CYCLONE TRACK VARIABILITY OVER TURKEY IN ASSOCIATION WITH REGIONAL CLIMATE

Transcription

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 20: (2000) CYCLONE TRACK VARIABILITY OVER TURKEY IN ASSOCIATION WITH REGIONAL CLIMATE MEHMET KARACA a, *, ALI DENIZ b and METE TAYANÇ c a ITU, Eurasia Institute of Earth Sciences, Maden Fak. Genel Jeoloji ABD, Maslak, Istanbul, Turkey b ITU Department of Meteorology, Maslak, Istanbul, Turkey c MU Department of En ironmental Engineering, Kuyubaşı, Istanbul, Turkey ABSTRACT In this study a set of cyclone frequency statistics is developed and the prevailing tracks of cyclones are derived for a region of the world that has not been previously investigated in detail. Analysis reveals that Turkey experiences the effects of five dominant cyclone trajectories. Investigation of the seasonal variability of the cyclone frequencies shows that the highest number of cyclones occurs during winter. The variability of the subtropical jetstream latitude is analysed by multi-variable regression involving cyclone numbers, the numbers of occurrences of the Mediterranean Persistence High Pressure (MPHP), which is an extension of the Azore High Pressure and their periods. This analysis revealed that the most important factor among the three factors chosen is the number of observed blocking cases over Turkey. Although the northern parts of Turkey are accepted as having a transitional type climate between the Mediterranean and temperate regions, our results showed that they are more influenced by cyclones of Mediterranean origin. Copyright 2000 Royal Meteorological Society. KEY WORDS: cyclone track; regional climate; Turkey; variability 1. INTRODUCTION Relatively small-scale features (such as changes in cyclonicity, changes in cyclone tracks, spatial variations related to topography, etc.) have been the focus of many recent climate studies. Generally speaking, synoptic-scale climatological studies are carried out in the following two steps: (i) determination of the categories of atmospheric circulation, and (ii) determination of relationship between weather elements and those categories. Analyses of surface low pressure systems, and their relationship with atmospheric variables such as precipitation, both regionally and globally, have long been of interest to scientists. Many studies can be found, mainly for regions in the developed countries that deal with the determination of cyclonic tracks focused on seasonal variations, in particular winter versus summer or spring versus autumn cases and their frequencies. The objectives of such studies are first, to obtain the major interseasonal variations and second, to fulfill the desire to have a concise picture of the cyclonic track climatology. Important studies carried out on this subject are Petterssen (1956), Sanders and Gyakum (1980), Whittaker and Horn (1984), Lambert (1988), Wallace et al. (1988), Fraedrich and Müller (1992), and Rogers (1997). Reiter (1975) and Alpert (1984) focused on cyclones in the Mediterranean Basin. In some studies, trajectory calculations are based upon wind analyses produced at the European Center for Medium Weather Forecast (ECMWF), employing a trajectory analysis based on the air mass transportation model of Reiff and Velds (1984). Among the earlier studies on cyclones, Alpert (1984) has suggested that conditional instability of the second kind (CISK) may play an important role in the eastern Mediterranean in keeping local depressions stabilized at certain locations for relatively long periods. These consistent perturbations can bring abundant precipitation to many regions of Turkey. In another study that was done by Hayden and Smith (1982), the authors developed a model which essentially produces a * Correspondence to: Istanbul Technical University, Eurasia Institute of Earth Sciences, Maden Fak., Maslak, Istanbul, Turkey; karaca@itu.edu.tr Copyright 2000 Royal Meteorological Society

