Réf. Brunetti & al 2006 - A

Référence bibliographique complète
BRUNETTI M., MAUGERI M., NANNI T., AUER I., BÖHM R., SCHÖNERW. Precipitation variability and changes in the greater Alpine region over the 1800–2003 period. Journal of Geophysical Research, 2006, Vol. 111, 29 p.

Abstract: The paper investigates precipitation variability in the greater Alpine region (GAR) (4–19°E, 43–49°N) based on 192 instrumental series of homogenized and outlier checked monthly precipitation and on the 1° gridded version of the same data set. Compared to the previous data sets, the one used in this paper adds a full century of data (earliest series starting in 1800) by exploiting the early instrumental period as much as possible in terms of series length and spatial density. The records were clustered into climatically homogeneous subregions, by means of a principal component analysis, and average subregional series were calculated. The principal component analysis was applied also in T-mode to investigate the most recursive precipitation patterns that characterize the examined area. Yearly and seasonal trend analysis was performed both on subregional average series and on the mean GAR series. It was also applied to moving windows, of variable width ranging from 2 decades to 2 centuries, in order to investigate any trends over decadal to secular timescales. Beside trends in total precipitation, precipitation seasonality was also analyzed as an important indicator of climate changes. Links between precipitation variability in the Alpine region and atmospheric circulation, and the North Atlantic Oscillation in particular, were also studied.

Mots-clés
 

Organismes / Contacts
Central Institute for Meteorology and Geodynamics, Institute of Atmospheric Sciences and Climate,Instituto di Fisica Generale Applicata.m.brunetti@isac.cnr.it

(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
Precipitation      

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Alpine Arc Greater Alpine Region (GAR): The Alps and their wider surroundings (4–19°E, 43–49°N) 112 available grid points    
1800–2003

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
Time Evolution of Regional and Subregional Average Precipitation Series and Trend Analysis:

Large variability on interannual to centennial timescales is evident. Besides few common features in the different subregions, such as the wet earliest 2 decades, the minima in the second half of the 19th century and around 1950, and the distinct autumn-wetting in the recent 2 decades, there are many differences in the long-term behavior, as well as in the interannual variability.

The amplitude of the interannual variability differs considerably among the subregions. This seems to be independent of the number of available stations involved in each subregional mean, because the number of stations per region is quite similar (52, 41, 48, and 51 series for NW, NE, SW, and SE respectively), and the subregion with the lowest number of available stations is that with the lowest variability (the NE region). The reason seems to be related more to geographical aspects, the northern subregions having a lower interannual variability than the southern ones. The interannual variability differs markedly also among seasons, winter and autumn being the seasons with the highest interannual variability.

The annual precipitation amounts show the already mentioned general drying trend from the wet early 1800s to the dry mid 19th century in all subregions (with the exception of the SE with no data in the first 4 decades of the 19th century). Then, there is a marked splitting between a long-lasting wetting in NW (less pronounced in NE) and a long-term drying in the Mediterranean, which is more pronounced in SE, not really significant in SW. These long-term features of the annual precipitation amounts are the result of a few similar seasonal evolutions but numerous different ones.

In northern subregions (NW and NE) there is an initial tendency toward a decrease in winter precipitation amount, from the beginning up to 1850s, followed by a long increasing tendency that stops in the 1970s. This is not evident in southern subregions, where no relevant long-term variations are visible up to the 1970s. On the contrary, the winter precipitation decrease, that characterizes the recent decades from the 1980s up to today, is a common feature of all subregions, being evident in both northern and southern ones (but weakest in the NW).

In autumn, the most interesting aspects are the long decrease in precipitation amount that characterizes southern subregions from the beginning up to 1970s (in northern subregions there is also a decrease in this period, but it is not as clear as in southern subregions), and then a rapid increase that is evident in all subregions.

The most prominent feature which distinguishes spring precipitation from the other three seasons is the complete absence of the wet earliest decades in the first part of the 19th century.

As already mentioned, summer precipitation does not show prominent and long-lasting trends. They are more characterized by ups and downs on a decadal scale, more pronounced in SW.

According to the described subregional differences, the all GAR-average analysis is not really representative of the entire area, when for example, inverse subregional long-term trends lead to a no-trend result for the average over the entire region. On the other hand, the few already mentioned ‘‘all-GAR-features’’ may well reflect the continental-to-global-scale background (after the elimination of the topographically forced enhancements/reductions explainable through the existence of the Alpine chain).

Northern subregions show a positive trend in the total annual precipitation amount, even if it does not reach significant values. On the contrary, total annual precipitation has a negative tendency in southern subregions with highly significant values (5% and 4% per century in SW and SE respectively).

On a seasonal basis, northern subregions reach significant positive trend values in winter (only for NW, with an increase of +9% per century) and in spring (both for NW and NE, with trends of +6% and +4% per century respectively). In summer and autumn the sign of the trend is negative but not significant.

The strongest contribution to the negative trend of total annual precipitation for southern subregions comes from the autumn season, where the trends are significant for both southern subregions (14% and 10% per century for SW and SE respectively). SE shows a significant negative trend also in spring (9% per century). North


Atlantic Oscillation (NAO) and GAR Precipitation:


The most relevant results are for the winter season (DJF), with the well-known negative correlation between NAO and precipitation southward of the Alps. In the northern part of the region, the winter correlation is also negative, but not significant. The transition between strong and weak NAO influence is very sharp along the zonal part of the Alpine chain, less pronounced west of the Alps, and smoothest (and turning northward) in the eastern, more continental parts of the GAR. This points to an existing marked influence of the Alps with respect to NAO precipitation correlation.

