cyclone track variability over turkey in association with regional climate

INTERNATIONAL JOURNAL OF CLIMATOLOGY

Int. J. Climatol. 20: 1225–1236 (2000)

CYCLONE TRACK VARIABILITY OVER TURKEY IN ASSOCIATIONWITH REGIONAL CLIMATE

MEHMET KARACAa,*, ALI DENIZb and METE TAYANCc

a ITU, Eurasia Institute of Earth Sciences, Maden Fak. Genel Jeoloji ABD, Maslak, 80626 Istanbul, Turkeyb ITU Department of Meteorology, Maslak, 80626 Istanbul, Turkey

c MU Department of En6ironmental Engineering, 81040 Kuyubası, Istanbul, Turkey

ABSTRACT

In this study a set of cyclone frequency statistics is developed and the prevailing tracks of cyclones are derived fora region of the world that has not been previously investigated in detail. Analysis reveals that Turkey experiences theeffects of five dominant cyclone trajectories. Investigation of the seasonal variability of the cyclone frequencies showsthat the highest number of cyclones occurs during winter. The variability of the subtropical jetstream latitude isanalysed by multi-variable regression involving cyclone numbers, the numbers of occurrences of the MediterraneanPersistence High Pressure (MPHP), which is an extension of the Azore High Pressure and their periods. This analysisrevealed that the most important factor among the three factors chosen is the number of observed blocking cases overTurkey. Although the northern parts of Turkey are accepted as having a transitional type climate between theMediterranean and temperate regions, our results showed that they are more influenced by cyclones of Mediterraneanorigin. 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 variationsrelated 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 thecategories of atmospheric circulation, and (ii) determination of relationship between weather elements andthose categories.

Analyses of surface low pressure systems, and their relationship with atmospheric variables such asprecipitation, both regionally and globally, have long been of interest to scientists. Many studies can befound, mainly for regions in the developed countries that deal with the determination of cyclonic tracksfocused on seasonal variations, in particular winter versus summer or spring versus autumn cases andtheir frequencies. The objectives of such studies are first, to obtain the major interseasonal variations andsecond, to fulfill the desire to have a concise picture of the cyclonic track climatology. Important studiescarried out on this subject are Petterssen (1956), Sanders and Gyakum (1980), Whittaker and Horn(1984), Lambert (1988), Wallace et al. (1988), Fraedrich and Muller (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 Centerfor Medium Weather Forecast (ECMWF), employing a trajectory analysis based on the air masstransportation model of Reiff and Velds (1984). Among the earlier studies on cyclones, Alpert (1984) hassuggested that conditional instability of the second kind (CISK) may play an important role in the easternMediterranean in keeping local depressions stabilized at certain locations for relatively long periods. Theseconsistent perturbations can bring abundant precipitation to many regions of Turkey. In another studythat 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, 80626 Istanbul,Turkey; e-mail: [email protected]

Copyright © 2000 Royal Meteorological Society

M. KARACA ET AL.1226

prognostic chart of the expected spatial cyclone frequency distribution. Tayanc et al. (1998b) simulatedthe cyclogenesis process of the March 1987 blizzard over the Mediterrranean Basin and Balkan Regionusing the NCEP/ETA model with high resolution. In general, these previous studies indicated that themain regions of cyclogenesis are primarily baroclinic zones. Katsoulis et al. (1998) studied monthlyanticyclonicity in southern Europe and the Mediterranean region and Kassomenos et al. (1998) focusedon the meso-scale patterns over the Mediterranean Basin. According to their findings the MediterraneanBasin and southern Europe are influenced during the year by semi-permanent large-scale anticyclones, theAzores 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 Turkeyand its vicinity between 1979 and 1994. The data and methodology for the analysis of cyclone tracks aredescribed in the second section. In Section 3, the climate and the cyclogenesis process in the Basin issummarized. In Section 4, cyclone tracks and their relationship with precipitation at selected stations areanalysed in terms of monthly, seasonal and yearly variations. Finally, a brief summary of the study isprovided 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 residingon the cyclone tracks; and

(iv) to obtain a concise picture of the cyclone track climatology over Turkey in relation to the subtropicaljetstream 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 ofcyclones, the daily surface and 500-hPa charts in the ‘Meteorologische Abhandlungen’ Bulletin publishedby Berlin University, Meteorology Institute are used (Berliner Wetterkarte, 1979–1994). Cyclonic tracksare 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 thesame time (Alpert et al., 1990).

To find the subtropical jetstream core over the eastern Mediterranean, the monthly mean 250-hPageopotential heights (ECMWF gridded on 2°×2°) from 1979 to 1994 are used. The investigated arearesides between 20°–76°N and 40°W–80°E. The region occupied by these coordinates includes Europeand the Mediterranean Sea where the main cyclones affecting Turkey are generated.

Precipitation data over Turkey is obtained from the State Meteorological Office of Turkey. Homogene-ity analysis of this data set is done by O8 zcelik (1996). Precipitation data from 20 stations are used to findany changes introduced in station precipitation series owing to changes in the dominant cyclone tracksduring the period considered. Figure 1 demonstrates the locations of these 20 precipitation stations. Dailyprecipitation data are obtained from the State Meteorological Office (DMI), and monthly, seasonal andannual averages are computed. The methodologies of estimating these averages, applying homogeneitytests and correcting erroneous series are explained in O8 zcelik (1996), Tayanc and Toros (1997), Tayanc etal. (1997, 1998a).

