MARCH 2002 TRIGO ET AL. 549 Climatology of Cyclogenesis Mechanisms in the Mediterranean ISABEL F. T RIGO* Climatic Research Unit, University of East Anglia, Norwich, United Kingdom GRANT R. BIGG School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom TREVOR D. DAVIES Climatic Research Unit, University of East Anglia, Norwich, United Kingdom (Manuscript received 2 November 2000, in ®nal form 19 July 2001) ABSTRACT A general climatology of the main mechanisms involved in Mediterranean cyclogenesis is presented. A diagnostic study of both composite means and case studies is performed to analyze processes occurring in different seasons, and in different cyclogenetic regions within the same season. It is shown that cyclones that developed over the three most active areas in winterÐthe Gulf of Genoa, the Aegean Sea, and the Black SeaÐ are essentially subsynoptic lows, triggered by the major North Atlantic synoptic systems being affected by local orography and/or low-level baroclinicity over the northern Mediterranean coast. It is also suggested that cyclones in two, or all three, of these regions often occur consecutively, linked to the same synoptic system. In spring and summer, thermally induced lows become progressively more important, despite the existence of other factors, such as the Atlas Mountains contributing to lee cyclogenesis in northern Africa, or the extension of the Asian monsoon into the eastern part of the Mediterranean. As a consequence, the behavior of Mediterranean cyclones becomes modulated by the diurnal forcing; the triggering and mature stages are mostly reached by late afternoon or early nighttime, while cyclolysis tends to occur in early morning. 1. Introduction cyclone distributions and genesis mechanisms than that of Petterssen (1956) is appropriate now that adequate The spatial distributions and general cyclogenesis data are available. Certainly there is growing interest in mechanisms in the Northern Hemisphere have been an- these particular weather systems, since the number of alyzed by Petterssen (1956), in his climatology of sur- studies of Mediterranean cyclone formation has in- face cyclones. Over the Mediterranean region, he iden- creased in recent years, andÐin recognition of the high ti®ed two main centers of activity in winter, over the spatial variability over the regionÐmost have focused western and eastern basins, respectively; and one main on case studies occurring in speci®c areas, such as in center over Iberia in the summer. However, the Medi- the Sahara (e.g., Thorncroft and Flocas 1997), in the terranean Sea and its immediate environs experience a Aegean Sea (e.g., Flocas and Karacostas 1996), and in high spatial variability of weather conditions, leading the lee of the Alps (e.g., Buzzi and Tibaldi 1978; Gomis to large arid areas (e.g., Thornes 1998, 2±4) and, yet, et al. 1990). However, there has been no such study for still accommodating the greatest annual precipitations the Mediterranean Basin (Fig. 1). This paper addresses totals in Europe, in the Dinaric Alps [see Fig. 1 for that de®ciency through the study of the main cyclogen- location; Radinovic (1987)]. Such high variability sug- esis processes using a detailed surface cyclone database gests that a rather more detailed study of Mediterranean (Trigo et al. 1999, hereafter TDB), complemented by upper-level information, thus allowing an examination * Current af®liation: Departamento de FõÂsica, Faculdade de CieÃn- of the interactions between lows developing in different cias de Lisboa, Lisboa, Portugal. parts of the basin and large-scale features. As suggested by TDB, the traditional four meteoro- logical seasons (December±February, DJF; March± Corresponding author address: Isabel Franco Trigo, Departamento de FõÂsica, Faculdade de CieÃncias de Lisboa, Campo Grande, Ed. C8, May, MAM; July±August, JJA; September±November, Piso 6, 1749-016 Lisboa, Portugal. SON) do not ®t well the patterns of cyclone occurrence E-mail: [email protected] in the Mediterranean, mainly due to its high inter- q 2002 American Meteorological Society Unauthenticated | Downloaded 10/07/21 05:48 AM UTC 550 MONTHLY WEATHER REVIEW VOLUME 130 FIG. 1. The Mediterranean Sea and topography (m) of the surrounding regions. monthly variability. The annual distributions of cyclone nual cycle over the Black Sea. The western Mediter- events per 105 km2 (i.e., the cyclones' spatial densities) ranean has a much more marked seasonality than the in the most active cyclogenetic regions (de®ned in TDB, eastern basin. Frequency of occurrence should not be and shown in Table 1) between 1987 and 1996 are rep- simply equated with strength. An example is over the resented in Figs. 2a (western Mediterranean) and 2b Gulf of Genoa where, although cyclones are a constant (eastern Mediterranean); only cyclone events lasting at feature over the whole year, they are generally deeper, least 12 h have been