It is interesting to note that this situation is not representative of the whole 1866–2003 period, the correlation not being constant over time, in particular in the northern part of the Alps. The sequence of maps shows that there is a negative sign of the correlation over all the region, in the first map (late 19th century), which progressively assumes a north-south dipolar pattern that becomes clearer and clearer in the following maps from the 20th century.
Modélisations
 
Hypothèses
 

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

The 192 station series of the GAR were regionalized (via S-mode PCA, performed on the correlation matrix of normalized monthly/seasonal/annual precipitation totals) into four approximately equally sized principal subregions (NW, NE, SW, and SE).

The precipitation evolution of the different subregions was stressed by means of a moving window technique (called running trend analysis here), with variable window width, that allowed the identification of some relevant tendencies on a wide range of timescales (from 2 decades to 2 centuries) involving the whole GAR, in some cases, or a few subregions, in many others. The different time evolution of precipitation in the various subregions was also highlighted by a T-mode PCA. This technique highlighted the existence of two leading general and long-term dipole structures, throughout north-south and west-east main directions. Series, representative of these two patterns, were constructed from the differences between northern and southern regional average series, and western and eastern ones.Besides changes in total precipitation amount, precipitation distribution over the year was also analyzed to identify changes in precipitation seasonality.

The final section of this work was aimed at making an initial step into the ‘‘understanding business’’ which has to rely strongly on forcing via circulation [via the NAO] – especially in a region with such prominent orographic features like the Alps.


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

As a consequence of the already described subregional similarities/discrepancies the strongest visible feature in the annual mean GAR-precipitation series is the drying trends from the wet first 3 decades (1800 to 1830 and the years around 1850) to the leading minimum centered around the 1860s. Afterward, the splitting into different subregional evolutions leads to general deletion of significant all-GAR long-term trends. Only the wet 1910s should be mentioned, mainly because of their importance for the last general glacier advance during this most maritime decade of the entire instrumental period in the Alps.

This feature, together with the similar effect of the synchronicity of the wet early instrumental period, with the most pronounced Alpine glacier advances before 1820 and 1850, highlights the great application potential of the new HISTALP precipitation data set for climate impact studies. Attempts to understand the pre-1850 glacier advances with models based on temperature alone have not really been successful so far.

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
 

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

(4) - Remarques générales

 


(5) - Syntèses et préconisations

This study is aimed at describing precipitation behavior over the last 2 centuries in a wide region centered on the European Alps. Moreover, it describes what can be analyzed if the full capacity of existing instrumental data is used, for a relatively small but climatologically interesting region at the border between different continental-scale European climate regimes.

By ‘‘full capacity exploitation’’ [the authors] mean the highest achievable spatial density of at least centennial series, which have been subjected to careful detection and elimination of nonclimatic noise.

[The authors] are confident of having fulfilled this requirement [compare Auer et al., 2005], and thus can rely on the results of [their] analysis.

Selected examples served to show and discuss the principal features of 2 centuries’ precipitation variability in the study region.

The 192 station series of the GAR were regionalized (via S-mode PCA, performed on the correlation matrix of normalized monthly/seasonal/annual precipitation totals) into four approximately equally sized principal subregions (NW, NE, SW, and SE).

Precipitation average series of these subregions not only evolved through different interannual variability, but are also characterized by significantly different long-term trends: the most relevant one being an increase in the total precipitation amount north of the Alpine chain, and a highly significant decrease south of the Alps.

The precipitation evolution of the different subregions was stressed by means of a moving window technique (called running trend analysis here), with variable window width, that allowed the identification of some relevant tendencies on a wide range of timescales (from 2 decades to 2 centuries) involving the whole GAR, in some cases, or a few subregions, in many others.

The different time evolution of precipitation in the various subregions was also highlighted by a T-mode PCA. This technique highlighted the existence of two leading general and long-term dipole structures, throughout north-south and west-east main directions. Series, representative of these two patterns, were constructed from the differences between northern and southern regional average series, and western and eastern ones. The N-S dipole shows the most stable and significant 2-century trend in the study region: a general strengthening of the mentioned dipole structure, due to a relative wetting of the northern GAR (temperate westerly), versus the southern GAR (Mediterranean parts).

Besides changes in total precipitation amount, precipitation distribution over the year was also analyzed to identify changes in precipitation seasonality. A regular and smooth signal toward an increase in the relative contribution of winter and spring precipitation to the total annual amount, with respect to that coming from summer and autumn months, was observed. This tendency, however, must be interpreted with caution, because it could be partly attributed to wind-induced instrumental measuring errors, a bias that the intensive homogenization and adjustments applied on the series have reduced but not completely eliminated. This possibility must be investigated in the light of a wider availability of long-term wind and snow series.

The final section of this work was aimed at making an initial step into the ‘‘understanding business’’ which has to rely strongly on forcing via circulation – especially in a region with such prominent orographic features like the Alps. The long-term correlative comparison with the NAO index showed existing spatial patterns, as well as an interesting variability of the NAO correlation over time.

[Author's] future study objectives will concentrate on increasing our understanding of the multiple climate variability structure in the GAR. This will be based on combined analyses of the existing climate elements in HISTALP (temperature, precipitation, air pressure with full spatial coverage, sunshine and cloudiness with nearly full spatial coverage, and humidity for parts of the GAR, compare Auer et al. [2006]) in relation to larger-scale (continental to global) background features, and finding dependencies of the local (GAR or smaller) climate variability residuals on circulation.

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