The European summer of 2012 was marked by strongly contrasting rainfall anomalies, which led to flooding in northern Europe and droughts and wildfires in southern Europe. This season was not an isolated event, rather the latest in a string of summers characterized by a southward shifted Atlantic storm track as described by the negative phase of the SNAO. The degree of decadal variability in these features suggests a role for forcing from outside the dynamical atmosphere, and preliminary numerical experiments suggest that the global SST and low Arctic sea ice extent anomalies are likely to have played a role and that warm North Atlantic SSTs were a particular contributing factor. The direct effects of changes in radiative forcing from greenhouse gas and aerosol forcing are not included in these experiments, but both anthropogenic forcing and natural variability may have influenced the SST and sea ice changes

Alexander, LV

Allen, MR

Anel, JA

Barriopedro, D

Black, MT

Carey-Smith, T

Castillo, R

Cattiaux, J

Chen, X

Chevallier, M

Christidis, N

Ciavarella, A

de Vries, H

Dean, SM

Deans, K

Diffenbaugh, NS

Doblas-Reyes, F

Donat, MG

Dong, B

Eilerts, G

Funk, C

Galu, G

Garcia-Herrera, R

Germe, A

Gill, S

Gimeno, L

Guemas, V

Herring, SC

Hoell, A

Hoerling, MP

Huntingford, C

Husak, G

Imada, Y

Ishii, M

Karoly, DJ

Kimoto, M

King, AD

Knutson, TR

Lewis, SC

Lin, R

Lyon, B

Massey, N

Mazza, E

Michaelsen, J

Mollard, J

Mori, M

Mote, PW

Nieto, R

Otto, FEL

Park, J

Perkins, SE

Peterson, TC

Rosier, S

Rowland, J

Rupp, DE

Salas y Melia, D

Scherer, M

Scott, PA

Shiogama, H

Shukla, S

Song, F

Sparrow, S

Sutton, R

Sweet, W

Tett, SFB

Trigo, RM

van Oldenborgh, GJ

van Westrhenen, R

Verdin, J

Watanabe, M

Wittenberg, AT

Woollings, T

Yiou, P

Zeng, F

Zervas, C

Zhang, R

Zhou, T

Peterson, T.C.

Alexander, L.V.

Allen, M.R.

Anel, J.A.

Barriopedro, D.

Black, M.T.

Carey-Smith, T.

Castillo, R.

Cattiaux, J.

Chen, X.L.

Chen, X.Y.

Chevallier, M.

Christidis, N.

Ciavarella, A.

de Vries, H.

Dean, S.M.

Deans, K.

Diffenbaugh, N.S.

Doblas-Reyes, F.

Donat, M.G.

Dong, B.

Eilerts, G.

Funk, C.

Galu, G.

Garcia-Herrera, R.

Germe, A.

Gill, S.

Gimeno, L.

Guemas, V.

Herring, S.C.

Hoell, A.

Hoerling, M.P.

Huntingford, C.

Husak, G.

Imada, Y.

Ishii, M.

Karoly, D.J.

Kimoto, M.

King, A.D.

Knutson, T.R.

Lewis, S.C.

Lin, R.P.

Lyon, B.

Massey, N.

Mazza, E.

Michaelsen, J.

Mollard, J.

Mori, M.

Mote, P.W.

Nieto, R.

Otto, F.E.L.

Park, J.

Perkins, S.E.

Rosier, S.

Rowland, J.

Rupp, D.E.

Salas y Melia, D.

Scherer, M.

Shiogama, H.

Shukla, S.

Song, F.F.

Sparrow, S.

Scott, P.A.

Sutton, R.

Sweet, W.

Tett, S.F.B.

Trigo, R.M.

van Oldenborgh, G.J.

van Westrhenen, R.

Verdin, J.

Watanabe, M.

Wittenberg, A.T.

Woollings, T.

Yiou, Pascal

Zeng, F.R.

