Réf. Wang & al. 2011 - A

Référence bibliographique complète

WANG, X.L., WAN, H., ZWIERS, F.W., SWAIL, V.R., COMPO, G.P., ALLAN, R.J., VOSE, R.S., JOURDAIN, S., YIN, X. (2011). Trends and low-frequency variability of storminess over western Europe, 1878–2007. Climate Dynamics, Volume 37(11-12), 2355-2371. [Etude en ligne]

Abstract: This study analyzes extremes of geostrophic wind speeds derived from sub-daily surface pressure observations at 13 sites in the European region from the Iberian peninsula to Scandinavia for the period from 1878 or later to 2007. It extends previous studies on storminess conditions in the Northeast (NE) Atlantic-European region. It also briefly discusses the relationship between storminess and the North Atlantic Oscillation (NAO). The results show that storminess conditions in the region from the Northeast Atlantic to western Europe have undergone substantial decadal or longer time scale fluctuations, with considerable seasonal and regional differences (especially between winter and summer, and between the British Isles-North Sea area and other parts of the region). In the North Sea and the Alps areas, there has been a notable increase in the occurrence frequency of strong geostrophic winds from the mid to the late twentieth century. The results also show that, in the cold season (December–March), the NAO-storminess relationship is significantly positive in the north-central part of this region, but negative in the south-southeastern part.

Mots-clés

 

 

Organismes / Contact

Climate Research Division, Science and Technology Branch, Environment Canada, Toronto, ON, Canada (Xiaolan.Wang@ec.gc.ca)

CIRES, Climate Diagnostics Center, University of Colorado, Boulder, CO, USA

NOAA Earth System Research Laboratory, Physical Sciences Division, Boulder, CO, USA

Hadley Centre, Met Office, Exeter, UK

NOAA’s National Climatic Data Center, Asheville, NC, USA

Meteo-France, Direction de la Climatologie, Toulouse, France

STG Inc., Asheville, NC, USA

• Present address: Pacific Climate Impacts Consortium, University of Victoria, Victoria, BC, Canada

 

(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

Storminess

 

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

Europe

 

 

 

 

 1878–2007

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

Long-term trends and low-frequency variability of storminess

Similar to what was noted in W09, decadal or longer time scale variability in storminess is very profound in this region. The previously reported trends seen in the second half of the twentieth century (McCabe et al. 2001; Gulev et al. 2001; Wang et al. 2006a, b) seem to have continued into the early twenty-first century. For the North Sea area in winter, the 1960s–1970s is the calmest period, while the 1990s is the roughest period in the record. There are also large seasonal and regional differences in trends and variability. The most striking seasonal differences are seen between winter (DJF) and summer (JJA), which is not surprising given the different circulation regimes dominating in different seasons. In terms of Kendall’s linear trend estimates, the most significant winter storminess trend in western Europe is seen in the Alps region. Namely, winter storminess shows a steady increasing trend in the region from Paris to Kremsmuenster to Barcelona to Madrid. It is also shown that winter storminess appears to have slightly declined over northern Europe (east of Denmark) and southeastern Iberia, while there is an unprecedented maximum in the early 1990s in the North Sea-British Isles area. The latter is also very clearly shown in the series of 20-yr mean storminess conditions. Note that the geostrophic wind speeds and the ERA40 surface wind speeds have been shown to have very similar trends and low-frequency variations in the extremes (99th percentiles; see W09).

In the North Sea area, there has been a notable increase in the occurrence frequency of moderate-strong geostrophic winds from the mid to the late twentieth century, while the increase in the earlier half of the century is mostly in the occurrence frequency of weaker geostrophic winds. The peak in the early 1990s is unprecedented only in the high (99th) percentiles; it is not the highest peak in the 50th and 25th percentiles. In terms of linear trend, only the 50th percentiles show a marginally significant downward trend, with no significant trend in the other quantiles. In the Alps, however, an increase has been observed in all percentiles shown; the distribution has a heavier tail than that of the APTB triangle [North Sea area]. Note that there has been a notable increase in the upper tail (≥20 m s−1) in the recent decades in both triangle regions.

