6 days in the scenario climate, whereas CW days are reduced by almost half, from 11 to 6 days. In sharp contrast, the warm modes increase significantly, by 50% in the case of the WD and WW modes, from about 38 to 55 days and 15 to 24 days per winter, respectively. The change in WD mode and the quasi-disappearance of the cold modes in the scenario climate will certainly weigh heavily on Alpine snow amount and duration in the last 30 years of this century, but will there still be room for an occasional snow-abundant winter?

It was discussed above that snow-abundant winters are today associated with a low number of WD days, less than about 30–35 days per winter (or at least 1 standard deviation below the mean number of WD days). Exploration of individual winters in the 30-year set simulated by the RCM shows only one winter exhibiting less than 30 WD days and two winters with less than 40 days; in comparison, the control climate shows 8 winters with less than 30 WD days and 21 winters with less than 40 WD days. These statistics incorporate the model-simulated seasonal shifts in precipitation regimes that are likely to increase quite substantially in winter in the Swiss Alps, as reported in a number of studies including those of the EU PRUDENCE project. It is assumed in this study that the statistical relationships between the four modes and the characteristics of the snow pack will not change in the future. This assumption is justified by the fact that in the baseline climate, certain winters have already exhibited conditions similar to those that are expected to occur more frequently in the future. These situations that are characteristic of warm winters have been well described by the correlations between a particular mode and the duration and thickness of the snow pack, and there is thus no reason to believe that this will change substantially in the future.

The change in the range of snow duration and mean snow depth that can be expected for 2071–2100 for the four Alpine stations was calculated using the information inferred from current climate where similar numbers of WD days occur. […] The changes between control and enhanced greenhouse climates are quite marked, particularly below 2000 m altitude, where winters without snow are projected to occur […]. Analysis of the threshold beyond which a winter in the control climate can be considered as snow abundant, as defined in the text, shows that such winters can still occur at medium to high elevations in the future, and even at low elevations under specific conditions of topography. The results discussed in this study are reasonably consistent with other studies using more detailed approaches to estimate future snow conditions in the Alps (Abegg and Froesch, 1994; Uhlmann et al., 2009).

The conditions for snow-abundant winters almost disappear in the future, but 1–2 of the 30 winters may still see significant amounts of snow, mainly at elevations above 1500 m where precipitation will continue to fall in the solid phase, as opposed to predominantly in the liquid phase beneath this level. By 2100, many winters will probably be at least as mild and snow sparse as the record 2006–2007 winter, but the occasional snow-abundant winter may be somewhat similar to the 2008–2009 winter. The response of environmental systems to rare snow-abundant events may, however, be different in the future compared with that of today, because many systems may have adapted at least partially to a warmer climate and may thus be less well geared to coping with a long and snowy winter season and the high runoff conditions that could occur in the spring.

Hypothèses

 

 

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

Joint distributions of two weather variables, such as temperature and precipitation, have been shown inter alia by Beniston and Goyette (2007) and Beniston (2009) to be reasonable proxies for weather patterns and their persistence. This is because particular combinations of temperature and precipitation are closely related to the underlying synoptic circulations that may or may not bring snowy winters to the Alps. Joint distributions will be shown in this study to better reflect the weather conditions that lead to snow-abundant or snow-sparse winters than temperature or precipitation statistics taken separately. This is because cold winters are not necessarily snow-abundant winters, and vice versa. In addition, because snow is a variable that is difficult to reproduce in global and even regional climate models (Räisanen, 2008), use of joint distributions can help estimate the duration and amount of snow at a particular location without necessarily applying detailed energy-balance models, snow models or other downscaling techniques to infer snow characteristics.

This study firstly provides an insight into the behaviour of snow in relation to specific modes of temperature and precipitation since the middle of the last century to establish the relationships between the snowpack thickness and its duration for the observed climate. In a second step, an assessment is made of the ability of regional climate models (RCMs) to reproduce the observed temperature–precipitation relationships for the reference climate (or ‘control climate’) of the period 1961–1990. Finally, the changes in the frequency of occurrence of particular combinations of temperature and precipitation for the period 2071–2100 are analysed to determine whether, despite the much warmer Alpine climate projected for the end of the century, snow-abundant winters may still occur.­­

The observed climate data have been provided by the Swiss weather service, MeteoSwiss. For this study, and to highlight the methodology applied in this study, daily mean temperature, precipitation and snow-depth statistics have been compiled for four weather stations that span an altitudinal range of 1000–2500 m above sea level, and cover the period from 1 January 1951 through 30 April 2009. Stretching from west to northeast Switzerland, the stations include, respectively, Château d’Oex (980 m), Andermatt (1440 m), Arosa (1850 m) and Saentis (2500m).