Zervas, C.

Zhang, R.

Zhou, T.J.

HAL-INSU

Explaining Extreme Events of 2012 from a Climate Perspective

Yiou, P.

HAL UVSQ

HAL-CEA

King AD,,

Lewis SC,,

Perkins SE,,

Alexander LV,,

Donat MG,,

Karoly DJ,,

Black MT,

UNSWorks

Limited Evidence of Anthropogenic Influence on the 2011-12Extreme Rainfall over Southeast Australia. [in ³Explaining ExtremeEvents of 2012 from a Climate Perspective²]

Attribution of extreme events is a challenging science and one that is currently undergoing considerable evolution. In this paper are 19 analyses by 18 different research groups, often using quite different methodologies, of 12 extreme events that occurred in 2012. In addition to investigating the causes of these extreme events, the multiple analyses of four of the events, the high temperatures in the United States, the record low levels of Arctic sea ice, and the heavy rain in northern Europe and eastern Australia, provide an opportunity to compare and contrast the strengths and weaknesses of the various methodologies. The differences also provide insights into the structural uncertainty of event attribution, that is, the uncertainty that arises directly from the differences in analysis methodology. In these cases, there was considerable agreement between the different assessments of the same event. However, different events had very different causes. Approximately half the analyses found some evidence that anthropogenically caused climate change was a contributing factor to the extreme event examined, though the effects of natural fluctuations of weather and climate on the evolution of many of the extreme events played key roles as well

IssueLab

English

Explaining Extreme Events of 2012 From a Climate Perspective

© 2013 American Meteorological Society. Artículo firmado por 78 autores.Attribution of extreme events is a challenging science and one that is currently undergoing considerable evolution. In this paper are 19 analyses by 18 different research groups, often using quite different methodologies, of 12 extreme events that occurred in 2012. In addition to investigating the causes of these extreme events, the multiple analyses of four of the events, the high temperatures in the United States, the record low levels of Arctic sea ice, and the heavy rain in northern Europe and eastern Australia, provide an opportunity to compare and contrast the strengths and weaknesses of the various methodologies. The differences also provide insights into the structural uncertainty of event attribution, that is, the uncertainty that arises directly from the differences in analysis methodology. In these cases, there was considerable agreement between the different assessments of the same event. However, different events had very different causes. Approximately half the analyses found some evidence that anthropogenically caused climate change was a contributing factor to the extreme event examined, though the effects of natural fluctuations of weather and climate on the evolution of many of the extreme events played key roles as well.Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEpu

