Research Collection Journal Article Winter synoptic-scale variability over the Mediterranean Basin under future climate conditions as simulated by the ECHAM5 Author(s): Raible, Christoph Cornelius; Ziv, Baruch; Saaroni, Hadas; Wild, Martin Publication Date: 2010-08 Permanent Link: https://doi.org/10.3929/ethz-b-000156425 Originally published in: Climate Dynamics 35(2-3), http://doi.org/10.1007/s00382-009-0678-5 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Clim Dyn (2010) 35:473–488 DOI 10.1007/s00382-009-0678-5 Winter synoptic-scale variability over the Mediterranean Basin under future climate conditions as simulated by the ECHAM5 Christoph Cornelius Raible • Baruch Ziv • Hadas Saaroni • Martin Wild Received: 7 January 2009 / Accepted: 28 September 2009 / Published online: 15 October 2009 Ó Springer-Verlag 2009 Abstract Changes of the winter climate in the Mediter- climatic changes. The Mediterranean cyclones, which are ranean Basin (MB) for future A2 conditions are investi- individually detected and tracked, decrease by 10% in the gated for the period 2071–2100 and compared with the Western Mediterranean (WM) whereas no significant control period 1961–1990. The analysis is based on time- change is found in the Eastern Mediterranean. The cyclone slice simulations of the latest version of the ECHAM intensity is slightly reduced in the entire region. To model. First, the control simulation is evaluated with understand these changes, the underlying dynamical reanalysis data. The emphasis is given to synoptic and background is analyzed. It is found that changes in baro- large-scale features and their variability in the MB. The clinicity, static stability, transformation from eddy kinetic model is found to be capable of reproducing the main energy to kinetic energy of the mean flow and stationary features of the MB and southern Europe in the winter wave activity are significant in particular in the WM and season. Second, the A2 simulation is compared with the the coastline of North Africa. The reduction of cyclonic control simulation, revealing considerable changes of the activity severely impacts the precipitation mainly in the synoptic variability. Focusing on the synoptic spatio-tem- southern part of the WM. poral scale aims to unfold the dynamic background of the Keywords Cyclones Á Eddy kinetic energy Á Mediterranean Á A2 scenario Electronic supplementary material The online version of this article (doi:10.1007/s00382-009-0678-5) contains supplementary material, which is available to authorized users. 1 Introduction C. C. Raible (&) Climate and Environmental Physics, Physics Institute, All global climate models, applied to emission scenarios of University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland e-mail: [email protected] the twenty-first century, project a general warming (IPCC 2001). Besides the Arctic and the Antarctic, the IPCC C. C. Raible (2007) suggests that the Mediterranean Basin (MB) is Oeschger Centre for Climate Change Research, another key region of high anthropogenic climate impact, University of Bern, Bern, Switzerland in particular on the hydrological cycle in winter, the rain- B. Ziv iest season of the region (HMSO 1962; Gibelin and De´eque´ Department of Natural Sciences, Open University, 2003; Lionello et al. 2006; Giorgi and Lionello 2008). Tel Aviv, Israel The MB is located close to the subtropics (partly in the H. Saaroni subtropics) and is heavily influenced by the surrounding Department of Geography and the Human Environment, regions and possibly by long distance teleconnections. Tel Aviv University, Tel Aviv, Israel Obviously, the North Atlantic, located upstream, has an important impact on the climate of the MB, by, e.g., M. Wild Institute for Atmospheric and Climate Science ETH, cyclones traveling into the MB (Raible 2007), or blocking Zurich, Switzerland events (Martin et al. 2004), which prevent cyclones in the 123 474 C. C. Raible et al.: Winter cyclonic activity over Mediterranean Basin western part of the MB and lead to polar intrusions further hydrological cycle in the MB. Due to the complexity of the downstream in the eastern part (Ziv et al. 2006). Other terrain in this region and because rainfall changes involve a factors affecting the MB are the North Atlantic Oscillation wide range of scales, it is necessary to study possible future (Jacobeit et al. 2003) and the West Atlantic–East Russia changes by highly resolved model simulations. To that end oscillation (Krichak and Alpert 2005). In addition to the we use simulations of a higher resolution model—the above mentioned mid-latitude factors, the MB is affected ECHAM5 (Roeckner et al. 2003). The simulations are by episodes of tropical–extra-tropical interactions. These performed for two periods: a control period from 1961– include the Red-Sea trough (Dayan et al. 2001), the North 1990 to evaluate the model’s ability to realistically simu- African Sharav cyclones (Alpert and Ziv 1989), and