2 1226 M. KARACA ET AL. prognostic chart of the expected spatial cyclone frequency distribution. Tayanç et al. (1998b) simulated the cyclogenesis process of the March 1987 blizzard over the Mediterrranean Basin and Balkan Region using the NCEP/ETA model with high resolution. In general, these previous studies indicated that the main regions of cyclogenesis are primarily baroclinic zones. Katsoulis et al. (1998) studied monthly anticyclonicity in southern Europe and the Mediterranean region and Kassomenos et al. (1998) focused on the meso-scale patterns over the Mediterranean Basin. According to their findings the Mediterranean Basin and southern Europe are influenced during the year by semi-permanent large-scale anticyclones, the Azores anticyclone in the west and Eurasiatic (Siberian) anticyclone in the northwest. The main aim of this paper is to determine the main routes of cyclone tracks that have affected Turkey and its vicinity between 1979 and The data and methodology for the analysis of cyclone tracks are described in the second section. In Section 3, the climate and the cyclogenesis process in the Basin is summarized. In Section 4, cyclone tracks and their relationship with precipitation at selected stations are analysed in terms of monthly, seasonal and yearly variations. Finally, a brief summary of the study is provided and conclusions drawn in the light of the results. 2. OBJECTIVES, DATA AND METHODOLOGY The objectives of producing a data set of cyclone frequency statistics are (i) to derive the dominant cyclone tracks over the region; (ii) to investigate the seasonal variability of cyclone frequencies; (iii) to detect the effects of apparent changes introduced in the precipitation series of the stations residing on the cyclone tracks; and (iv) to obtain a concise picture of the cyclone track climatology over Turkey in relation to the subtropical jetstream and frequency of the persistance high pressure centre over the Mediterranean. Different data sets are used for different purposes in this study. To determine tracks and frequencies of cyclones, the daily surface and 500-hPa charts in the Meteorologische Abhandlungen Bulletin published by Berlin University, Meteorology Institute are used (Berliner Wetterkarte, ). Cyclonic tracks are found subjectively by marking down the centre of the low pressure system on the surface charts. Verification of cyclone paths is done by comparing with the geopotential heights at 500-hPa charts at the same time (Alpert et al., 1990). To find the subtropical jetstream core over the eastern Mediterranean, the monthly mean 250-hPa geopotential heights (ECMWF gridded on 2 2 ) from 1979 to 1994 are used. The investigated area resides between N and 40 W 80 E. The region occupied by these coordinates includes Europe and the Mediterranean Sea where the main cyclones affecting Turkey are generated. Precipitation data over Turkey is obtained from the State Meteorological Office of Turkey. Homogeneity analysis of this data set is done by O zçelik (1996). Precipitation data from 20 stations are used to find any changes introduced in station precipitation series owing to changes in the dominant cyclone tracks during the period considered. Figure 1 demonstrates the locations of these 20 precipitation stations. Daily precipitation data are obtained from the State Meteorological Office (DMI), and monthly, seasonal and annual averages are computed. The methodologies of estimating these averages, applying homogeneity tests and correcting erroneous series are explained in O zçelik (1996), Tayanç and Toros (1997), Tayanç et al. (1997, 1998a). 3. CLIMATOLOGY OF THE BASIN The Koppen s definition of Mediterranean climate is, in simple terms, one in which winter rainfall is more than three times summer rainfall, but for much of the region summer rainfall is practically zero. The seasonal cycle is well pronounced and can be defined. July, August and September (JAS) are characterized

3 CYCLONE TRACK VARIABILITY OVER TURKEY 1227 Figure 1. The locations of precipitation stations by warm and dry weather in a large part of the basin; a consequence of a strong high pressure ridge extending from the Azores subtropical high. The ridge further extends southward to Egypt. To the northwest, the ridge extends to Turkey, even further down to the Persian Gulf. During JAS, the eastern Mediterranean is affected by an extension of the Indian summer monsoon depression. In October the rainy season begins. Winter is characterized by cyclonic disturbances and low pressure in the Mediterranean with higher pressure to the east associated with the Siberian high. In spring, the rainy season continues. By May, the polar front and associated strong jetstream is sufficiently far north that its influence is diminished, and the subtropical highs and associated ridges once more exert their influence (Chang, 1972; Barry and Perry, 1973; Deniz and Karaca, 1995). The formation of Mediterranean cyclones is partly determined by excursions of the polar front jet and the European trough, modified by the land sea temperature contrast which favours cyclogenesis over the area (Wigley and Farmer, 1982). Owing to large topographical differences in the area, the formation of eastern basin cyclones, affected by lee cyclogenesis and is associated with cold northerly flows (Alpert, 1984; Deniz and Karaca, 1995; Karaca and Dobricic, 1997; Tayanç et al., 1998b). The movement of cyclones is not well understood. In the western Mediterranean, depressions are frequently steered along the Mediterranean front by the temperature contrast which results from colder continental air moving over the warmer sea (Wigley and Farmer, 1982; Palutikoff et al., 1992; Halpert et al., 1993). This front is particularly strong in spring. Roughly half of the central Basin depressions are steered over the Black Sea, and there is some evidence of steering by the upper flow along the axis of the subtropical jet. The movement of the eastern Basin depressions may be determined by the zonality of the upper flow and by the strength of the Siberian high (Wigley and Farmer, 1982). 4. RESULTS As mentioned above our study can be grouped into three parts: (i) developing a cyclone track climatology; (ii) its relationship with station based precipitation; and (iii) determining the latitudinal variation of the subtropical jet core over the eastern Mediterranean and frequency of Mediterranean Persistence High Pressure (MPHP) which is an extension of the Azore High Pressure Cyclone track climatology Analysis of the 15-year period daily surface and 500-hPa weather maps revealed four main cyclone tracks that have affected Turkey. Although there are other cyclones originating at various places and affecting Turkey, in general they are small in number and weaker in cyclone strength. In this paper, cyclones which have affected Turkey are classified into four groups in terms of their tracks. In this classification the number and the deepness of cyclones are used as weighting factors. Figure 2 shows the

