Analysis of an ensemble of present day and future regional climate simulations for Greece

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 29: (2009) Published online 8 December 2008 in Wiley InterScience ( Analysis of an ensemble of present day and future regional climate simulations for Greece P. Zanis, a * I. Kapsomenakis, b,c C. Philandras, b K. Douvis, b,c D. Nikolakis, c E. Kanellopoulou, c C. Zerefos c,d,e and C. Repapis b a Department of Meteorology-Climatology, School of Geology, Aristotle University of Thessaloniki, Greece b Research Centre for Atmospheric Physics and Climatology, Academy of Athens, Greece c Laboratory of Climatology and Atmospheric Environment, School of Geology and Geoenvironment, National and Kapodistrian University of Athens, Greece d National Observatory of Athens, Athens, Greece e Foundation for Biomedical Research, Academy of Athens, Greece ABSTRACT: This study investigates the simulated changes in temperature and precipitation over Greece from nine Regional Climate Models (RCMs) for the period under the A2 emission scenario and evaluates their performance during the control period using Greek gridded datasets of temperature and precipitation. In winter, most RCMs show a bias towards warmer and dryer conditions and a bias towards higher inter-annual temperature variability and lower inter-annual variability of precipitation than the Greek gridded datasets during the control period In summer, all RCMs show a cold bias for the maritime sub-regions and generally all sub-regions show very small biases in precipitation. Concerning the future projections of the RCMs for Greece the mean change of the nine RCMs for the mean air temperature, T 2mean, between the future period and the control period for the integrated Greek domain is 3.4 C for winter and 4.5 C for summer with the changes being larger in continental than in the marine sub-regions. The inter-annual temperature variability in the future simulations generally increases in summer and decreases in winter almost for all RCMs with these changes being larger in the continental sub-regions than in maritime sub-regions of Greece. Almost all RCMs simulate a decrease of the precipitation for the future climate for both winter and summer with the inter-annual variability of summer precipitation decreasing for the majority of RCMs. The reported future climatic changes will have important impacts for the region of Greece in vital sectors such as water resources, agriculture, tourism, forest fire risk and energy demand. Copyright 2008 Royal Meteorological Society KEY WORDS Greece; regional climate; regional climate models (RCMs); future simulations Received 4 July 2007; Revised 9 October 2008; Accepted 11 October Introduction In recent decades there is growing interest in the past, present and projected in future climate variability and climate change related to the study of the global warming as a consequence of the anthropogenic enhancement of the greenhouse effect (IPCC, 2007). Decision makers in governments, non-governmental organizations and industry as well as the general public need detailed information on future regional climate in order to evaluate the risks of climate change. Coupled Atmospheric-Ocean General Circulation Models (AOGCMs) and General Circulation Models (GCMs) are the modelling tools traditionally used for generating projections of climatic changes as a result of anthropogenic forcing. However, because of limited computational resources, the horizontal resolution of present day coupled AOGCMs are still of the order of * Correspondence to: P. Zanis, Department of Meteorology- Climatology, School of Geology, Aristotle University of Thessaloniki, University Campus, Thessaloniki 54124, Greece. zanis@auth.gr a few hundred kilometres (Mearns et al., 2001). At this resolution, the effects of local and regional topographic characteristics are not fully captured. Regional climate models (RCMs) have been developed for the application of dynamical downscaling methods to enhance the regional information provided by (GCMs) or by the largescale reanalysis fields (NCEP/ERA-40) (Dickinson et al., 1989; Giorgi et al., 1990). RCMs represent in more detail surface features, such as complex mountain topographies and coastlines as well as small islands and peninsulas, which in a global model could not even appear because of their smaller size relative to the GCM gridbox. When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation (Jacob et al., 2007). Greece is situated at the southern part of the Balkan Peninsula in a region characterized by complex topography and steep orography of a land stripe embedded into the Mediterranean Sea and hence regionalization Copyright 2008 Royal Meteorological Society