Winter statistics have been derived, including mean winter snow-pack depth, continuous snow duration (using a 10-cm snow-depth threshold to avoid counting dispersed intermittent snow fall recorded at the beginning or end of the winter season) and joint temperature/precipitation quantiles. For this study, the 25 and 75% joint quantiles were used to define winter situations that are cold/dry (CD; when the joint quantiles are equal to or below T25p25, where the subscript refers to the quantile threshold), cold/wet (CW; T25p75), warm/dry (WD; T75p25) and warm/wet (WW; T75p75). The threshold values are computed on the basis of daily mean temperature and precipitation statistics for the ‘winter half-year’ (i.e. beginning of November through the end of April) for the reference period 1961–1990. A 6-month period, rather than the more usual definition of the 3 winter months December–January–February, is used in this study because at many Alpine sites, snow is likely to appear and remain on the ground for more than 3 months.

In this study, snow-sparse or snow-abundant winters are defined as those where mean snow depth is below or beyond 1 standard deviation around the mean. Because snow depth and snow duration are generally well correlated (Beniston et al., 2003), a snow-sparse winter is also likely to be one with a fairly short period of snow cover and vice versa.

The regional scenario climate data were obtained from regional climate model (RCM) simulations undertaken in the context of the EU ‘PRUDENCE’ project (http://prudence.dmi.dk), and in particular the HIRHAM model of the Danish Meteorological Institute (Christensen et al., 1998). Simulations of the reference climatic period 1961–1990 have shown that the HIRHAM model exhibits genuine skill in reproducing contemporary climate, including mean and extreme climate in the Alps, thereby providing some confidence as to its capability for simulating the characteristics of temperatures and precipitation in the future (Beniston, 2006; Beniston et al., 2007). The model operates at a 50-km resolution and has completed two 30-year simulations, i.e. ‘current climate’ or the ‘control simulation’ for the period 1961–1990, and the future ‘greenhouse-gas climate’ for the period 2071–2100 with the IPCC SRES A2 emission scenario (Nakicenovic et al., 2000). The fully coupled ocean–atmosphere general circulation model (GCM) of the UK Hadley Centre, HADCM3 (Johns et al., 2003), has been used to drive the higher-resolution atmospheric HadAM3H model (Pope et al., 2000), which in turn provides the initial and boundary conditions for the RCMs used in the PRUDENCE project, including HIRHAM4.

 

(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 des paramètres de l'aléa à des paramètres climatiques

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

 

 

 

 

(4) - Remarques générales

A warmer climate will undoubtedly reduce the general abundance of snow, because the zero-degree isotherm will be displaced on average towards higher latitudes and altitudes. A number of studies have shown how the average conditions of mountain snow packs may change in a ‘greenhouse climate’ by the end of the 21st century in the Alps (Beniston et al., 2003; Uhlmann et al., 2009), but little attention has been paid to the variability of winter snow conditions (Lopez-Moreno et al., 2009). It is interesting to note that today in the Alps, periods of snow abundance still occur despite the fact that winter temperatures in many parts of the alpine domain are 1–2 °C warmer than they were a century ago (Beniston, 2004). Over the past decade, sharply contrasting winters have been observed (Scherrer et al., 2004), from the exceptionally mild snow-sparse conditions experienced in 2006–2007 (up to 4 standard deviations above the mean in terms of winter temperature anomalies) or 2001–2002, to the snow abundance of 1998–1999 and 2008–2009 (up to 3 standard deviations below the long-term temperature average).

It is thus of interest to investigate whether snow-abundant winters could still occur in a greenhouse climate, because abundant snow has many environmental (e.g. hydrology, vegetation and natural hazards) and socio-economic (e.g. hydropower, agriculture and tourism) implications in the Alps. It is a key element for the timing and amount of runoff for rivers flowing off the alpine domain, and any long-term reductions in snow amount would significantly impact water use in major rivers such as the Rhone and the Rhine, not only within the mountains themselves but also far downstream in the populated lowlands (Beniston et al., 2003). The presence or absence of snow is an important determinant of the vegetation cycle of many plant species, some of which commence their vegetation cycle as soon as the snow cover has been removed; some species can cope with an earlier start to the season, whereas others cannot. There will thus be competition leading to shifts in species distribution (Keller et al., 2005). […]

 

(5) - Syntèses et préconisations

This study has shown that the behaviour of snow in the Alps is very sensitive to one of four temperature–precipitation modes, computed on the basis of joint exceedances of the lower and upper quartiles of these two variables. These four modes are closely related to synoptic weather patterns that can be either favourable or detrimental to snow in the mountains; as a result, the mode most closely correlated to snow amount and duration (WD) can be used as a proxy for these parameters in climate models that do not adequately simulate snow.

