On the Relationships Between Circulation Types and Changes in Rainfall Variability in Greece
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INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 24: 1695–1712 (2004) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1088 ON THE RELATIONSHIPS BETWEEN CIRCULATION TYPES AND CHANGES IN RAINFALL VARIABILITY IN GREECE PANAGIOTIS MAHERAS,a,* KONSTANTIA TOLIKA,a CHRISTINA ANAGNOSTOPOULOU,a MARGARITIS VAFIADISb IOANNIS PATRIKASc and HELENA FLOCASd a Department of Meteorology and Climatology, University of Thessaloniki, 54124 Thessaloniki, Greece b Division of Hydraulics, Faculty of Technology, University of Thessaloniki, Thessaloniki Greece c Division of Hydraulics, Faculty of Technology, University of Thessaloniki, Thessaloniki, Greece d Laboratory of Meteorology, Department of Applied Physics, University of Athens, Athens, Greece Received 17 December 2003 Revised 28 June 2004 Accepted 30 June 2004 ABSTRACT An attempt is made to examine rainfall variability over the Greek area in relation to 500 hPa atmospheric circulation. Daily precipitation series from 22 evenly distributed Greek stations have been used for the period 1958–2000, along with the classification scheme of daily circulation types at 500 hPa for the same period. The seasonal frequency and the trends of circulation types have been calculated. It was found that there is a general positive trend of anticyclonic circulation types and a negative one for cyclonic types. The seasonal trends of rainy days and the precipitation totals have also been calculated and analysed. A general decreasing tendency of winter rainfall is observed; the decreasing trend during autumn and spring is less significant. Concerning the frequency and intensity of rainfall per circulation type, a decreasing tendency becomes evident for the majority of the stations during winter, whereas there is an increasing tendency during autumn. A multiple regression–cross-validation model was developed using the seasonal frequency of circulation types as predictors and the seasonal rainfall totals as predictants. Only the winter modelled precipitation shows a good agreement with the observed precipitation, whereas for the other seasons the agreement is relatively poor. This could be caused by the influence of different factors that are not captured by the classification scheme used. The proposed model could serve as a circulation-based downscaling method that could be further applied to geopotential data available from general circulation models in order to study regional climatological consequences of future climate scenarios. Copyright 2004 Royal Meteorological Society. KEY WORDS: Greece; circulation types; trend analysis; multiple regression–cross-validation analysis; daily rainfall; seasonal rainfall, downscaling method 1. INTRODUCTION The precipitation regime in Greece presents highly irregular behaviour, both on spatial and temporal scales, namely in rainfall amount and rainfall distribution. In fact, the precipitation pattern over Greece shows a strong gradient between the western part (where precipitation is two or three times higher) and the other regions (Xoplaki et al., 2000; Maheras and Anagnostopoulou, 2003). Furthermore, the highest precipitation totals of western Greece were found to be related to the atmospheric circulation in association with Mediterranean sea- surface temperature distribution and the complex topography of the region, as imposed by the orography of the Pindus Mountains in northwestern and central Greece and the mountains of Olympus and Crete (Metaxas, 1978; Xoplaki et al., 2000; Maheras and Anagnostopoulou, 2003). It has been demonstrated that local or regional changes of meteorological parameters in mid-latitudes, including rainfall, are mainly controlled by the atmospheric circulation (Parker et al., 1994; Steinberger and * Correspondence to: Panagiotis Maheras, Department of Meteorology and Climatology, School of Geology, Aristotle University of Thessaloniki, 54006 Greece; e-mail: [email protected] Copyright 2004 Royal Meteorological Society 1696 P. MAHERAS ET AL. Figure 1. Geographical distribution of the stations over the Greek area Gazit-Yaari, 1996; Turke¸¨ s, 1998), although observed changes in rainfall cannot always be explained by changes in circulation (Frei et al., 1998; Goodess and Jones, 2002). Krichak et al. (2000) and Kutiel et al. (2001) studied synoptic patterns associated with dry and wet conditions in the eastern Mediterranean and Turkey respectively. They found that dry conditions in the eastern Mediterranean are associated with a large- scale positive 500 hPa anomaly over eastern Europe, whereas a negative anomaly normally prevails over southwestern and western Europe. Brunetti et al. (2002), who have investigated the atmospheric circulation