2 ENSEMBLE OF PRESENT AND FUTURE REGIONAL CLIMATE SIMULATIONS FOR GREECE 1615 techniques to enhance the regional climatic information are necessary. The assessment of potential effects and adaptations for climate change in the area of Greece has so far been drawn from available published material, typically based on GCM scenario simulations in which Greece is represented by only two grid-boxes (Parry, 2000; Mitchell et al., 2002). Furthermore Mediterranean is recognized among the most responsive regions to climate change (Giorgi, 2006). There are a number of studies which have investigated changes in temperature and precipitation since the middle of the 20th century for eastern Mediterranean and Greece. Repapis and Philandras (1988) showed that eastern Mediterranean air temperature time series follow the Northern Hemisphere secular trend from late 19th Century to 1970s. Nevertheless the warming trend of the last 30 years that is well documented for the Northern Hemisphere and the global average appears only since early 1990s in the eastern Mediterranean region and Greece in particular (Saaroni et al., 2003; Feidas et al., 2004; Repapis et al., 2007). Feidas et al. (2004) showed an overall cooling trend in winter for the period for Greece. Xoplaki et al. (2003) reported that variations of a mode related to a more meridional circulation at the upper tropospheric levels are found to be responsible for the occurrence of extreme events and decadal trends in summer air temperature over Greece, the latter being characterized by a cooling in the early 1960s and a warming in the early 1990s. Recent studies of 20th century Mediterranean precipitation trends show large precipitation decreases in the region starting in the 1960s (e.g. Giorgi, 2002; Dünkeloh and Jacobeit, 2003; Maheras et al., 2004; Xoplaki et al., 2004; Krichak and Alpert, 2005; Feidas et al., 2007). As far as the future climate projections in Mediterranean are concerned, several studies based on GCMs and RCMs predict mainly a warming and drying of the Mediterranean region for the last decades of the 21st century for various emission scenarios (Gibelin and Déqué, 2003; Pal et al., 2004; Giorgi and Bi, 2005; Giorgi and Lionello, 2008). Gao et al. (2006), based on high-resolution climate change simulations with RegCM3, showed that in winter the mean precipitation change is positive in the northern Mediterranean and negative in the southern Mediterranean, while in the other seasons precipitation mostly decreases. Hertig and Jacobeit (2007), using statistical downscaling methods to assess Mediterranean precipitation changes for the period , indicated that the eastern and southern parts of the Mediterranean area will exhibit mainly negative precipitation changes from October to May. Other recent studies have focused on future changes in the extremes of temperature and precipitation. Diffenbaugh et al. (2007), on the basis of a high-resolution simulation with RegCM3, reported that elevated greenhouse gas concentrations dramatically increased the heat stress risk in the Mediterranean region, with the occurrence of hot extremes increasing by % throughout the region. Goubanova and Li (2007) assessed the potential future changes in climate extremes around the Mediterranean in an ensemble of future climate scenario simulations and they concluded that the Mediterranean basin will experience in general a warmer and dryer climate. As far as the near future projections are concerned, multi-model GCM experiments show a global warming trend of 0.2 C per decade for the period with a mean annual near surface air temperature increase of C forthe Mediterranean region which is insensitive to the choice among Special Report on Emission Scenarios (SRES) (IPCC, 2007). Even if all greenhouse gases and aerosols were held constant at year 2000 levels, a global warming trend of about 0.1 C per decade would occur as a result of the thermal inertia of the oceans and ice sheets and their long time scales for adjustment (IPCC, 2007). For Greece in particular there are only a few studies so far on future climate change, and these are mainly based on data from GCMs. Tolika et al. (2006) demonstrated that the spatial distribution of winter precipitation in Greece is not well reproduced by the Hadley Center atmospheric GCM (HadAM3P) with the model underestimating also the rainfall amounts. In another study Anagnostopoulou et al. (2006) found that HadAM3P model underestimates the frequency of severe cyclones in the Mediterranean region. The European project Prediction of Regional Scenarios and Uncertainties for Defining European Climate change risks and Effects (PRUDENCE) provided a series of highresolution climate change scenarios for Europe at the end of the 21st century using dynamical downscaling methods with 10 different RCMs. Regional multi-model ensembles are valuable because not only they increase the spatial resolution of the global simulations, but also they moderate the uncertainties arising from different parameterisations and dynamical cores in different RCMs. The aim of this work is to contribute to the few studies so far on the projected climate changes in the region of Greece, by including detailed regional information from these state-of-the-art RCMs for different climatic subregions of Greece. 2. Data and methods The model performance of the nine RCMs was investigated during the control period using gridded temperature and precipitation data interpolated from a network of about 74 stations covering Greece and neighbouring countries. All the data from the Greek stations have been officially provided by the Hellenic Meteorological Service where they follow standard quality control procedures except for four stations with precipitation data provided by another governmental organisation. Furthermore the homogeneity of the temperature and precipitation data in a large number of our stations have also been studied in previous publications and they are considered homogeneous (Peterson et al., 1998; Xoplaki et al., 2000, 2003, 2004; Feidas et al., 2004, 2007). Among the stations with temperature data, 40 stations had no gaps, 16

3 1616 P. ZANIS ET AL. stations had gaps of less than 5% and only 4 stations had gaps between 5 and 15%. As far as the precipitation data availability is concerned, 46 stations had no gaps, 9 stations had gaps of less than 5% and only 2 stations had gaps between 5 and 10%. These gaps in monthly values were filled using recorded monthly values in neighbouring stations with high correlation following procedures similar to those suggested by Feidas et al. (2004) and Feidas et al. (2007) for temperature and precipitation, respectively. These Geek gridded temperature and precipitation data were compiled as yearly and seasonal means for the period on the RCM s model grid. The methodology applied for the interpolation from irregularly distributed surface station data at coordinates xi, yi, zi (where xi = lon, yi = lat and zi = alt from mean sea level of the i station) to surface gridded points Xj,Yj,Zj (where Xj = lon, Yj = lat and Zj = alt from mean sea level of the j grid) was based on 2-D kriging procedure in association with a back fitting loop to estimate the elevation component. The kriging calculations were done with the statistical package R-project ( In the first part of this study, the mean and the inter-annual variability of the Greek gridded temperature and precipitation data (shown in Table I) were compared with temperature and precipitation for each one of the nine RCMs for winter, summer and the whole year for individual sub-regions of Greece and the integrated Greek domain. For clarity reasons the Greek gridded temperature and precipitation data were also compared with the Climatic Research Unit (CRU) TS 2.0 dataset supplied on a 0.5 grid (Mitchell and Jones, 2005) in a similar way. The Greek domain and the selected sub-regions are shown in Figure 1. The sub-regions selected mainly with geographical criteria but climatic information was also considered taking into account the regional division followed by the Hellenic Meteorological Service and previous studies (Balafoutis and Arseni-Papadimitriou, 1992; Anagnostopoulou, 2003). In the second part of this study we have investigated the simulated changes for each one of the nine RCMs between the future period and the control period in the mean and the inter-annual variance of (1) mean air temperature at 2 m above the ground (T 2mean), (2) maximum air temperature at 2 m (T 2max), (3) minimum air Temperature at 2 m (T 2min) and precipitation (Prec) for each individual season and the whole year for the individual sub-regions of Greece as well as for the intergraded Greek domain. The RCMs used within this paper are introduced in alphabetical order together with their main references and the Institute of their origin in Table II. All nine RCMs cover Europe with a km resolution, they have been forced with six hourly lateral boundary conditions provided from the same global model HadAM3H of the Hadley Center (Buonomo et al., 2007) and run for both a control period and a future scenario period following the A2 emission scenario from IPCC (the only scenario used consistently from the full list of PRUDENCE RCMs). Hence the differences among these RCM simulations originate from differences in the dynamical core, the physical parameterizations of sub-grid processes and solving techniques among the different RCMs. The A2 scenario is among the worse IPCC future emission scenarios characterized by an independently operating world with continuously increasing population, regionally oriented economic development and slower and more fragmented technological changes and improvements to per capita income. The emission scenarios are described in the IPCC (SRES) (Nakićenović et al., 2000). It should be noted that the use of one GCM (e.g. HadAM3H) and one emission scenario (e.g. A2) for driving the different RCMs is a dominant factor for the RCM results. In a study assessing the uncertainties in PRUDENCE RCM projections it was found that the uncertainty introduced by the choice of the driving GCM is generally larger than those introduced by choice of the RCM, by the choice of the scenario and the choice of the member of the ensemble (Déqué et al., 2007). More details for the overall experiment set-up that was utilized within PRUDENCE are described by Christensen and Christensen (2007). 3. Results 3.1. Comparison between Greek gridded data and CRU The Greek gridded data of temperature and precipitation were compared first with the respective values from the CRU TS 2.0 dataset for clarity reason and because the evaluation of the PRUDENCE RCMs has been done in previous studies with respect to the CRU data-set. As it is shown in Table III, the yearly averages of the CRU data are in general colder than our gridded Greek database for all sub-regions in a range of C. The integrated Greek domain show that the CRU data are colder than our Greek gridded dataset by 0.7 C for the whole year, 0.6 C for winter and 0.9 C for summer. In winter season the CRU data are colder than our gridded Greek database for all maritime influenced regions (except Crete) in a range of C while for the continental regions the temperature bias is smaller and ranges between 0.5 and 0.4 C. In summer season the CRU data are colder than our gridded Greek database for the continental regions in a range of C while the maritime influenced regions (except Crete) show a much smaller temperature bias ranging between 0.3 and 0.3 C. As far as the interannual temperature variability is concerned both datasets show similar values. The yearly averages of the CRU precipitation data show lower values than the Greek gridded precipitation for most of the sub-regions, ranging from 2.6% in Dodekannese to 37.2% in West Greece, except in Cyclades and East Aegean where CRU data show more precipitation. This behaviour is also observed for the winter season. In summer season the precipitation of the