Barriopedro Cepero, David

García Herrera, Ricardo

Docta Complutense

Bulletin of the American Meteorological Society

Explaining extreme events of 2012 from a climate perspective

Oxford University Research Archive

EXPLAINING EXTREME EVENTS OF 2012 FROM A CLIMATE PERSPECTIVE

Dong, Buwen

Sutton, Rowan

Woollings, Tim

Central Archive at the University of Reading

The extreme European summer 2012 Article Published Version Dong, B., Sutton, R. and Woollings, T. (2013) The extreme European summer 2012. Bulletin of the American Meteorological Society, 94 (9). s28­s32. ISSN 1520­0477 doi: https://doi.org/10.1175/BAMS­D­13­00085.1 Available at http://centaur.reading.ac.uk/34441/ It is advisable to refer to the publisher’s version if you intend to cite from the work. Published version at: http://dx.doi.org/10.1175/BAMS­D­13­00085.1 To link to this article DOI: http://dx.doi.org/10.1175/BAMS­D­13­00085.1 Publisher: American Meteorological Society All outputs in CentAUR are protected by Intellectual Property Rights law, including copyright law. Copyright and IPR is retained by the creators or other copyright holders. Terms and conditions for use of this material are defined in the End User Agreement . www.reading.ac.uk/centaur CentAUR Central Archive at the University of Reading Reading’s research outputs onlineS28 SEPTEMBER 2013|since 1950 (Kattenberg et al. 2008). This is actually a conservative estimate as the cold spells warm more strongly because of the stronger warming of Siberia (de Vries et al. 2012a). The resulting ice growth curve leads to a marginal situation where the ice thick-ness would have been almost equal to the historic threshold.Conclusions. We have analyzed the February 2012 cold spell from the perspective of its ability to generate sufficient ice thickness to organize the classic 200-km ice-skating 11-City Tour in the Netherlands. Despite very low tempera-tures, the ice was too thin in many locations. We investi-gated whether global warming could be responsible for this. A simulation with the KNMI ice-growth model, however, points to another more im-portant cause. Snowfall on the thin ice that had just formed is shown to limit the ice growth more strongly than the ef-fect of warming. This study emphasizes that interpreting the role of global warming in the cases of extreme winter European cold spells has to be carried out with care, as the natural variability in these events is large, leading to low signal-to-noise ratios. A remaining question is whether future cold spells will be accompanied by increasingly frequent snowfall events. There are in-dications from both observations and global climate models that the opposite is the case (de Vries et al. 2012b), but this has to be verified in more detail using regional climate models.10. THE EXTREME EUROPEAN SUMMER 2012BUWEN DONG, ROWAN SUTTON, AND TIM WOOLLINGSIntroduction. The summer of 2012 was marked by strongly contrasting precipitation anomalies across Europe. For example, the United Kingdom experi-enced its wettest summer (JJA) since 1912, which lead to widespread f looding. Spain, in contrast, suffered drought and wildfires associated with the second lowest summer rainfall in the last 60 years (see “The Record Winter Drought of 2011–12 in the Iberian Peninsula” in this report). These extremes were associated with a clear dipole in precipitation anomalies, indicating a northward concentration of European precipitation (Fig 10.1b). Here we show that the precipitation anomalies can be understood as consequences of anomalies in the large-scale at-mospheric circulation. We also present preliminary investigations into the potential role of anomalous SSTs in forcing the atmospheric circulation. Large-scale circulation. The JJA 2012 anomalies in European precipitation were related to a large-scale dipole pattern in sea level pressure (SLP) over the North Atlantic (Fig 10.1a), with low-pressure anoma-lies stretching from the Atlantic across the British Isles and Scandinavia (tracking the region of high FIG. 9.2. The cold spell 2012 as simulated by the KNMI ice model (de Bruin and Wessels 1988), showing observed snow depth (green), 12-hourly tem-perature (thin black), five-day average (thick black), 1981–2010 climatology (purple), simulated water temperature (blue), and ice thickness (red). The dashed lines indicate a simulation in a colder climate, while the dotted line indicates the ice growth without snow. (Source: KNMI.)S29SEPTEMBER 2013AMERICAN METEOROLOGICAL SOCIETY |precipitation) and high pressure over Greenland. This pattern projects strongly onto