4 1228 M. KARACA ET AL. four main trajectory systems of cyclones affecting the region. The tracks are assigned a number according to their points of origin from north to south. These are: 1. The path which originates from north of Turkey over the southwestern parts of Russia and passes from the Black Sea region (Path 1). 2. The path which originates from the Balkans and affects Marmara and the Black Sea region, and also partly affects inner parts of Anatolia (Path 2). 3. The path which is generated in the Genoa Gulf and affects Turkey. It is possible to investigate this path in two sections: both of the sections extend to the western Aegean Sea on the same track but later they split up into: (a) The path which moves to the northeast direction and affects the northern Aegean region, all the Marmara region and western and middle Black Sea region (Path 3a). (b) The path which moves towards the east and affects western Turkey and passes over middle Anatolia. Later it obtains a northeasterly direction and middle-eastern Black Sea region (Path 3b). 4. The path which originates from the western or middle Mediterranean with some cases originating from south of the Genoa Gulf and some cases from north of the Sahara Desert which move towards the eastern Mediterranean. It affects the southern parts of Turkey, Crete, Cyprus and Middle East (Path 4). The cyclogenesis process of these weather systems can sometimes occur over the Sahara and North Africa, when there is a large amplitude of the upper air flow. In such cases, cold air surge to the low latitudes brings low temperatures over Turkey (Danuta, 1992). Paths 1 and 2 are typical summer-time trajectories that bring summer storms over the northern parts of Turkey. They bring abundant rain with them, and in some cases, flooding is not a surprise. In winter, cold air masses from the Balkan Region and northern Europe are associated with these trajectories. They are characterized by substantial amplification of the planetary-scale flow waves during the development phase of the cyclone that can bring cold air to very low latitudes. The frequency of Paths 3 and 4 is greater in winter months than in other months. These type of cyclones are generally associated with above normal temperatures in their warm sector and normal or a little below-normal temperatures at the back of the cold front. Thus, it is not common to have snow blizzards when the cyclones are of these types. Air sea interaction during the cyclogenesis process has a contribution to the moisture content of the cyclones, generally causing them to become unstable leading to thunderstorms. Figure 2. The paths of atmospheric cyclones over Turkey

5 CYCLONE TRACK VARIABILITY OVER TURKEY Monthly, seasonal and annual ariability of cyclones The annual total number of cyclones that have affected Turkey from 1979 to 1994 are illustrated in Figure 3(a). The highest number of cyclones with 68 occurrences can be seen in In contrast, the lowest number of cyclones with 46 occurrences belongs to 1989 and It is obvious that during the 15-year period there is a decreasing trend in the number of perturbations, but that a decreasing trend is not significant. Figure 3(b) presents the monthly variability of the average number of the cyclones for the period of As is expected due to Turkey s Mediterranean type of climate, the number of cyclones is the lowest in the summer and highest in the winter. December has got the highest number of cyclones with a value of 96 and August has got the least with a number of 40. Seasonal variability of the cyclones is given in Figure 4. The seasons are defined as: December of previous year, January and February for winter; March, April and May for spring; June, July and August for summer; September, October and November for Autumn. There is no data for December 1978, so the number of cyclones in the winter of 1979 is excluded from the analysis. Therefore, winter data begins from Although in both winter and summer the number of cyclones is decreasing with respect to time, in spring no trend can be established and in autumn there is a slightly increasing trend. None of those trends are statistically significant. It is important to note here that only 12 cyclones affected the region in the winter of 1989, when the majority of the precipitation is expected to fall, and only four in the summer of the same year. Widespread droughts were experienced during this year, leading to famines in certain regions of Turkey (Deniz and Unal, 1998). To make generalizations about drought, effective cyclone life-periods must be assessed besides the cyclone numbers. Making drought analysis and determining their impacts are beyond the scope of this article. Figure 3. (a) Frequency of cyclones over Turkey from 1979 to 1994; (b) cyclone numbers in months