4 ENSEMBLE OF PRESENT AND FUTURE REGIONAL CLIMATE SIMULATIONS FOR GREECE 1617 Table I. Mean and the inter-annual variance of temperature and precipitation of the reference period for winter, summer and the whole year for individual sub-regions of Greece and the integrated Greek domain based on the Greek gridded temperature and precipitation. Greek gridded dataset Crete Dodekannese Peloponnese Cyclades E. Aegean N. Aegean Ionian W. Greece CE Greece W-C Macedonia E. Maced./Thrace Greece Year T 2m ( C) T 2m-stdev ( C) Prec (mm/day) Prec-stdev (mm/day) DJF T 2m ( C) T 2m-stdev ( C) Prec (mm/day) Prec-stdev (mm/day) JJA T 2m ( C) T 2m-stdev ( C) Prec (mm/day) Prec-stdev (mm/day)

5 1618 P. ZANIS ET AL. Figure 1. Map with the selected Greek sub-regions indicated with boxes while with points is indicated the station with precipitation and temperature data used to produce the gridded Greek datasets. Specifically, circles indicate stations with both temperature and precipitation data, triangles indicate stations with only precipitation data and asterisks indicate stations with only temperature data. The sub-regions include continental regions such as a) West Greece (WG), b) Central-eastern Greece (CEG), c) West-central Macedonia (WCM), d) Eastern Macedonia/Thrace (EMT) and e) Peloponnese (P) as well as maritime influenced regions such as f) Crete (C), g) Dodekannese (D), h) Cyclades (CY), i) East Aegean (EA), j) North Aegean (NA) and Ionian (I). This figure is available in colour online at Table II. Regional Climate Model acronyms, main references and institute of their origin. Acronym Institute of their origin Main references CHRM Swiss Federal Institute of Technology, Zurich, Switzerland (ETHZ) Vidale et al. (2003) CLM GKSS Research Center, Geesthacht GmbH, Germany (GKSS) Steppeler et al. (2003) HadRM3H Hadley Centre for Climate Prediction and Research, UK (HC) Buonomo et al. (2007) HIRHAM Danish Meteorological Institute, Denmark (DMI) Christensen et al. (1998) RACMO Royal Netherlands Meteorological Institute, the Netherlands (KNMI) Lenderink et al. (2003) RCAO Swedish Meteorological and Hydrological Institute, Sweden (SMHI) Döscher et al. (2002) RegCM2 The Abdus Salam International Centre for Theoretical Physics, Italy (ICTP) Giorgi and Mearns, (1999) REMO Max-Planck-Institute for Meteorology, Germany (MPI) Jacob, (2001) PROMES Universidad Complutense de Madrid, Spain (UCM) Castro et al. (1993) CRU dataset is similar to our Greek gridded dataset, although the percentage differences can be very high because of the dry summer conditions in Greece with low amounts of precipitation. For the integrated Greek domain the CRU data show less precipitation than our Greek gridded dataset by 8.3% for the whole year and 11% for winter while for summer the CRU data show more precipitation by only 2.0%. Finally, the inter-annual variability in precipitation is smaller in CRU data than in our Greek gridded dataset for almost all sub-regions for the whole year and for both winter and summer except for Cyclades and East Aegean. In conclusion, apart from a few regional differences the comparison of the two datasets canbe consideredsatisfactory. However, in some sub-regions such as West Greece, significant differences between Greek gridded dataset and CRU are observed for both the absolute and the percentage amount of precipitation, particularly during winter. It should be highlighted that for these sub-regions the total precipitation, especially in the winter season, increases with altitude as the westerly flow passing over the Adriatic Sea and Ionian Sea is forced to orographic uplift over the Pindos mountain range at West Greece (Xoplaki et al., 2000). The network of meteorological stations, on which the Greek gridded dataset is based, is denser than the network of the CRU database for the area of Greece (with only a few Greek stations), and additionally it includes high-altitude stations. Therefore, we anticipate that the Greek gridded dataset represents the total amount of precipitation at West Greece more realistically Validation of the control simulations Near surface air temperature The differences in temperature and inter-annual temperature variability between the nine RCMs and the Greek gridded dataset for the winter and summer seasons during the control period for the individual Greek sub-regions are shown in Figures 2 and 3, respectively. In the winter season the air temperature biases for most RCMs range between 1 and2 C for most of the subregions and for the integrated Greek domain except the CLM model which shows colder biases. In general the majority of RCMs are warmer than the observed values in winter (Figure 2). An outstanding regional feature in winter season is the North Aegean where all RCMs show