the negative phase of the summer North Atlantic Oscillation (SNAO; Fol-land et al. 2009). As in winter, this dominant pattern of variability describes meridional shifts of the storm track and the associated eddy-driven jet. However, the sea-sonal migration of the storm tracks means that over Europe, the precipitation response is opposite to that in winter, so that a northward shift of European precipitation is associated with a negative SNAO and a southward shift of the Atlantic storm track and jet (Dong et al. 2013, manuscript submitted to Environ. Res. Lett.). These features are illustrated for JJA 2012 in Figs. 10.1e–h. Compared to climatology, the jet in summer 2012 was displaced south over the eastern North Atlantic and extend-ed into central Europe. The climatology of cyclone track density (Fig. 10.1g) shows a split into two preferred cyclone paths, one passing to the south of Iceland and into the Nordic Seas and the other across the Brit-ish Isles and into southern Scandinavia. In the summer of 2012, almost all cyclones took the southern path, fol-lowing the northern f lank of the jet across the British Isles (Fig. 10.1h). The Euro-pean precipitation anoma-lies are, therefore, consis-tent with the modulation of preferred cyclone paths and the large-scale circulation. Summer 2012 in the context of recent variability. The At-lantic/European summer of 2012 should not be viewed in isolation but as the lat-est in a succession of very similar summers. A dipole in precipitation anomalies resembling that in Fig. 10.1b was very clear in all of the summers from 2007 to 2011, with the exception of sum-mer 2010 (Supplementary Fig. S10.1). Similarly, the SNAO has been negative for all of the last six summers (e.g., Allan and Folland 2012). Taking JJA means of the daily SNAO index from NOAA’s Climate Prediction Center gives values of -0.8, -1.9, -1.6, -1.1, -2.0, and -2.2 FIG. 10.1. Anomalies for JJA 2012 from the climatological period 1964–93 for (a) SLP (hPa) from HadSLP2, and (b) percentage precipitation change (%) from the daily gridded E-OBS precipitation (version 7.0) over Europe (Haylock et al. 2008). (c), (d) SSTs (°C) from HadISST (Rayner et al. 2003). 500-hPa zonal wind (m s-1) from the NCEP reanalysis for (e) the climatological period 1964–93 and (f) 2012. Cyclone track density as in Hoskins and Hodges (2002) for (g) the climatological period and (h) 2012. Track density is in unit of numbers per month per unit area, where the unit area is equivalent to a 5° spherical cap (about 106 km2). Note that this climatological period is dominated by cold AMO conditions and is the period used for the climatological model simula-tions. Thick lines in (a) highlight regions where the differences are statistically significant at the 90% confidence level using a two-tailed Student’s t test.S30 SEPTEMBER 2013|standard deviations for the summers from 2007 to 2012. These summers contain four out of the five in the record since 1950 that have an index with less than -1.5 standard deviations. Similarly, in the England and Wales precipitation record (Alexander and Jones 2001), 2012 was the wettest summer since 1912, and the average anomaly of the last six summers is 1.15 standard deviations (with respect to the last 100 years, 1913–2012). This string of recent European summers is consistent with the high importance of decadal variability in shaping European summer climate. Sutton and Dong (2012) demonstrated clear varia-tions on decadal timescales in SLP and precipitation patterns across Europe, which are very similar to those seen in summer 2012. This is consistent with the low-frequency variability of the SNAO (Folland et al. 2009), which suggests the influence of some factor external to the atmosphere. These decadal variations in large-scale circulation are well correlated with basin-wide fluctuations in Atlantic Ocean SSTs, known as the Atlantic Mul-tidecadal Oscillation (AMO). The recent summers occurred during a period of warm SSTs not seen since the 1950s, and it is likely that these Atlantic tem-peratures have played a role in influencing the SNAO (Knight et al. 2006; Folland et al. 2009; Sutton and Dong 2012). The North Atlantic SST anomalies for JJA 2012 (Fig. 10.1c) show a similar structure to that associated with the AMO and with interannual variations of the summer storm track (Dong et al. 2013, manuscript submitted to Environ. Res. Lett.). Warm SSTs are evident across the subtrop-ics but especially in the subpolar North Atlantic, where anomalies exceed 2°C. This anomaly pattern reflects the superposition of the mixed layer