In contrast, summer storminess trends are characterized by a decreasing trend in the region from the Bay of Biscay to the North Sea to central Europe, with a significant increasing trend over southern and eastern Iberia and the French Alps, and no significant change in the other parts of the region analyzed. Spring storminess appears to have an increasing trend in the Alps region, but a decreasing trend in the region from Iberia to the Bay of Biscay to northern Europe; it also appears to have increased slightly in the northern most part of the region. Autumn storminess shows a decreasing trend in the region from northern Europe to the North Sea to the Bay of Biscay, and over Central Iberia, with an increasing trend over the Alps and southern Iberia. In all four seasons, the trends are field significant at 5% level, for the field of the 24 triangles, according to the Walker’s test.

Analyzing extremes of 3-hourly SLP changes derived from in-situ sub-daily pressure observations, Alexander and Tett (2005) and Allan et al. (2009) also reported that the British Isles experienced the most severe storm activity in the 1990s over the period from 1920 to 2004. Allan et al. (2009) further reported that in this region severe storms in autumn (OND) and winter (JFM) respond to different physical mechanisms (namely, tropical to mid-latitude North Atlantic and lesser Pacific “ENSO-like” influences dominate in OND, whereas NAO influences predominate in JFM). The seasonal differences in storminess trends and low-frequency variability could be associated with the seasonal variation in the dominating physical mechanisms.

Since individual storms are generally accompanied by precipitation, changes in storminess should be associated with changes in precipitation (Compo and Sardeshmukh 2004). Using data for the period from 1961 to 2000, Fowler and Kilsby (2003a, b) estimated that the recurrence of 10-day precipitation totals with a 50-year return period (based on data for 1961–1990) had increased by a factor of two to five by the 1990s in northern England and Scotland. Trenberth et al. (2007) reported that annual precipitation increased in northern Europe and decreased in the Mediterranean region during the period from 1901 to 2005, and that central and northern Europe exhibited changes primarily in winter, with insignificant changes in summer. These precipitation trends are in general agreement with the above-described historical storminess trends.

Storminess conditions and the NAO

European surface air temperature and precipitation are strongly affected by the NAO (Hurrell and van Loon 1997; Hurrell 1995; Alexandersson et al. 1998). It is also associated with the tendency for inverse variations in precipitation between northern Europe and the Mediterranean (Dickson et al. 2000; Hurrell and van Loon 1997; Trenberth et al. 2007). For instance, a more positive NAO in the 1990s was associated with wetter conditions in northern Europe and drier conditions over the Mediterranean and northern African regions (Dickson et al. 2000; Trenberth et al. 2007). Significant relationships between the NAO and storminess conditions in the North Atlantic domain have also been reported in several studies (e.g., Wang et al. 2009; Chang 2009; Wang et al. 2006b; Allan et al. 2009; Jung et al. 2003; Ulbrich and Christoph 1999).

Simultaneous correlations between seasonal NAO index series and the P95 and P99 storm indices were calculated for the period from 1878 or later to 2007, along with the corresponding significance level and field significance. For the P99 storm index, the significance level is also shown. In winter, highly significant positive correlations are seen over northern and central parts of the region analyzed, with negative correlations for the southern part (Iberia and Alps; DJF). A similar pattern, but with slightly weaker correlations, is also seen in spring (MAM). However, the correlations are much weaker in summer and autumn (JJA and SON). Over Iberia, the correlations in summer are very different from those in the other seasons, featuring marginally significant positive correlations in summer but mostly negative correlations in the other seasons. For the field of the 24 triangles, the correlations are field significant at 5% level in winter, spring, and autumn, but insignificant in summer, according to the Walker’s test.