6 ENSEMBLE OF PRESENT AND FUTURE REGIONAL CLIMATE SIMULATIONS FOR GREECE 1619 Table III. Differences in the mean and the inter-annual variance of temperature and precipitation between the Greek gridded dataset and the CRU TS 2.0 dataset for winter, summer and the whole year of the reference period for the individual sub-regions of Greece and the integrated Greek domain. Differences Crete Dodekannese Peloponnese Cyclades E. Aegean N. Aegean Ionian W. Greece CE Greece W-C Macedonia E. Maced./Thrace Greece Year T 2m ( C) T 2m-stdev ( C) Prec (%) Prec-stdev (%) DJF T 2m ( C) T 2m-stdev ( C) Prec (%) Prec-stdev (%) JJA T 2m ( C) T 2m-stdev ( C) Prec (%) Prec-stdev (%) Because of lack of CRU data there are no values given for North Aegean and Ionian.

7 1620 P. ZANIS ET AL. Figure 2. Differences in mean temperature at 2 m in winter and summer seasons between each one of the nine RCMs and the Greek gridded dataset during the control period for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at warm biases between 2 and 3.5 C except RegCM2 which shows a warm bias of only 0.5 C (Figure 2). It should be noted that REMO model shows consistently the warmest bias, whereas CLM model shows the coldest bias among the nine RCMs. As far as the inter-annual variability of air temperature in winter is concerned most of the RCMs show higher variability than the Greek gridded dataset (Figure 3). We also detect a tendency of the continental sub-regions towards a larger positive bias in the interannual variability of air temperature compared with the maritime sub-regions. In summer, all RCMs show a cold bias for the maritime sub-regions and the intergraded Greek domain ranging from 0 to 2.5 C (Figure 2), whereas in the continental sub-regions there are models showing a small warm bias (e.g. HadRM3P, RCAO and REMO) and other models showing a cold bias (e.g. CLM and PROMES). In summer the most RCMs under-predict the inter-annual air temperature variability in the maritime sub-regions and over-predict it in continental sub-regions. The CLM model shows the largest underestimation of the interannual air temperature variability while HadRM3P shows the highest overestimation among the nine RCMs. In general the range of the temperature changes of the nine RCMs is smaller in winter than in summer and it is also smaller at the marine than at the continental sub-regions of Greece for both winter and summer seasons. The temperature trend for the nine RCMs, the Greek gridded dataset and the CRU dataset during the control period , for the winter and summer seasons, of the individual Greek sub-regions and Greece as a whole are shown in Figure 4. During winter the majority of PRUDENCE RCMs reproduce well the negative temperature trends observed in the Greek gridded dataset and in CRU dataset. The observed summer temperature trends in the Greek gridded and CRU datasets are relatively smaller than in winter. The summer temperature trend is also reproduced reasonably by the PRUDENCE RCMs. The only exception is PROMES, which continuously reproduces a warming temperature trend for all Greek sub-regions for both the winter and summer seasons. However, the majority of the trend calculations for summer and winter over the period from both

8 ENSEMBLE OF PRESENT AND FUTURE REGIONAL CLIMATE SIMULATIONS FOR GREECE 1621 Figure 3. Differences in inter-annual temperature variability in winter and summer seasons between each one of the nine RCMs and the Greek gridded dataset during the control period for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at observed and modelled temperature data are not significant in the 95% significance level. The residual mean square error (RMSE) of the differences in temperature between each one of the nine RCMs and the Greek gridded dataset during the control period for the individual Greek sub-regions and the integrated Greek domain was also calculated. In winter, the RMSE for the integrated Greek domain ranges from 1.1 to 1.9 C for the different RCMs with a mean RMSE value of 1.3 C. In summer, the RMSE for the integrated Greek domain ranges from 0.5 to 2.2 C for the different RCMs with a mean RMSE value of 1.1 C. The RMSE at the continental sub-regions is slightly higher (by 0.1 C) than at the maritime sub-regions in winter while in summer no differences are seen Precipitation As it can be noted from Figure 5, all RCMs in winter underestimate the precipitation by 25 to 55% for the integrated Greek domain with the smallest biases from REMO and RegCM2. For the individual sub-regions, the smallest biases are noted for the Aegean Sea, whereas the largest underestimation is observed for Crete except for the REMO model. In summer, all RCMs give very small differences from the observed gridded data in absolute amounts of precipitation, but the percentage precipitation biases are very large especially for the maritime influenced sub-regions and the Southern Greece sub-regions because of the fact that the precipitation in South Greece in summer is very low. Practically the models RACMO, RCAO and CLM simulate almost zero amount of precipitation. All RCMs underestimate the inter-annual variability of precipitation compared with the Greek gridded dataset in winter (Figure 6). The largest underestimation is seen at the most southern Greek sub-domains at Crete and Dodecanese. For the integrated Greek domain the bias of the inter-annual variability of precipitation ranges from 5 to 42%. The smallest biases in winter for the integrated Greek domain are observed for the models REMO and RegCM2. In summer, the nine RCMs show discrepancies concerningthe inter-annualvariability of precipitation but this can also be attributed to the fact of the low amounts of precipitation at this time of the year.