ocean response to the atmospheric anomalies and the low-frequency warming as-sociated with the AMO (Dong et al. 2013, manuscript submitted to Environ. Res. Lett.). Global SST anomalies for the same season are given in Fig. 10.1d, showing close to neutral ENSO conditions in the tropical Pacific. Outside of the North Atlantic, the strongest anomalies are in the subtropical/midlatitude North Pacific. Warm anomalies are evident in the Arctic, consistent with the continuing decline of sea ice. The rate of this decline might have been enhanced by recent atmospheric circulation anomalies related to the SNAO (Overland et al. 2012), and these sea ice extent anomalies may have had an influence on the atmospheric circulation (Balmaseda et al. 2010). Investigating the potential role of external forcing. We now briefly report on preliminary numerical model experiments to assess the importance of the SST and sea ice anomalies seen in summer 2012. We use the atmosphere configuration of the UK Met Office Hadley Centre Global Environment Model version 3 (HadGEM3-A), similar to the version used by Hewitt et al. (2011), with a resolution of 1.875° longitude by 1.25° latitude and 85 levels in the vertical. The experi-ments performed are summarized in Table 10.1; the last 25 years of each experiment are used for analysis. The CONTROL experiment reproduces the observed SLP and precipitation patterns very realistically for JJA (Supplementary Fig. S10.2).The SLP changes in the GLOBAL simulation show a significant low-pressure response around 30°N over North America, the Atlantic Ocean, and North Table 10.1. Summary of Numerical ExperimentsExperiments Boundary Conditions Length of RunCONTROLForced with monthly climatological SST and sea ice averaged over the period 1964–93 using HadISST (Rayner et al. 2003)32 yearsGLOBALForced with monthly SST and sea ice from Dec 2011 to Nov 201227 yearsATLANTICForced with SST and sea ice from Dec 2011 to Nov 2012 over the Atlantic sector (including the southern At-lantic Ocean) in the longitude range 32.5°W–17.5°E and climatological SST and sea ice outside the Atlantic27 yearsNORTH ATLANTICForced with SST and sea ice extent from Dec 2011 to Nov 2012 over the North Atlantic (north of 35°N with SST linearly smoothed southward to the climatology at 30°N) but clima-tological SST and sea ice outside the North Atlantic 27 yearsS31SEPTEMBER 2013AMERICAN METEOROLOGICAL SOCIETY |Africa, also extending into Southern Europe (Fig. 10.2a). Over Northern Europe, a pattern resembling the negative phase of the SNAO is simulated, but the anomaly is much weaker than that observed (Fig. 10.1a) and is not significant. The pattern of simulated European precipitation anomalies (Fig. 10.2d) is consistent with the negative phase of the SNAO, and is, therefore, very similar to the observations (Fig. 10.1b), but the magnitudes are again much weaker.The ATLANTIC experiment shows a much stron-ger SLP response in the tropical North Atlantic (Fig. 10.2b). This is part of a baroclinic response (vertical structure not shown) that is not consistent with the observations and suggests that the influence of SST anomalies outside the Atlantic damps the direct response to Atlantic SSTs in this region. Negative pre-cipitation anomalies are again simulated in southern Europe, but the pattern further north is very noisy (Fig. 10.2e). In the NORTH ATLANTIC experiment, the model simulates a substantial low-pressure anomaly over the midlatitude North Atlantic, with a weak extension into western Europe (Fig. 10.2c). This is similar to the response found (using a different model) by Sutton and Hodson (2005) and suggests a significant response to the warm SST anomalies in the northwest Atlantic (Fig. 10.1c), but one which is sensitive to the influence of lower latitude Atlantic SST anomalies (implied by comparison Figs. 10.2b and 10.2c). The response is equivalent barotropic (not shown), which indicates a significant role for eddy-mediated processes (e.g., Kushnir et al. 2002; FIG. 10.2. (a)–(c) SLP anomalies (hPa) and (d)–(f) precipitation changes (%) in the model simulations forced by different configurations of SST and sea ice in 2012 relative to the control simulation forced by climatological SST and sea ice (1964–93 mean). (a) and (d) Forced by global SST and sea ice in 2012; (b) and (e) forced by SST and sea ice in 2012 over the Atlantic sector but with climatological SST outside the Atlantic; and (c) and (f) forced by North Atlantic (northward of 35°N) SST and sea ice in 2012, with climatological SST and sea ice outside the North Atlantic. Thick