considered. It should be noted that and have more severe weather in winter than during the the values shown in Table 1 represent the total number summer, when they are, in fact, more frequent. Hence, of events detected in each area and, hence, differ from in order to overcome the problems presented by these the values per unit area represented in Fig. 2 for the differing seasonalities in the various regions, the anal- same months. As expected in an area where cyclogenesis ysis in the present work is performed for representative is highly determined by geographical features, the lower winter, spring and summer months, namely January, curve in both diagrams, which corresponds to the whole April, and August (see TDB, section 3, for argument). basin, re¯ects a much lower number of cyclone events Since October is characterized by a rather sudden tran- than any of the other curves. The relatively small annual sition from summer to winter conditions, which are es- cycle over the whole basin re¯ects the completely dif- tablished by November (TDB; HMSO 1962), the tra- ferent seasonalities in the separate cyclogenetic regions. ditional autumn period is considered to be part of the For example, there is a peak of activity in spring over winter season. the Sahara, in summer over Iberia, and a smoother an- The data used in this work, including the database of TABLE 1. Number of cyclogenesis events detected in each of the most cyclogenetic regions for Jan, Apr, and Aug, between 1987 and 1996. Absence of a ®gure means the region is not a major area for cyclogenesis during the season in question. Sahara Iberian Peninsula Gulf of Genoa Aegean Sea Middle East Black Sea 288±328N 368±428N 408±458N 368±418N 328±388N 418±458N Month 7.58W±2.58E 108W±08 7.58±12.58E 22.58±27.58E 37.58±42.58E 32.58±42.58E Jan Ð Ð 30 22 Ð 30 Apr 60 Ð 45 Ð 53 55 Aug 23 89 68 Ð 55 68 Unauthenticated | Downloaded 10/07/21 05:48 AM UTC MARCH 2002 TRIGO ET AL. 551 FIG. 2. Total number of cyclogenesis events detected per 105 km2 within the most active regions in the (a) western and (b) eastern Mediterranean, during the 1987±96 period. The total number of cyclogenesis events detected in the whole basin per unit area is represented by the bottom line. Mediterranean cyclones, and the diagnostic techniques night with clear sky and calm conditions. However, di- are brie¯y described in section 2. The diurnal cycle of agnosis of 10-m wind and 10-m vorticity [obtained from cyclogenesis/cyclolysis is analyzed in section 3, where National Centers for Environmental Prediction±Nation- we will show that the development of Mediterranean al Center for Atmospheric Research (NCEP±NCAR) re- lows becomes progressively more dependent on the time analyses] tend to con®rm most of our results. of day throughout spring and summer. This is a feature The cyclone tracking was based on a nearest neighbor that clearly separates winter from spring/summer cy- search in the previous ®eld, within an area de®ned by clogenesis processes. imposing thresholds to the maximum cyclone velocity. To achieve our goal of a global description of cyclo- If no cyclone was found within that area, cyclogenesis genesis mechanisms in the Mediterranean, we will then was assumed to have occurred (see the appendix for perform a composite analysis for the most active regions further details; a full description of the detecting and in January (section 4), and in April and August (section tracking techniques may be found in TDB). The reso- 5), respectively. For each of these two cases, the com- lution of the ECMWF dataset proved to be adequate for posite results will be complemented by case studies, in a climatology of Mediterranean systems, including the order to con®rm the physical reality underlying the com- detection of the initial stages of development, often as- positing procedure and its interpretation. sociated with subsynoptic scales (e.g., Buzzi and Tibaldi 1978). The spatial resolution of the data strongly limits the detection of systems with radii of less than 150± 2. Data and methodology 200 km (Fig. 3 in TDB). The present work is based on the analysis of a subset The composite analysis and case studies presented of an 18-yr (1979±96) climatology of Mediterranean here are performed for the 1987±96 period, for which cyclones derived from European Centre for Medium- upper-level ECMWF reanalyses were also available. Range Weather Forecasts (ECMWF) Re-Analyses These include geopotential height, temperature, relative [1.125831.1258 horizontal and 6-hourly temporal res- vorticity, and vertical motion, at 1000, 850, 700, 500, olution; Gibson et al. (1999)]. Cyclones were detected 400, 300, and 200 hPa. The data cover the area from by identifying 1000-hPa height minima (thresholds for 24.758 to 50.6258N, and 15.758Wto458E (Fig. 1). corresponding sea level pressure and pressure gradient The objective of the composite analysis is to char- were used).