9 1622 P. ZANIS ET AL. Figure 4. Temperature trend for each one of the nine RCMs, the Greek gridded dataset and CRU dataset in winter and summer seasons during the control period for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at As far as the precipitation trend for the control period is concerned, both the Greek gridded dataset and the CRU dataset show a negative precipitation trend for all Greek sub-regions during winter (Figure 7). The nine RCMs also reproduce the negative trend for all subregions but with smaller (absolute) values. In summer, the observed precipitation trends in the Greek gridded and CRU datasets are close to zero and the same feature is also reproduced by the majority of RCMs for most sub-regions. The RMSE of the differences in precipitation between each one of the nine RCMs and the Greek gridded dataset during the control period for the individual Greek sub-regions and the integrated Greek domain was also calculated. In winter, the RMSE for the integrated Greek domain ranges from 1.5 to 2.1 mm/day for the different RCMs with a mean RMSE value of 1.7 mm/day. In summer, the RMSE for the integrated Greek domain ranges only from 0.2 to 0.3 mm/day for the different RCMs. The RMSE at the continental sub-regions is greater than at the maritime sub-regions in both winter and summer by 0.2 and 0.3 mm/day, respectively Future projections Future projections of near surface air temperature As it can be inferred from Table IV the mean change of the nine RCMs for the mean temperature at 2 m ( T 2mean) between the future period for scenario SRES A2 and the control period for the integrated Greek domain for the whole year is 3.7 C, for winter 3.4 C and for summer 4.5 C. As far as the individual Greek sub-regions are concerned, there are regional differences within Greece for the simulated increase of air temperature at 2 m for A2 scenario with respect to the control run. The main difference is between the continental and the marine parts of Greece with the marine sub-regions showing temperature changes lower by C compared with the continental sub-regions. This feature is seen in all seasons but is more distinct for summer. The differences in mean air temperature at 2 m and in the inter-annual temperature variability in winter and summer seasons between future period for scenario SRES A2 and the control period for

10 ENSEMBLE OF PRESENT AND FUTURE REGIONAL CLIMATE SIMULATIONS FOR GREECE 1623 Figure 5. Percentage differences in total precipitation in winter and summer seasons between each one of the nine RCMs and the Greek gridded dataset during the control period for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at each one of the nine RCMs for the individual Greek subregions and the integrated Greek domain are shown in Figures 8 and 9. Figure 8 indicates that the range of the nine RCMs is smaller in winter than in summer, and it is also smaller at the marine than at the continental subregions of Greece for both winter and summer seasons. In winter the T 2mean becomes largest in North Greece (West-central Macedonia and eastern Macedonia/Thrace) ranging from 3.3 to 4.5 C from the nine RCMs. The lowest values are seen at Ionian See and Aegean See (Cyclades and Crete) ranging from 2.8 to 3.5 C. Integration for the whole Greek domain shows a T 2mean increase ranging from 3 to 3.6 C from the nine RCMs. In summer the T 2mean is clearly larger in continental subregions compared with marine sub-regions ranging from 4to6.5 C for the continental sub-regions and from 3.5 to 5 C for the maritime sub-regions. Integration for the whole Greek domain shows a T 2mean increase ranging from 4 to 5 from the nine RCMs. In order to examine if the future projected temperatures are significantly different from the present time values, the two-sample Welch t-test was applied. The results of the Welch s t-test indicated that the changes in temperature are statistically significant at the 95% significance level for winter, summer and the whole year, for all RCMs, and for all Greek sub-regions. Temperature trends during the future period have also been calculated for each one of the nine RCMs for all Greek sub-regions and the integrated Greek domain. In all cases there is a warming trend for both winter and summer seasons. In winter the RCMs predict a positive temperature trend ranging from 0.01 to 0.08 C/year for the different sub-regions with a mean positive trend of 0.04 C/year for the integrated Greek domain. In summer the RCMs predict a stronger positive temperature trend ranging from 0.03 to 0.14 C/year for the different sub-regions with a mean positive trend of 0.07 C/year for the integrated Greek domain. In general during this season the RCMs predict higher trends for the continental than for the marine subregions. The inter-annual temperature variability generally increases in summer in the future simulations almost for all RCMs (Figure 9). The increase in inter-annual

11 1624 P. ZANIS ET AL. Figure 6. Percentage differences in inter-annual precipitation variability for winter and summer seasons between each one of the nine RCMs and the Greek gridded dataset during the control period for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at temperature variability in summer between the A2 scenario and the control run is larger in the continental sub-regions than in maritime sub-regions of Greece (Figure 9). In contrast, in winter, the standard deviation of T 2mean between the A2 scenario and the control run shows a decrease for almost all RCMs except HadRM3P (Figure 9). This decrease is largest and is clearly seen in continental sub-regions of Greece, but it is also evident for the integrated Greek domain. Concerning the mean changes in T 2max and T 2min of the nine individual RCMs between the future period for scenario SRES A2 and the control period , we get qualitatively similar results to T 2mean but in general the T 2max is slightly higher than T 2mean and T 2min is slightly lower than T 2mean especially over the continental parts of Greece for summer season when we notice the biggest differences among T 2mean, T 2max and T 2min. For example Table IV indicates that T 2mean, T 2max and T 2min in summer are 4.5, 4.7 and 4.5 C, respectively, for the integrated Greek domain while for the continental sub-regions such as West-central Macedonia and eastern Macedonia/Thrace the respective values are 5.2, 5.5 and 5.0 C. In maritime sub-regions we notice negligible differences among T 2mean, T 2max and T 2m for both winter and summer seasons Future projections of precipitation Almost all nine RCMs simulate a decrease of the precipitation for the future climate for both summer and winter and for all sub-regions of Greece. For the integrated Greek domain in winter we calculated an overall precipitation decrease of 15.8% (about 0.20 mm/day) for the whole year, 14.2% (about 0.30 mm/day) for winter and 57.3% for summer (about 0.18 mm/day) from the nine RCMs (Table IV). More specifically, in winter the precipitation change between A2 scenario and control run shows a percentage decrease for all Greek sub-regions up to 40% for the majority of the nine RCMs. Especially at the southern sub-regions of Greece (Crete, Peloponesse Dodekannese and Cyclades) we note the largest percentage decreases of precipitation ranging from 10 to 35% (Figure 10).

