Mediterranean Cyclones and Windstorms in a Changing Climate
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Mediterranean cyclones and windstorms in a changing climate Article Published Version Creative Commons: Attribution 3.0 (CC-BY) Open Access Nissen, K. M., Leckebusch, G. C., Pinto, J. G. and Ulbrich, U. (2014) Mediterranean cyclones and windstorms in a changing climate. Regional Environmental Change, 14 (5). pp. 1873- 1890. ISSN 1436-378X doi: https://doi.org/10.1007/s10113-012- 0400-8 Available at http://centaur.reading.ac.uk/32732/ It is advisable to refer to the publisher's version if you intend to cite from the work. To link to this article DOI: http://dx.doi.org/10.1007/s10113-012-0400-8 Publisher: Springer 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 online Reg Environ Change (2014) 14:1873–1890 DOI 10.1007/s10113-012-0400-8 ORIGINAL ARTICLE Mediterranean cyclones and windstorms in a changing climate Katrin M. Nissen • Gregor C. Leckebusch • Joaquim G. Pinto • Uwe Ulbrich Received: 27 April 2012 / Accepted: 20 December 2012 / Published online: 19 January 2013 Ó The Author(s) 2013. This article is published with open access at Springerlink.com Abstract Changes in the frequency and intensity of positive shift in the NAO Index on the cyclone decrease is cyclones and associated windstorms affecting the Medi- restricted to the Western Mediterranean region, where it terranean region simulated under enhanced Greenhouse explains 10–50 % of the simulated trend, depending on the Gas forcing conditions are investigated. The analysis is individual simulation. With respect to windstorms, based on 7 climate model integrations performed with two decreases are simulated over most of the Mediterranean coupled global models (ECHAM5 MPIOM and INGV basin. This overall reduction is due to a decrease in the CMCC), comparing the end of the twentieth century and at number of events associated with local cyclones, while least the first half of the twenty-first century. As one of the the number of events associated with cyclones outside of models has a considerably enhanced resolution of the the Mediterranean region slightly increases. These systems atmosphere and the ocean, it is also investigated whether are, however, less intense in terms of their integrated the climate change signals are influenced by the model severity over the Mediterranean area, as they mostly affect resolution. While the higher resolved simulation is closer to the fringes of the region. In spite of the general reduction in reanalysis climatology, both in terms of cyclones and total numbers, several cyclones and windstorms of inten- windstorm distributions, there is no evidence for an influ- sity unknown under current climate conditions are identi- ence of the resolution on the sign of the climate change fied for the scenario simulations. For these events, no signal. All model simulations show a reduction in the total common trend exists in the individual simulations. Thus, number of cyclones crossing the Mediterranean region they may rather be attributed to long-term (e.g. decadal) under climate change conditions. Exceptions are Morocco variability than to the Greenhouse Gas forcing. Neverthe- and the Levant region, where the models predict an less, the result indicates that high-impact weather systems increase in the number of cyclones. The reduction is will remain an important risk in the Mediterranean Basin. especially strong for intense cyclones in terms of their Laplacian of pressure. The influence of the simulated Keywords Climate change Á Mediterranean Á Windstorms Á Cyclones K. M. Nissen (&) Á U. Ulbrich Institute for Meteorology, Freie Universita¨t Berlin, Carl-Heinrich-Becker Weg 6-10, 12165 Berlin, Germany Introduction e-mail: [email protected] The Mediterranean region (MR) has been recognised as G. C. Leckebusch one of the areas that will be strongly affected by climate School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, change (e.g. Giorgi 2006). Among the natural hazards Birmingham B15 2TT, UK affecting this region, extreme cyclones play an important role: these high-impact weather events may cause wind- J. G. Pinto storms, storm surges, landslides and flooding (De Zolt Institute for Geophysics and Meteorology, University of Cologne, Kerpener Str. 13, et al. 2006; Liberato et al. 2011; Lionello et al. 2010; 50937 Cologne, Germany Nissen et al. 2010). Thus, changes in cyclone occurrence 123 1874 K. M. Nissen et al. and characteristics are expected to play a main role in past, a different way by climate change than the cyclones present and future changes of the MR climate. Here, we located inside the MR (e.g. Pinto et al. 2007, 2009). In this focus on cyclones and the occurrence of windstorms as one study, we apply the same technique