lines highlight regions where the differences are statistically significant at the 90% confidence level using a two-tailed Student’s t test.S32 SEPTEMBER 2013|Feldstein 2007). Consistent with the observations (Fig 10.1a), the simulated SLP response implies enhanced westerly winds to the southwest of the United King-dom, but the response is weaker than the observed anomalies and is displaced northwards. Overall, the model results suggest that the at-mospheric circulation over the North Atlantic and European region in summer 2012 (negative phase of the SNAO), which was largely responsible for the observed extreme anomalies in European precipita-tion, was influenced by global SST and sea ice extent anomalies, and that it is likely that SST anomalies in the North Atlantic played a particularly important role (consistent with Dong et al. 2013, manuscript submitted to Environ. Res. Lett.; Sutton and Hodson 2005). Differences between the simulated responses and observed anomalies—in terms of both spatial patterns and the much weaker magnitude of the simulated anomalies—require some explanation. The simplest explanation would be internal variabil-ity. However, the fact that 2012 is only the latest in a series of negative SNAO European summers makes this possibility unlikely. Furthermore, the level of internal variability in the model simulations is insuf-ficient to account, with significant likelihood, for the magnitude of the observed anomalies (not shown). Other possible factors include an important role for coupled ocean-atmosphere interactions in shaping the response (Sutton and Mathieu 2002; Dong et al. 2013, manuscript submitted to Environ. Res. Lett.) and the direct impact of changes in radiative forcings from greenhouse gases and aerosols, which were not considered in the experiments discussed here. Model biases may also be a factor. Investigating these pos-sibilities is the subject of ongoing research.Conclusions. The European summer of 2012 was marked by strongly contrasting rainfall anoma-lies, which led to flooding in northern Europe and droughts and wildfires in southern Europe. This season was not an isolated event, rather the latest in a string of summers characterized by a southward shifted Atlantic storm track as described by the negative phase of the SNAO. The degree of decadal variability in these features suggests a role for forc-ing from outside the dynamical atmosphere, and preliminary numerical experiments suggest that the global SST and low Arctic sea ice extent anomalies are likely to have played a role and that warm North Atlantic SSTs were a particular contributing factor. The direct effects of changes in radiative forcing from greenhouse gas and aerosol forcing are not included in these experiments, but both anthropogenic forcing and natural variability may have influenced the SST and sea ice changes. 11. ARE RECENT WET NORTHWESTERN EUROPEAN SUMMERS A RESPONSE TO SEA ICE RETREAT?SIMON F. B. TETT, KIRSTEN DEANS, EDOARDO MAZZA, AND JAMES MOLLARDIntroduction. Since 2007, UK and northwestern (NW) European summers have been anomalously wet, with summer 2007 being notable for signifi-cant f looding in southern England. Arctic sea ice extent had record low values in September 2007 and 2012 (Parkinson & Comiso, 2013). To explore the potential impact of these changes in sea ice on precipitation, we carried out a set of numerical simulations of the high-resolution version of Ha-dAM3 (Pope et al. 2000) driven with different sea surface temperature and sea ice boundary conditions.  Observational data and experimental design. We focus on percentage of 1961–90 precipitation from the Glob-al Precipitation Climatology Centre GPCC reanalysis V5 1° × 1° product for which we use 1950–2006. From 2007 to 2012, we use the GPCC monitoring product (Schneider et al. 2011) to give a homogeneous dataset from 2007 and allow consideration of 2011 and 2012 in near-real time. We processed the GPCC datasets so that in each 1° × 1° grid box where there are no stations the data was set as 'missing'. Precipitation was con-servatively regridded to the 1.25° × 1.875° grid of the model and converted to percentage of the 1961–90 sea-sonal average. We then area-averaged the percentage of normal precipitation for the northwest European region—the western half (48°N–75°N, 10°E–15°W) of the northern European region of Giorgi and Francisco (2000). We find that the GPCC monitoring dataset is biased with respect to the reanalysis dataset, so we correct the monitoring product by the difference between it and the reanalysis product for the 2007–10 