12 ENSEMBLE OF PRESENT AND FUTURE REGIONAL CLIMATE SIMULATIONS FOR GREECE 1625 Figure 7. Precipitation trend for each one of the nine RCMs, the Greek gridded dataset and CRU dataset in winter and summer seasons during the control period for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at Interestingly, at the eastern Macedonia and Thrace subregion we note that some RCMs (HIRHAM, CHRM, RACMO and PROMES) demonstrate an increase of winter precipitation up to 4%. The inter-annual variability of winter precipitation generally increases in the future climate for the majority of RCMs at maritime sub-regions while in the continental sub-regions there is no robust tension from the various RCMs as a few of them show an increase and others a decrease. In summer the precipitation change between A2 and control run shows a percentage decrease for all nine RCMs. This percentage decrease is similar for the individual Greek sub-regions ranging from 20 to 80% (Figure 10). However, in absolute values there is larger decrease of summer precipitation at the northern parts of Greece since at the southern parts there is small amount of total precipitation both in present and future simulations. The inter-annual variability of summer total precipitation generally decreases in the future climate for all Greek sub-regions almost for all RCMs (Figure 11). The Welch s t-test was used to examine if the future projected precipitation ( ) is significantly different from the present time values ( ). It was found that for the majority of the nine RCMs the changes in precipitation for winter season are not statistically significant at the 95% significance level for almost all Greek sub-regions and the integrated Greek domain which is related to large variability in precipiatation. The main exceptions are Crete for the majority of RCMs and other southern Greek sub-regions (Peloponnese, Dodekannese, Cyclades) for HadRM3P and RegCM2 showing statistical significant changes in future winter precipitation. On the contrary, the changes in future summer precipitation are statistically significant at the 95% significance level for the majority of the nine RCMs for all Greek sub-regions. Precipitation trends during the future period have also been calculated for each one of the nine RCMs and for all Greek sub-regions. All RCMs predict a positive precipitation trend for the winter season, where the intra-model variability is high. For the summer season all RCMs except PROMES predict a negative precipitation trend for all the sub-regions of Greece except for the south maritime sub-regions where the future projected summer precipitation is close to zero.

13 1626 P. ZANIS ET AL. Table IV. Mean changes for mean temperature ( T 2mean), minimum temperature ( T 2min), maximum temperature ( T 2max) and total precipitation ( Prec) from nine RCMs between the future period and the control period for the integrated Greek domain and the Greek sub-regions for the whole year as well as for winter and summer seasons. Crete Dodekannese Peloponnese Cyclades E. Aegean N. Aegean Ionian W. Greece CE Greece W-C Macedonia E. Maced./Thrace Greece Year T 2mean ( C) T 2min ( C) T 2max ( C) prec (%) DJF T 2mean ( C) T 2min ( C) T 2max ( C) prec (%) JJA T 2mean ( C) T 2min ( C) T 2max ( C) prec (%) Discussion and conclusions The PRUDENCE dataset of high-resolution simulations from nine RCMs gave us the opportunity to evaluate their model performance for the Greek area during the control period against gridded observations of temperature and precipitation interpolated from a network of 74 stations covering Greece and neighbouring countries and to investigate the regional characteristics of their projected future climate for Greece. Regarding the mean near surface air temperature, apart from a few regional differences the comparison between Greek gridded and CRU datasets can be considered satisfactory. However, in most Greek sub-regions where the differences between the two datasets are notable, the RCM results are closer to the Greek gridded dataset. The majority of RCMs are warmer than the observational gridded data for all Greek sub-regions during winter season which is in line with a general tendency of the PRUDENCE ensemble for warm bias in the European area during winter in an intercomparison study with respect to CRU data (Jacob et al., 2007). In the summer season, all RCMs show a cold bias for the maritime sub-regions. The tendency of RCMs in the maritime subregions for a warm bias in winter and a cold bias in summer, might be partially attributed to the fact that maritime observations originate from island stations in the Aegean Sea where the air could be warmer in summer and colder in winter than the surrounding air above the sea while the models consider the Aegean islands mainly as sea. Despite the bias of the RCMs results the temperature pattern that they produce is similar to that of the observational gridded datasets. In this respect the RCMs largely improve the results of the GCM because of their higher analysis which allows them to better represent the smaller-scale effects as well as the land sea and orographic characteristics. The CRU and Greek gridded dataset show similar values of the inter-annual temperature variability with their differences being small for all seasons and sub-regions. In contrast most of the RCMs show higher inter-annual temperature variability than both gridded datasets for winter and summer seasons in the continental sub-regions. The overestimation of inter-annual temperature variability in summer by most of the RCMs in the Greek continental sub-regions has been also reported in other studies for the European domain with respect to the CRU TS 2.0 observational dataset (Jacob et al., 2007; Lenderink et al., 2007). This was linked to the behaviour of the different RCMs in the partitioning of the different terms to the surface energy budget. It appeared that the overestimation of the temperature variability has no unique cause but the effect of short-wave radiation dominates in some RCMs, whereas in others the effect of evaporation dominates (Lenderink et al., 2007). In the maritime sub-regions of Greece most RCMs overestimate the inter-annual temperature variability for winter while underestimate it for summer. We also note a tendency that the continental sub-regions show a larger positive bias in the inter-annual