6 1230 M. KARACA ET AL. Figure 4. Seasonal variation of total number of cyclones for all paths; (a) winter, (b) spring, (c) summer and (d) fall It can be deduced from Table IV that the majority of the cyclones which affect Turkey originate from the Genoa Gulf, and then follow path 3b which passes over the middle Aegean, then middle Anatolia and the East Black Sea regions, respectively. The lowest number of cyclones can be seen in the first path. Although there is a decreasing trend in Paths 1, 2, 3a and 4, there is a slightly increasing trend in Path 3b. One of the first studies trying to group the main routes followed by cyclones in the Mediterranean Basin was conducted by Trewartha (1961). The study concluded that the cyclones can travel large distances over land and they are relatively infrequent in summer. According to Trewartha around cyclonic systems, occurring during October through April, follow three main tracks: one over the Balkans to the Marmara region, another one from the Aegean Sea to western Turkey, the other one from the Aegean Sea to southern Turkey. Our results are in close agreement with the findings of Trewartha. Reiter (1975) concluded that the eastern Mediterranean region is frequented by eastward travelling depressions especially during the October March period. About 30 cyclones per year arrive from the central Mediterranean region. It is also common that a cyclone moving along the Mediterranean coast of Turkey produces a secondary lee trough along the Black Sea coast (Brody and Nestor, 1980). Our results also demonstrate that the number of cyclones affecting Turkey in a year can range from 20 to 25 during drought periods and from 40 to 50 during very wet periods Analysis of precipitation series Turkish precipitation stations are categorized according to their location on the cyclone trajectories. A classified list of four precipitation stations for each path is presented in Table I. This has enabled comparison of the precipitation data with the cyclone trajectories. Figure 5 depicts the total of four selected station precipitation time series, thus, a relationship between total number of cyclones for each path with the precipitation data can be investigated. It is intuitively clear from the figures that the amount of precipitation is positively correlated with the frequency of low pressure systems. Precipitation data in winter, spring and fall seasons are highly correlated with the frequency of cyclones for each path. However, the trends are different, especially for the precipitation in the inland stations due to convective precipitation in summer.

7 CYCLONE TRACK VARIABILITY OVER TURKEY 1231 Table I. The selected list of precipitation stations classified according to their location on the cyclone paths Station Latitude (N) Longitude (E) Height (m) Path 1 Samsun Sinop Giresun Trabzon Path 2 Edirne Lüleburgaz Tekirdağ Göztepe Path 3a Çanakkale Bandırma Balıkesir Bursa Path 3b Akhisar I zmir Kütahya Uşak Path 4 Dörtyol Gaziantep I skenderun Mersin The methodology for the determination of latitudinal ariation of subtropical jetstream and the generation of the jetstream index A concise picture of the cyclone track climatology over Turkey can only be obtained when subtropical jetstream and the frequency of the persistence of high pressure centres over the Mediterranean is considered. This is because the cyclones affecting the region are mainly due to the interactions between polar and subtropical jetstreams. The meridional oscillation of subtropical and polar jetstreams is very important in bringing warm and cold air together for cyclogenesis. That s why the authors tried to determine the latitudinal variation of the subtropical jetstream as a primary step. In order to estimate the variation of the axis of the subtropical jetstream, ECMWF 250-hPa geopotential heights for the period obtained for the 1769 grid points lying on the latitudinal interval N and longitudinal interval 40 W 80 E is used. From the data, the geostrophic wind patterns are obtained, and the reference latitude for the estimations is accepted as 40 N by taking into account the approximate latitude of Turkey. The following formula is used: ref = n i, j=1 V i, j d i, n V i, j i, j=1 where is the reference of latitudinal variation of subtropical jetstream, V i, j is the geostrophic velocity at level of 250 mb, d is the distance from reference latitude, 40 N. The resolution of the grid mesh is 2 2. The variation of the subtropical jetstream is estimated by using 40 N as the reference latitude and a main interval window located on N and E. This main interval window is illustrated in Figure 6. To detect whether there is a structure disorder interrupting the general variability in the windowing system or not, six different windows in the main window are taken. The variability of the subtropical jetstream in each window is investigated and it is found that the general variability is kept intact.