and precipitation in Italy for the last 50 years, demonstrated that two indices (western European circulation index (WERCI) and Mediterranean circulation index (MCI)) are mainly correlated with precipitation totals. Furthermore, both indices show evidence of a signal connected to a strong increase in winter air pressure in the Mediterranean area starting around 1980 (Brunetti et al., 2002). Goodess and Jones (2002) found a general decreasing tendency of the mean seasonal rainfall over the Iberian Peninsula for the period 1958–97. Comparison of the trends in rainfall and circulation-type frequency suggests possible links. Turke¸¨ s et al. (2002) showed a statistically significant negative relationship between precipitation anomalies and 500 hPa geopotential height anomalies in winter and autumn for almost all of Turkey. These types of study are highly relevant to the construction of empirical downscaling procedures that permit large-scale information to be related to regional or local parameters. According to Goodess and Jones (2002), ‘inherent to all empirical downscaling approaches are the two assumptions that firstly, these relationships will be in time invariant (i.e. unchanged in a future warmer climate regime) and secondly, that rainfall changes are driven largely by changes in circulation’. Maheras et al. (1999), who investigated wet and dry monthly anomalies across the Mediterranean basin and their relationship with circulation over the last 130 years, found that the diminution of winter precipitation in Greece could be attributed to the increased frequency of northwesterly or northeasterly dry and cold continental airflows over Greece. Xoplaki et al. (2000) studied the connection between the large-scale 500 hPa geopotential height fields and precipitation over Greece during wintertime with the aid of a canonical correlation analysis. They found that there were more wintertime blocking situations over the last 30 years and, thus, a reduced frequency of depression activity over Mediterranean areas was the main reason for dry winters over Greece. Additionally, Maheras and co-workers (Maheras, 2002; Maheras et al., 2002a; Maheras and Copyright 2004 Royal Meteorological Society Int. J. Climatol. 24: 1695–1712 (2004) CIRCULATION TYPES AND RAINFALL VARIABILITY CHANGE 1697 Figure 2. The location of the centre of (a) anticyclonic and (b) cyclonic types Anagnostopoulou, 2003), following a circulation-type approach based on statistical downscaling, developed a multiple regression model in order to simulate the annual cycle of rainy days and the corresponding rainfall amount, as well as selected extreme precipitation events for Greek stations. However, the relationship between circulation types and rainfall, including rainfall patterns and trends, focusing not only on mean changes but on more detailed characteristics (such as wet-day probability and intensity) has not been extensively studied yet. Such a study would help greatly in constructing a robust downscaling procedure for the development of future precipitation scenarios in Greece. Therefore, the objectives of this study are: • To explore the spatial and temporal variations in the circulation–rainfall relationship over Greece, in order to consider whether the trend of rainfall in the Greek area that has been identified in previous studies (Anagnostopoulou, 2003; Maheras and Anagnostopoulou, 2003) can be explained, in whole or in part, by circulation changes. • To identify the best potential circulation based on predictor variables for Greek rainfall. Copyright 2004 Royal Meteorological Society Int. J. Climatol. 24: 1695–1712 (2004) 1698 P. MAHERAS ET AL. Table I. Description of the 14 circulation types, six anticyclonic and eight cyclonic circulation types Anw The anticyclonic centre is located in the west or northwest of the Greek area, over central, western or northern Europe. The wind over Greece prevails from the northerly sector, with higher intensity in the cold period Ane The anticyclonic centre is located in the northeast of the Greek area, over eastern Europe. The winds are principally from the northerly sector, similar to Anw, but with lower speed A The anticyclonic centre is located over the Greek area, producing weak variable winds or calms in the warm period and relatively strong winds or calms in the cold period Asw The anticyclonic centre is located in the west or southwest of the Greek area, usually over the central or southwestern Mediterranean basin or northern African coast. This type is characterized mainly by westerly or northwesterly flow Ase The anticyclonic centre is located in the southeast of Greece, in the area of Cyprus or the Middle East. The wind prevails from the southerly sector Ae The anticyclonic centre is located in the east of Greece, over Turkey, resulting mainly in easterly flow at