[The authors] speculate that the seasonality of the NAO-storminess relationship arises from the seasonal migration of the storm track and the seasonal variations in the two poles of the NAO, the Azores High and the Icelandic Low. In winter, the Azores High center locates around 30°N latitude (south of the Azores). The Iberian peninsula lies in the northern periphery of the high pressure ridge, and north-central Europe lies in the region of largest pressure gradient between the two poles of the NAO. Thus, a stronger positive NAO is associated with stormier winters in north-central Europe but less stormy winters in the Iberian peninsula (a stronger Azores High could even control the Iberian peninsula) (DJF). In summer, while the Icelandic Low weakens, the Azores High center moves northward to around 35°N, often building a ridge across France and the Alps region, bringing hot and dry weather to these areas. In this situation, the Iberian peninsula lies in the southern periphery of the Azores High, where African easterly waves are impelled, favouring tropical cyclonegenesis. Therefore, a stronger positive NAO is associated with stormier summers in the Iberian peninsula (JJA). (…)

Modélisations

 

Hypothèses

 

 

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

This study extends the region analyzed by Wang et al. 2009 (W09) and Alexandersson et al. (1998, 2000) southward to Iberia and eastward to central Europe. The authors augmented the geostrophic wind dataset of W09 by deriving sub-daily geostrophic wind speeds and their extremes for an additional 14 triangles in this region for the period from 1878 or later to 2007, to assess the historical storminess conditions and whether trends have continued into the early twenty-first century. Similar to W09, they also briefly discuss the relationship between the storminess conditions and the North Atlantic Oscillation (NAO).

The sub-daily sea level pressure (SLP) data analyzed in this study were obtained from the International Surface Pressure Databank (Yin et al. 2008). (…) In this study 14 new pressure triangles are formed and analyzed to augment the triangles of W09. The authors also analyze the triangles analyzed in W09, using a better sampling technique to reduce aliasing effects (as detailed in the study). As noticed in W09, the configuration of the triangles is important; triangles that are too large tend to mask differences among different parts of the triangle area. Extreme geostrophic wind speeds from smaller triangles should correspond better with the areal maximum surface wind speed than those from larger triangles, because a geostrophic wind speed represents an average wind condition over the triangle region. These factors were considered when constructing the triangles; the triangles [used] are the smallest and most comparable in size that can be constructed from the available sites with long term sub-daily pressure observations. They should represent the long term trend in storm activity, although the degree of approximation of geostrophic wind speeds to the corresponding surface wind speeds could be compromised in regions of complex terrain (better approximation is expected over smooth surfaces, such as oceanic areas).

For each triangle, sub-daily instantaneous geostrophic wind speeds are calculated from the sub-daily instantaneous SLP values for the same hour at the three sites that form the triangle when none of the three values are missing (see W09 for details). (…)

Aliasing occurs when a time series is subsampled at regular intervals, with the result that high-frequency variation not resolved by the longer subsampling interval is “folded” onto lower frequencies. (…) To diminish aliasing effects, the authors first calculate the 95th and 99th percentiles (P95, P99) of all sub-daily geostrophic winds in moving 91-day windows, obtaining a daily series of moving seasonal quantiles. (…)

In the last section the authors briefly discuss the relationship between the NAO and the storminess conditions over the western European region analyzed in this study. (…) They use the NAO index of Hurrell (1995), as updated by W09, because the pressure records from both Stykkisholmur and Lisbon are included in the geostrophic winds (a better match in location).

 

(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

 

 

(5) - Syntèses et préconisations

[The authors] have analyzed extremes of geostrophic wind speeds derived from sub-daily SLP observations at 13 sites in the western European region from the Iberian peninsula to Scandinavia for the period from 1878 or later to 2007, as an extension of the previous studies on storminess conditions in the Northeast (NE) Atlantic-European region. [They] have also updated the results for the 10 triangles analyzed in W09 using a re-sampling technique to reduce aliasing effects, which are found to be very small and do not change the conclusions. [They] have also briefly discussed the relationship between the storminess conditions and the North Atlantic Oscillation (NAO).