The study has shown that since the 1950s, there have been significant changes in the four modes, reflecting shifts in the frequency of occurrence of the underlying weather patterns. Although the WD episodes have almost doubled during winters over the past half-century, some exceptions where the WD mode is low and snow-abundant winters have occurred in recent years still remain, despite the overall warmer climate that the Alps currently experiences compared with that several decades ago.

A comparison of the four modes observed for the control period 1961–1990 and modelled by the HIRHAM RCM in the context of the EU PRUDENCE project has shown that the results compare sufficiently favourably for the RCM simulations to be applied to an enhanced greenhouse climate. The model shows that snow-abundant winters as experienced even in recent years of this century may occur less than 5% of the time in the latter 30 years of this century, but they will not totally disappear even in a much warmer climate. Although they will be rare events, it may be necessary to consider appropriate measures to adapt to the consequences of such winters, whose impacts on hydrology or vegetation may be different to today because the environment itself may have changed to the extent that it would no longer be geared towards unusual abundant snow events.

Références citées :

Abegg B, Froesch R. 1994. Climate change and winter tourism, Impact on transport companies in the Swiss canton of Graubünden, Mountain Environments in Changing Climates, Routledge: London; 328–348.

Begert M, Schlegel T, Kirchhofer W. 2005. Homogeneous temperature and precipitation series of Switzerland from 1864 to 2000. International Journal of Climatology 25: 65–80.

Beniston M. 2004. Climatic Change and its Impacts. An Overview Focusing on Switzerland. Kluwer Academic Publishers: Dordrecht/The Netherlands and Boston/USA (now Springer Publishers), 296 pp.

Beniston M. 2005. Warm winter spells in the Swiss Alps: strong heat waves in a cold season? Geophysical Research Letters 32: L01812.

Beniston M. 2006. The August 2005 intense rainfall event in Switzerland: not necessarily an analog for strong convective events in a greenhouse climate. Geophysical Research Letters 33: L5701.

Beniston M. 2009. Trends in joint quantiles of temperature and precipitation in Europe since 1901 and projected for 2100. Geophysical Research Letters 36: L07707.

Beniston M, Goyette S. 2007. Changes in variability and persistence of climate in Switzerland; exploring 20th century observations and 21st century simulations. Global and Planetary Change 57: 1–20.

Beniston M, Keller F, Koffi B, Goyette S. 2003. Estimates of snow accumulation and volume in the Swiss Alps under changing climatic conditions. Theoretical and Applied Climatology 76: 125–140.

Beniston M, Stephenson DB, Christensen OB, Ferro CAT, Frei C, Goyette S, Halsnaes K, Holt T, Jylhä K, Koffi B, Palutikoff J, Schöll R, Semmler T, Woth K. 2007. Future extreme events in European climate; an exploration of Regional Climate Model projections. Climatic Change 81: 71–95.

Johns TC, Gregory JM, Ingram WJ, Johnson CE, Jones A, Lowe JA, Mitchell JFB, Roberts DL, Sexton DMH, Stevenson DS, Tett SFB, Woodage MJ. 2003. Anthropogenic climate change for 1860 to 2100 simulated with the HadCM3 model under updated emission scenarios. Climate Dynamics 20: 583–612.

Keller F, Goyette S, Beniston M. 2005. Sensitivity analysis of snow cover to climate change scenarios and their impact on plant habitats in Alpine Terrain. Climatic Change 72: 299–319.

Lopez-Moreno JI, Goyette S, Beniston M. 2009. Impact of climate change on snowpack in the Pyrenees: horizontal spatial variability and vertical gradients. Journal of Hydrology 374: 384–396.

Nakicenovic N, Alcamo J, Davis G, de Vries B, Fenhann J, Gaffin S, Gregory K, Gr¨ubler A, Jung TY, Kram T, La Rovere EL, Michaelis L, Mori S, Morita T, Pepper W, Pitcher H, Price L, Raihi K, Roehrl A, Rogner H-H, Sankovski A, Schlesinger M, Shukla P, Smith S, Swart R, van Rooijen S, Victor N, Dadi Z. 2000. IPCC Special Report on Emissions Scenarios, Cambridge University Press: Cambridge, United Kingdom and New York, USA; 599 pp.

Pope DV, Gallani M, Rowntree R, Stratton A. 2000. The impact of new physical parameterizations in the Hadley Centre climate model HadAM3. Climate Dynamics 16: 123–146.

Räisanen J. 2008. Warmer climate: Less or more snow. Climate Dynamics 30: 307–319.

Scherrer SC, Appenzeller Ch, Laternser M. 2004. Trends in Swiss Alpine snow days: the role of local- and large-scale climate variability. Geophysical Research Letters 31: L13215.

Uhlmann B, Goyette S, Beniston M. 2009. Sensitivity analysis of snow patterns in Swiss ski resorts to shifts in temperature precipitation and humidity under condition of climate change. International Journal of Climatology 29: 1048–1055.