8 1232 M. KARACA ET AL. By using the German Meteorology Catalogue surface and 500-hPa height maps, the number of the cyclones affecting Turkey, the number of blocking cases and their duration were found. In this section the variation of the subtropical jetstream latitude is analysed by multi-variable regression with the factors of (i) the number of observed blocking cases over Turkey, (ii) the period of observed blocking cases over Turkey, and (iii) the number of observed cyclones. The factors important for the variation of the subtropical jetstream latitude are also important for the cyclogenesis process, the behaviour of the cyclone and its path. The established equation showing the variability of the jetstream axis on a monthly basis is as follows: Y= X X X 3, Figure 5. Seasonal variation of total precipitation for all paths

9 CYCLONE TRACK VARIABILITY OVER TURKEY 1233 Figure 5 (Continued) where X 1 is the number of observed blocking cases over Turkey, X 2 is the period of observed blocking cases over Turkey and X 3 is the number of observed cyclones. The formula obtained above is calibrated by comparing it with the equation of the variation of the jetstream axis as explained in Section 2. Thus, by putting the related variables into the equation, the monthly jetstream index can be generated (Table II). In the variability of jetstream axis, this formula shows us that among the three factors the most important one is the number of observed blocking cases over Turkey. It is clear that the effect of the number of blocking cases is more important than their duration and the number of observed cyclones (Table III). Another value in the equation of approximately 34.2 depicts that there are other factors playing a role in the variability of the jetstream axis and in turn cyclone formation and motion.

10 1234 M. KARACA ET AL. Figure 6. The main interval window that is used to obtain the variability of subtropical jetstream Table II. Jet index for Turkey Months Number of Duration of The axis of Number of blocking blocking (day) subtropical cyclones jet ( N) January February March April May June July August September October November December SUMMARY AND CONCLUSIONS In this study a data set of cyclone frequency statistics is produced and the prevailing tracks of cyclones are derived. The results revealed that Turkey is under the effect of five dominant trajectories. Investigation of the seasonal variability of the cyclone frequencies revealed that the highest number of cyclones detected in Turkey occur in winter. Although northern parts of Turkey are accepted as having a transition climate type between the Mediterranean and temperate regions, they are under the effect of low pressure systems originating in the Mediterranean more, and the above result of more cyclones in winter proves this. It is also illustrated that there is a positive impact of the number of cyclones on the precipitation amounts of the stations located on the trajectories of cyclones. It can be concluded that Paths 1 and 2 are typical summer-time trajectories that bring summer storms over the northern parts of Turkey. They bring abundant rain with them and in some cases flooding is not a surprise. In winter, cold air masses from the Balkans and northern Europe are associated with these

11 CYCLONE TRACK VARIABILITY OVER TURKEY 1235 Table III. Comparison of El Niño and non El Niño years for 20 precipitation stations in Turkey Duration of Number of Number of Precipitation (mm) blocking (day) blocking cyclones (winter) (winter) (ENSO year) 80 (minimum) 13 (minimum) 23 (higher) (higher) (non-enso year) 138 (maximum) 21 (maximum) 21 (lower) (lower) Table IV. Total numbers of cyclones according to the paths Years Path 1 Path 2 Path 3a Path 3b Path trajectories. They are characterized by a substantial amplification of the planetary-scale long waves during the development phase of the cyclone that brings cold air to very low latitudes. Thus, it is not uncommon to have snow blizzards when the cyclones are of this type. The frequency of Paths 3 and 4 are greater in winter months than in other months. These type of cyclones are generally associated with above normal temperatures in their warm sector and normal or a little below normal temperatures at the back of the cold front. Air sea interaction during the cyclogenesis process has a contribution to the moisture content of the cyclones, generally causing them to become unstable leading to thunderstorms. The analysis of the variability of the jetstream axis revealed that the most important factor among three chosen factors is the number of observed blocking cases over Turkey. It is also shown that the effect of the number of blocking cases is more important than their duration and the number of observed cyclones. It would seem that a classification system based upon synoptic weather frequencies could be adapted to a number of climatic and environmental applications. Assuming a longer period of homogeneous data, perhaps years, the inventory of mean weather type properties and frequencies could serve as a climatic baseline inventory for Turkey and the surrounding region. This inventory would permit evaluation of monthly or seasonal regimes of favourable or unfavourable weather for industries and activities that are strongly weather dependent. The inventory could also serve as a regional yardstick for the evaluation of local micro-scale climates in connection with meso-scale climates and their effects on standard data. If a synoptic analysis could be standardized, the inventories could provide a means for interregional comparisons by weather types along the margins of any area. These interregional comparisons could evaluate the changing frequencies and tracks of weather activity from place to place as related to weather types and the general circulation and also illustrate the modification of properties associated with the various regional settings. In order to understand the cyclogenesis processes in the Mediterranean Basin to a greater extent and establish their effects on Turkey and the relationship between precipitation and cyclone tracks, similar studies with longer data periods are needed.

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