14 ENSEMBLE OF PRESENT AND FUTURE REGIONAL CLIMATE SIMULATIONS FOR GREECE 1627 Figure 8. Differences in mean temperature at 2 m in winter and summer seasons between future period for scenario SRES A2 and the control period for each one of the nine RCMs for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at variability of air temperature compared with the maritime sub-regions which can be attributed to the fact that the variability is higher over the continents than over the sea. The summer and winter temperature trends observed in the Greek gridded dataset and in CRU dataset are also reproduced reasonably by the PRUDENCE RCMs. The winter cooling trend for Greece in the observed datasets and in the PRUDENCE RCMs is in line with several observational studies (Repapis and Philandras, 1988; Proedrou et al., 1997; Feidas et al., 2004). Regarding the precipitation, both the CRU dataset and the nine RCMs used in this study underestimate the winter rainfall amounts and the inter-annual variability of precipitation in comparison with the Greek gridded dataset. In summer, all RCMs and the CRU dataset show small differences from the Greek gridded dataset in precipitation and its inter-annual variability that can be attributed to the low amounts of precipitation at this time of the year. The Greek gridded dataset represents more realistically than CRU the total amount of precipitation at West Greece during winter where the presence of the Pindos mountain range enforces the prevalent westerly flow to uplift and produce the larger portion of the winter precipitation. We anticipate that the underestimation of winter precipitation in the CRU data can be explained by the lack of high-altitude station data, whereas the Greek gridded dataset includes. The causes responsible for the precipitation underestimation by the RCMs are more complex. One possible reason is that the RCMs underestimate the rainfall amounts during winter season mainly because the driving GCM HadAM3H also underestimates it. HadAM3H exhibits a stronger pressure gradient across a large part of central to northern Europe than the ERA-40 reanalysis which is caused by too high pressure over the Mediterranean region and too deep Icelandic low (Jacob et al., 2007). The consequence is too high precipitation rates over northern Europe and too low precipitation rates over southern Europe. Another possible reason is that although the PRUDENCE RCMs reproduce better, the general features of the spatial distribution of precipitation for Greece than the course resolution driving GCM, even finer spatial resolution is needed to simulate more adequately the

15 1628 P. ZANIS ET AL. Figure 9. Differences in the inter-annual temperature variability in winter and summer seasons between future period for scenario SRES A2 and the control period for each one of the nine RCMs for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at effects of the complex orography of the Greek terrain on the precipitation. The summer and winter precipitation trends for the control period observed in the Greek gridded dataset and in CRU dataset (negative trend in winter and close to zero trend in summer) are reproduced reasonably by the PRUDENCE RCMs. The negative precipitation trend is in line with a number of recent studies showing large precipitation decreases in the region starting in the 1960s associated with circulation patterns or with climatic indices such as the North Atlantic Oscillation (NAO) Index (Dünkeloh and Jacobeit, 2003; Maheras et al., 2004; Xoplaki et al., 2004; Krichak and Alpert, 2005; Feidas et al., 2007). It is well established that positive NAO phase is associated with less precipitation in the Mediterranean as storm tracks from the Atlantic shift to Northwest (Hurrell and Van Loon, 1997) even though the impact of the winter NAO on the Mediterranean precipitation regime is not homogeneously distributed (Xoplaki, 2002). After the investigation of the biases in the various RCMs used in this study against the Greek gridded and CRU datasets for the present day climate of Greece and taking into account the results from similar studies for the whole European domain (Jacob et al., 2007; Lenderink et al., 2007), we consider that the models performance is reasonable in representing the main climatic features of the overall mean, inter-annual variability and trend of temperature and precipitation. Hence their future simulation is a valid tool to investigate the future simulated changes in temperature and precipitation for different sub-regions of Greece. All nine RCMs predict a dramatic increase of the mean near surface air temperature for the future climate (SRES A2) during winter, summer and the whole year for all sub-regions of Greece. These air temperature future changes are higher during summer than during winter as well as for the continental regions than for the maritime ones. The mean temperature change of the nine RCMs ( T 2mean) between the future period and the control period for the integrated Greek domain for the whole year is 3.7 C, for winter 3.4 C and for summer 4.5 C. Almost all RCMs predict an increase of the inter-annual temperature variability in summer and decrease in winter.

16 ENSEMBLE OF PRESENT AND FUTURE REGIONAL CLIMATE SIMULATIONS FOR GREECE 1629 Figure 10. Percentage differences in total precipitation in winter and summer seasons between future period for scenario SRES A2 and the control period for each one of the nine RCMs for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at The estimated temperature increase results form the increase of the greenhouse gases (GHGs), which affect the earth s radiative balance. Furthermore, the changes in the radiative balance lead to changes in the general circulation thus redistributing synoptic patterns and associated temperature anomalies. In winter, the regional models closely follow the circulation changes simulated by the driving global model HadAM3H with a weak enhancement of southerly flows and a clear enhancement of the strength of westerly flows (van Ulden et al., 2007) which in turn can be associated with a shift to the north of the sub-tropical jet stream. The IPCC models predict a strengthening and a poleward shift of the tropospheric zonal jets in response to global warming (Lorenz and DeWeaver, 2007). This winter circulation change is also revealed by the future mean sea level pressure (SLP) fields (Figure 12(a)) and the differences between the future and the present mean SLP of RCM HIRHAM (Figure 12(c)). SLP decreases largely over northern Europe with maximum decrease over Northwest Europe. The HIRHAM SLP anomalies resemble the SLP anomalies of the driving GCM (HadAM3H). Furthermore we also note a decrease in the SLP inter-annual variability over Europe indicating, in association with Figure 12(a) and (c), a more zonal westerly flow with less dynamical instabilities in the future climate. As a consequence of the more stable circulation (more zonal) and the weak enhancement of southerly flows (also seen in the Balkan Peninsula) the frequency of cold air penetrations from the north in Greece will decrease leading to enhanced warming and smaller inter-annual temperature variability. The increase of the inter-annual temperature variability in summer for almost all RCMs and sub-regions of Greece is a common finding for the whole European domain. Climate models consistently predict an increase in the variability of summer temperatures in European mid-latitudes, but the underlying mechanisms responsible for this increase remain uncertain. Inter-annual temperature variability can be affected by large-scale circulation changes. In summer the most prominent circulation response in the RCMs is a weakening of westerlies or an enhancement of the frequency of the easterly flows even though this feature is less pronounced in some RCMs (van Ulden et al., 2007). This weakening of westerly flow

