Scenarios in the development of Mediterranean cyclones

The Mediterranean is one of the most cyclogenetic regions in the world. The cyclones are concentrated along its northern coasts and their tracks are oriented more or less west-east, with several secondary tracks connecting them to Europe and to North Africa. The aim of this study is to examine scenarios in the development of Mediterranean cyclones, based on five selected winter seasons (October– March). We detected the cyclones subjectively using 6hourly Sea-Level Pressure maps, based on the NCAR/NCEP reanalysis archive. HMSO (1962) has shown that most Mediterranean cyclones (58%) enter the Mediterranean from the Atlantic Ocean (through Biscay and Gibraltar), and from the southwest, the Sahara Desert, while the rest are formed in the Mediterranean Basin itself. Our study revealed that only 13% of the cyclones entered the Mediterranean, while 87% were generated in the Mediterranean Basin. The entering cyclones originate in three different regions: the Sahara Desert (6%), the Atlantic Ocean (4%), and Western Europe (3%). The cyclones formed within the Mediterranean Basin were found to generate under the influence of external cyclonic systems, i.e. as “daughter cyclones” to “parent cyclones” or troughs. These parent systems are located in three regions: Europe (61%), North Africa and the Red Sea (34.5%) and the Mediterranean Basin itself (4.5%). The study presents scenarios in the development of Mediterranean cyclones during the winter season, emphasizing the cyclogenesis under the influence of various external forcing. The large difference with respect to the findings of HMSO (1962) is partly explained by the dominance of spring cyclones generating in the Sahara Desert, especially in April and May that were not included in our study period. Correspondence to: H. Saaroni ([email protected]) 1 Background The Mediterranean Basin (MB) is one of the most cyclogenetic regions in the world (Petterssen, 1956; HMSO, 1962; Campins et al., 2000). This is in spite of its being south of the global mid-latitude cyclone belt and the fact that its southern end, latitude 30 N, denotes the northern border of the global desert belt (Trewartha and Horn, 1980; Ahrens, 2000). The southern location of the MB suppresses cyclonic activity in the summer, leaving the winter as the main active season. In the winter, the MB offers all the cyclogenetic factors, being highly baroclinic, warmer than its surroundings and positioned at the lee of mountain ridges, such as the Atlas ridge in Morocco, the European Alps and the Taurus ridge in Turkey (HMSO, 1962; Alpert et al., 1990, 1996; Trigo et al., 1999; Lionello et al., 2006). The MB winter cyclones have some special characteristics: they develop mainly at the northern Mediterranean coasts; their horizontal size varies between meso-scale and synoptic-scale (Alpert et al., 1990; Trigo et al., 1999; Campins et al., 2000; Trigo, 2006; Lionello et al., 2006); their life span is around one to four days; their typical speed is ∼5 ms−1; they reach the troposphere (Alpert and Ziv, 1989). In addition to the winter cyclones, another type of frontal cyclone, known as the “Sharav”, or “North African Cyclone” (Alpert and Ziv, 1989), is observed in the MB. These cyclones, most frequent in the spring season, develop and move along the southern coast of the MB. They owe their existence to the high temperature gradient that develops between the Mediterranean Sea and the African continent during that season (Alpert and Ziv, 1989). The winter cyclone track in the MB co-exists with the European track, which is an extension of the Atlantic stormtrack. It is therefore possible that the MB cyclones are influenced by the European ones (HMSO, 1962; Alpert et al., 1990; Lionello et al., 2006). Published by Copernicus Publications on behalf of the European Geosciences Union. 60 M. Romem et al.: Scenarios in the development of Mediterranean cyclones Fig. 1. Main winter cyclone track in the MB. Average annual frequencies are shown in brackets (HMSO, 1962, p. 33). HMSO (1962) documented the cyclone tracks in the MB, see Fig. 1. The main track is eastward, along the northern coast of the Mediterranean, down to the Levant region, with three branches toward the northeast, where cyclones leave the MB in favor of the European track; two in the Adriatic Sea and one in the Aegean Sea. An additional track extends from the lee of the Atlas along the North Africa coast. HMSO (1962) differentiated between cyclones that have formed within the MB and those entering that region. They found that 42% of the MB cyclones are formed outside the Basin. They divided the MB into three parts and emphasized the western MB as the most cyclogenetic. Other studies also described the typical track that starts from the western part of the MB and moves along it down to its eastern end. Many efforts have been made to map the MB cyclogenetic regions (Petterssen, 1956; Alpert et al., 1990; Trigo et al., 1999; Campins et al., 2000; Maheras et al., 2001; Genovés et al., 2006; Lionello et al., 2006; Trigo, 2006). The main regions are: the Gulf of Genoa, the Iberian Peninsula, Palos, the Atlantic coast of Morocco, southern Italy, the Aegean Sea, the Tyrrhenian Sea, south of Sardinia, the Ionic sea, the Balares Islands and Cyprus. Reiter (1975) mentions that the most cyclogenetic region is the western MB, but Alpert et al. (1990) and Petterssen (1956) found that the eastern MB is more cyclogenetic than the western part (the Gulf of Genoa) The present study attempts to re-evaluate those cyclones formed in the MB and those entering the region. In addition, a special effort is devoted to identifying and outlining the main scenarios of Mediterranean cyclogenesis. The motivation for this study is improvement of the atmospheric data base. Here, we present preliminary results, based on five winter seasons. Section 2 specifies the data and methodology used, Sect. 3 describes the results and Sect. 4 summarizes the implications of our findings and outlines the next stages for the research. 2 Methodology and data base Five winter seasons (October–March) were selected: 1986/7, 1991/2, 2001/2, 2002/3, 2003/4. These winters were selected as representing different types of cyclone formation. For instance in 1991/2 the cyclones were continuously concentrated in the eastern Mediterranean, whereas in 1986/7 there was a considerable shift in their location during the season. We chose the period October–March as the winter season since it is the period when most MB cyclones affect the region (HMSO, 1962). The database is the sea-level pressure (SLP) maps taken from the NCEP/NCAR CDAS-1 archive (Kalnay et al., 1996; Kistler et al., 2001) reanalysis gridded data, with a 2.5×2.5 spatial and 6 h temporal resolutions. The study region covers the domain 25 N–65 N and 20 W–45 E. The 6 hourly SLP maps were scanned subjectively and the center of each cyclone was detected and marked. Since eastern MB cyclones are characterized by a relatively high Adv. Geosci., 12, 59–65, 2007 www.adv-geosci.net/12/59/2007/ M. Romem et al.: Scenarios in the development of Mediterranean cyclones 61 pressure, we avoided using any threshold for the central pressure of cyclones or for the pressure gradient