tropi- late the climate conditions, and the period 2071–2100, cal plumes (Ziv 2001; Knippertz 2005). Tropical plumes based on the IPCC A2 scenario, which offers the most are mainly active in the winter season (Rubin et al. 2007). extreme potential changes, due to the highest expected Moreover, there are indications of a possible influence of anthropogenic forcing being applied. To better classify the El Nin˜o Southern Oscillation (ENSO) on the MB (Price changes of the A2 scenario we additionally analyzed a et al. 1998; Bro¨nnimann 2007), which is, however, still second set of time slice experiments including B2 scenario under debate. for the period 2071–2100 however with a slightly different A hint that the MB is sensitive with respect to the model setup (see Sect. 2 and supplementary material). The hydrological cycle is given by recent changes in the past main focus of this study is on synoptic-scale features and decades, based on observations. Alpert et al. (2002) found extreme events, assessing the following variables: a negative trend in rainfall for the last 60 years over the – the potential for cyclone formation by baroclinicity and majority of the MB, and suggested that this is part of global static stability, climate change. This negative trend in the MB and south- – the frequency and intensity of cyclones in the MB, ern Europe is clearly seen in the precipitation difference – the eddy kinetic energy transformation from the mean between the mean of the period 1978–2007 and 1949–1978 flow, (not shown). – the stationary wave activity, A major factor for generating precipitation in the MB is – polar outbreaks, and the cyclonic activity. Raible (2007) suggested a connection – precipitation. between the occurrence of extreme cyclones (either gene- rated in the MB or penetrating from the North Atlantic) and The outline of the study is as follows: The database precipitation events in Southern Europe (including the (model simulations and reanalysis data) and methods are MB). This relation was stable in a period of cold climate introduced in Sect. 2. Section 3 evaluates the control conditions in the past (Raible et al. 2007). For the Eastern simulation using a reanalysis data set. In Sect. 4, the A2 Mediterranean (EM) polar intrusions play an important role simulation is compared with the control simulation to (Saaroni et al. 1996). They are accompanied by a deep assess possible future changes. Finally, the results are upper-level trough and are associated with low-level discussed and major conclusions are highlighted in Sect. 5. cyclogenesis (Alpert and Reisin 1986). Observations show that cyclones, which are formed via this process, cause intense precipitation events in the EM (Alpert and Reisin 2 Data sets and methodology 1986). Recently, several attempts were made to assess future The study is based on model and reanalyses data. The main changes of the rainfall regime over the MB. The IPCC tool is the fifth version of the European Centre model of (2007), based on an ensemble of the climate models, and Hamburg (ECHAM5; Roeckner et al. 2003). This atmo- Lionello and Giorgi (2007), using the regional climate spheric general circulation model (GCM) is based on the model (RegCM), project an increase in winter rainfall over primitive equations and uses temperature, logarithm of Europe and the northern coast of the MB and decreased pressure, specific humidity, cloud-water mixing ratio precipitation over the southern and eastern MB for the (liquid and solid), vorticity, and divergence as prognostic period 2071-2100. Lionello and Giorgi (2007) explain variables. The horizontal resolution of this spectral model these trends by an increase in cyclone activity over Wes- is T106, corresponding to a longitude-latitude grid of tern Europe accompanied by a reduction of cyclonic approximately 1.125° 9 1.125° and 31 hybrid sigma- activity within the MB. Krichak et al. (2007) using the pressure levels in the vertical with the uppermost level at RegCM projected a reduction of the winter precipitation 10 hPa. over the eastern MB. Two time-slice simulations are performed with The purpose of this study is to assess the importance of ECHAM5. The control simulation (‘Ctrl simulation’, possible future changes in synoptic-scale features for the hereafter) for the period 1961–1990 is forced by observed 123 C. C. Raible et al.: Winter cyclonic activity over Mediterranean Basin 475 sea surface temperatures (SSTs) from the HadISST data set the characteristic time scale of synoptic cyclones; however, (Rayner et al. 2003) and the observed evolution of CO2, a considerable amount of this synoptic-scale variability is other well-mixed greenhouse gases, and ozone. The future related to high pressure systems and large-scale waves. The scenario simulation (A2) for the period 2071–2100 is second is a state-of-the-art cyclone detection and tracking forced by the IPCC A2 scenario (IPCC 2001) and uses technique (Blender et al.