The results show that storminess conditions in this European region have undergone substantial decadal or longer time scale fluctuations, with considerable seasonal and regional differences (especially between winter and summer, and between the British Isles-North Sea area and other parts of the region). The previously reported trends seen in the second half of the twentieth century (McCabe et al. 2001; Gulev et al. 2001; Wang et al. 2006a, b) seem to have continued into the early twenty-first century. The winter storminess trends are characterized by increases in the Alps region, with slight decreases in northern Europe and in the region from northwestern Iberia northeastward to the southern UK. In particular, there has been a notable increase in the upper tail of the distribution of geostrophic wind speeds in the recent decades in both the North Sea and the Alps areas; the occurrence frequency of strong geostrophic winds has increased notably from the mid to the late twentieth century. Decreases are also seen in spring storminess over the region from northwestern Iberia to the Bay of Biscay to northern Europe. In summer and autumn, storminess trends are characterized by decreases in the region from the Bay of Biscay to the North Sea to central Europe, with increases in the French Alps and southern Iberia. The results also show that, in the cold season (December–March), the NAO-storminess relationship is significantly positive in the north-central part of this region, but negative in the south-southeastern part. The NAO-storminess relationship revealed in this study is consistent with the results of previous studies (e.g. Chang 2009; Folland et al. 2009; Gulev et al. 2001; Serreze et al. 1997).

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Alexander LV, Tett SFB (2005) Recent observed changes in severe storms over the United Kingdom and Iceland. Geophys Res Lett 32:L13704. doi:10.1029/2005GL022371

 

Alexandersson H, Tuomenvirta H, Schmith T, Iden K (2000) Trends of storms in NW Europe derived from an updated pressure data set. Clim Res 14:71–73

 

Alexandersson H, Schmith T, Iden K, Tuomenvirta H (1998) Long-term variations of the storm climate over NW Europe. Glob Atmos Ocean Syst 6:97–120

 

Allan R, Tett S, Alexander LV (2009) Fluctuations of autumn-winter severe storms over the British Isles: 1920 to present. Int J Climatol 29:357–371. doi:10.1002/joc.1765

 

Chang EKM (2009) Are band-pass variance statistics useful measures of storm track activity? Re-examining storm track variability associated with the NAO using multiple storm track measures. Clim Dyn 33:277–296. doi:10.1007/s00382-009-0532-9

 

Compo GP, Sardeshmukh PD (2004) Storm track predictability on seasonal and decadal scales. J Clim 17:3701–3720

 

Dickson RR et al (2000) The Arctic Ocean response to the North Atlantic oscillation. J Clim 13:2671–2696

 

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Fowler HJ, Kilsby CG (2003) Implications of changes in seasonal and annual extreme rainfall. Geophys Res Lett 30:1720. doi:10.1029/2003017327

 

Fowler HJ, Kilsby CG (2003) A regional frequency analysis of United Kingdom extreme rainfall from 1961 to 2000. Int J Climatol 23:1313–1334

 

Gulev SK, Zolina O, Grigoriev S (2001) Extratropical cyclone variability in the Northern Hemisphere winter from the NNRs/NCAR reanalysis data. Clim Dyn 17:795–809

 

Hurrell JW (1995) Decadal trends in the North Atlantic oscillation: regional temperatures and precipitation. Science 269:676–679

 

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Jung T, Hilmer M, Ruprecht E, Kleppek S, Gulev SK, Zolina O (2003) Characteristics of the recent eastward shift of interannual NAO variability. J Clim 16:3371–3382

 

Leckebusch GC, Koffi B, Ulbrich U, Pinto JG, Spangehl T, Zacharias S (2006) Analysis of frequency and intensity of winter storm events in Europe on synoptic and regional scales from a multi-model perspective. Clim Res 31:59–74


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McCabe GJ, Clark MP, Serreze MC (2001) Trends in Northern Hemisphere surface cyclone frequency and intensity. J Clim 14:2763–2768

 

Serreze MC, Carse F, Barry RG, Rogers JC (1997) Icelandic low cyclone activity: climatological features, linkages with the NAO, and relationships with recent changes in the Northern Hemisphere circulation. J Clim 10(3):453–464

 

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