17 1630 P. ZANIS ET AL. Figure 11. Percentage differences in the inter-annual precipitation variability in winter and summer seasons between future period for scenario SRES A2 and the control period for each one of the nine RCMs for the individual Greek sub-regions and the integrated Greek domain. This figure is available in colour online at is also revealed by the differences between the future and the present mean SLP as SLP increases over the British Isles (Figure 12(d)). This SLP increase is associated with a ridging over the British Isles and Western Europe (Figure 12(b)). Many climate change simulations show such a ridging over Western Europe, which suggests an increase in the incidence of warm summers and more frequent heat waves in the future climate (Meehl and Tebaldi, 2004; Schär et al., 2004).van Ulden et al. (2007) suggested that depending on the sensitivity of a RCM to soil-moisture depletion, a positive feedback between future circulation changes (e.g. enhancement of the frequency of the easterly flows) and soil-moisture depletion may result in higher frequency of warm extreme events and very warm months. This may lead to an increase in the future inter-annual temperature variability. Vidale et al. (2007) also provided evidence that the increase of inter-annual temperature variability in future climate may be linked to the dynamics of soil-moisture storage and the associated feedbacks on the surface energy balance and precipitation. In a another recent study it was suggested that especially for Central and Eastern Europe the predicted increase in summer temperature variability is mainly due to feedback between the land surface and the atmosphere (Seneviratne et al., 2006). Almost all nine RCMs simulate a decrease of the precipitation for the future climate (SRES A2) during all seasons and for all sub-regions of Greece. For the integrated Greek domain in winter we calculated an overall precipitation decrease of 15.8% (about 0.20 mm/day) for the whole year, 14.2% (about 0.30 mm/day) for winter and 57.3% for summer (about 0.18 mm/day) from the nine RCMs. An overall decrease of mean precipitation was also noted in most of the Mediterranean regions for future climate (Pal et al., 2004; Tapiador et al., 2007). The reliability of the future projections of precipitation for Mediterranean can be assessed by the fact that the majority of the AOGCMs participated in IPCC AR4 show consistently strong precipitation decreases in the Mediterranean region for both winter and summer for the 21st century (Tselioudis et al., 2006). The decrease in precipitation in winter is linked with a clear enhancement of the strength of westerly flows

18 ENSEMBLE OF PRESENT AND FUTURE REGIONAL CLIMATE SIMULATIONS FOR GREECE 1631 Figure 12. Upper panel: Mean sea level pressure (SLP) fields (hpa) for the future period under scenario SRES A2 in (a) winter and (b) summer. Lower panel: Difference (hpa) of sea level pressure (SLP) between future period and the control period in (c) winter and (d) summer. The data are from the regional climate model HIRHAM. in the future simulation (van Ulden et al., 2007), which is linked with the intensification of the Icelandic low (Figure 12(c)). This is a common feature of the PRU- DENCE RCMs resembling the driving GCM HadAM3H, which causes an increase in the pressure gradient between the Azores high and the Icelandic low, thus resulting in an intensified zonal circulation over Europe that brings more storms into Northwestern Europe following the North Atlantic storm tracks (Figure 12(a)). The spatial characteristics of the winter SLP changes (Figure 12(c)) in future climate indicate a more intense NAO-positive phase and show similarities with the first canonical patterns (geopotential height at 1000 hpa or SLP vs precipitation) leading to present time winter precipitation anomalies obtained by Dünkeloh and Jacobeit (2003) as well as by Xoplaki et al. (2004) even though the future SLP anomalies are rather extending to the east. A study on the ability of the global circulation model HadAM3P to generate the frequency and intensity of severe cyclones in the Mediterranean region showed that the frequencies of the cyclones will decrease during the future period as a result of this northward shift of the location of the storm tracks (Anagnostopoulou et al., 2006). The various RCMs show a non-robust behaviour for the future changes in the inter-annual variability of winter precipitation for continental Greece. Goubanova and Li (2007) reported that the future winter extreme precipitation will increase around the Mediterranean over the areas where mean precipitation decreases, except in Greece with decreasing extremes. The future decrease in precipitation and its interannual variability in summer might be associated with the ridging over British Isles and Western Europe [SLP increases over the British Isles and Western Europe in Figure 12(d)] and the enhancement of the frequency of the easterly flows (or weakening of westerly flow). As it was also mentioned earlier in the text, this ridging situation (Figure 12(b)) and a positive feedback between future circulation changes and soil-moisture depletion may result in warmer and dryer conditions for Greece with more frequent heat waves and less frequent precipitation events. Giorgi et al. (2004) associate the decrease in mean precipitation over southern Europe with a more frequent anticyclonic circulation in this region in future climate. Concerning the decrease of the inter-annual precipitation variability, a recent study shows that Greece is an area where in the future the probability for the occurrence of extreme precipitation events will decrease (Goubanova and Li, 2007). Presumably we have also to consider that in future climate soil-moisture drying would also cause a less significant soil-moisture-precipitation feedback associated with a weaker land atmosphere coupling (Seneviratne et al., 2006). The summer and winter temperature and precipitation future projections of the ensemble of nine RCMs for the integrated Greek domain lie within the range of respective calculations from various GCMs