and investigate pos- hazardous phenomenon related to cyclones. sible future changes in windstorms, local Mediterranean Statistically significant negative trends in cyclone cyclones and non-local cyclones associated with Mediter- numbers affecting the MR have been detected for the ranean windstorms. The role of the NAO on present day recent past (e.g. Flocas et al. 2010; Maheras et al. 2001; variability and future changes is examined. The analysis Nissen et al. 2010; Trigo et al. 2000). On the other hand, evaluates a total of 7 global coupled model experiments while the number of explosive Mediterranean cyclones has performed using two different model configurations. All possibly decreased, this reduction is not statistically simulations are based on the same model for the atmo- significant (Kouroutzoglou et al. 2011a). sphere, but one simulation is especially designed for Research based on global climate model (GCM) simu- Mediterranean systems as it was conducted with a higher lations investigating Mediterranean cyclone activity under spatial resolution than other previously available AOGCM changing greenhouse gas (GHG) conditions has been simulations. The question is addressed, whether the simu- summarised in the review paper by Ulbrich et al. (2009). lated signals depend on the model resolution, which might For the MR, many GCM studies report a strong reduction be the case in the MR which is characterised by small-scale in winter cyclone activity (Lionello et al. 2002; Geng and features in orography and ocean–land contrast. Sugi 2003; Pinto et al. 2006; Leckebusch et al. 2006; The next section describes the model simulations used Bengtsson et al. 2006; Pinto et al. 2007;Lo¨ptien et al. for the present study and the methods employed for the 2008; Raible et al. 2010). With respect to cyclone inten- analysis. The model simulations are validated against sity, the results are more controversial, as some models observations in section ‘‘Simulation of the present day suggest a decrease in the frequency of the most intense climate’’ and the climate change signal is analysed in systems (Pinto et al. 2007; Raible et al. 2010), while other section ‘‘Climate change signal’’. Links with the NAO are models show more extreme events or an increase in the discussed in section ‘‘Links to NAO’’. Section ‘‘Summary intensity of extreme cyclones (Lionello et al. 2002; and discussion’’ summarises the results. Gaertner et al. 2007). For extreme windstorms, Bengtsson et al. (2009) predict a reduction in the number of events exhibiting wind speeds Data and methods above 35 m/s over the MR during winter at the end of the twenty-first century, under A1B conditions. They use the Data ECHAM5 model with a resolution of T213 (63 km) which shows a decrease in the 99.5 percentile of 925 hPa winds The study is based on coupled GCM simulations, which all over the entire MR. For the ECHAM5 MPIOM model with use the ECHAM5 model (e.g., Roeckner et al. 2006)as T63 resolution, Pinto et al. (2007) found a significant their atmospheric component. Results from two different decrease in the 98th percentile of the 10 m wind speed configurations are analysed: INGV CMCC and ECHAM5 during the winter half year only for the southernmost parts MPIOM. In both configurations, the ECHAM5 model is of the MR, while no significant changes are present in the integrated with 31 hybrid sigma-pressure levels in the northern part. A simulation with the HadCM3 model with a vertical and the highest vertical level at 10 hPa. resolution of 2.5° 9 3.75° latitude/longitude produces a In the ECHAM5 MPIOM configuration, the atmospheric reduction in winds restricted to the eastern MR (Lec- horizontal resolution is T63 (corresponding to 1.875° 9 kebusch and Ulbrich 2004). 1.875°). The ocean model MPIOM (e.g., Jungclaus et al. The distribution and evolution of cyclones and wind- 2006) comprises 40 vertical levels and is run at a horizontal storms in the MR for the ERA40 period has been analysed resolution of 1.5° 9 1.5°. in Nissen et al. (2010). A technique to match the wind- The INGV CMCC model (e.g., Scoccimarro et al. 2011) storms to the responsible cyclones was introduced. The uses a higher horizontal resolution for the atmosphere approach demonstrated that considering the link between (T159 corresponding to 0.75° 9 0.75°). The INGV CMCC pressure and wind features helps to understand the mech- system includes two ocean models. The global ocean anisms of windstorm variability. One particular result was component OPA8.2 (Madec et al. 1998) is run with 31 the finding that a considerable number of windstorms in the vertical levels and a horizontal resolution of approx. MR is associated with cyclones on a track completely 2° 9 2°. It uses a grid refinement in the equatorial region outside the MR.