3. CLIMATOLOGY OF THE BASIN

The Koppen’s definition of Mediterranean climate is, in simple terms, one in which winter rainfall is morethan three times summer rainfall, but for much of the region summer rainfall is practically zero. Theseasonal cycle is well pronounced and can be defined. July, August and September (JAS) are characterized

Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1225–1236 (2000)

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 ridgeextending from the Azores subtropical high. The ridge further extends southward to Egypt. To thenorthwest, the ridge extends to Turkey, even further down to the Persian Gulf. During JAS, the easternMediterranean is affected by an extension of the Indian summer monsoon depression. In October therainy season begins. Winter is characterized by cyclonic disturbances and low pressure in the Mediter-ranean with higher pressure to the east associated with the Siberian high. In spring, the rainy seasoncontinues. By May, the polar front and associated strong jetstream is sufficiently far north that itsinfluence 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 andthe European trough, modified by the land–sea temperature contrast which favours cyclogenesis over thearea (Wigley and Farmer, 1982). Owing to large topographical differences in the area, the formation ofeastern basin cyclones, affected by lee cyclogenesis and is associated with cold northerly flows (Alpert,1984; Deniz and Karaca, 1995; Karaca and Dobricic, 1997; Tayanc et al., 1998b). The movement ofcyclones is not well understood. In the western Mediterranean, depressions are frequently steered alongthe Mediterranean front by the temperature contrast which results from colder continental air movingover the warmer sea (Wigley and Farmer, 1982; Palutikoff et al., 1992; Halpert et al., 1993). This frontis particularly strong in spring. Roughly half of the central Basin depressions are steered over the BlackSea, and there is some evidence of steering by the upper flow along the axis of the subtropical jet. Themovement of the eastern Basin depressions may be determined by the zonality of the upper flow and bythe 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 thesubtropical jet core over the eastern Mediterranean and frequency of Mediterranean Persistence HighPressure (MPHP) which is an extension of the Azore High Pressure.

4.1. Cyclone track climatology

Analysis of the 15-year period daily surface and 500-hPa weather maps revealed four main cyclonetracks that have affected Turkey. Although there are other cyclones originating at various places andaffecting 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 thisclassification the number and the deepness of cyclones are used as weighting factors. Figure 2 shows the



four main trajectory systems of cyclones affecting the region. The tracks are assigned a number accordingto 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 passesfrom the Black Sea region (Path 1).

2. The path which originates from the Balkans and affects Marmara and the Black Sea region, and alsopartly 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 thispath in two sections: both of the sections extend to the western Aegean Sea on the same track but laterthey 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 (Path3b).

4. The path which originates from the western or middle Mediterranean with some cases originating fromsouth of the Genoa Gulf and some cases from north of the Sahara Desert which move towards theeastern Mediterranean. It affects the southern parts of Turkey, Crete, Cyprus and Middle East (Path4).

The cyclogenesis process of these weather systems can sometimes occur over the Sahara and NorthAfrica, when there is a large amplitude of the upper air flow. In such cases, cold air surge to the lowlatitudes brings low temperatures over Turkey (Danuta, 1992).

Paths 1 and 2 are typical summer-time trajectories that bring summer storms over the northern partsof 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. Theyare characterized by substantial amplification of the planetary-scale flow waves during the developmentphase 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 ofcyclones are generally associated with above normal temperatures in their warm sector and normal or alittle below-normal temperatures at the back of the cold front. Thus, it is not common to have snowblizzards when the cyclones are of these types. Air–sea interaction during the cyclogenesis process has acontribution to the moisture content of the cyclones, generally causing them to become unstable leadingto thunderstorms.

Figure 2. The paths of atmospheric cyclones over Turkey



4.2. Monthly, seasonal and annual 6ariability of cyclones

The annual total number of cyclones that have affected Turkey from 1979 to 1994 are illustrated inFigure 3(a). The highest number of cyclones with 68 occurrences can be seen in 1980. In contrast, thelowest number of cyclones with 46 occurrences belongs to 1989 and 1992. It is obvious that during the15-year period there is a decreasing trend in the number of perturbations, but that a decreasing trend isnot significant.

Figure 3(b) presents the monthly variability of the average number of the cyclones for the period of1979–1994. As is expected due to Turkey’s Mediterranean type of climate, the number of cyclones is thelowest in the summer and highest in the winter. December has got the highest number of cyclones witha 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 ofprevious year, January and February for winter; March, April and May for spring; June, July and Augustfor summer; September, October and November for Autumn. There is no data for December 1978, so thenumber of cyclones in the winter of 1979 is excluded from the analysis. Therefore, winter data begins from1980. Although in both winter and summer the number of cyclones is decreasing with respect to time, inspring no trend can be established and in autumn there is a slightly increasing trend. None of those trendsare statistically significant. It is important to note here that only 12 cyclones affected the region in thewinter of 1989, when the majority of the precipitation is expected to fall, and only four in the summer ofthe same year. Widespread droughts were experienced during this year, leading to famines in certainregions of Turkey (Deniz and Unal, 1998). To make generalizations about drought, effective cyclonelife-periods must be assessed besides the cyclone numbers. Making drought analysis and determining theirimpacts are beyond the scope of this article.

Figure 3. (a) Frequency of cyclones over Turkey from 1979 to 1994; (b) cyclone numbers in months



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 fromthe Genoa Gulf, and then follow path 3b which passes over the middle Aegean, then middle Anatolia andthe 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 Path3b.

One of the first studies trying to group the main routes followed by cyclones in the MediterraneanBasin was conducted by Trewartha (1961). The study concluded that the cyclones can travel largedistances over land and they are relatively infrequent in summer. According to Trewartha around 30–40cyclonic systems, occurring during October through April, follow three main tracks: one over the Balkansto the Marmara region, another one from the Aegean Sea to western Turkey, the other one from theAegean 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 depressionsespecially during the October–March period. About 30 cyclones per year arrive from the centralMediterranean region. It is also common that a cyclone moving along the Mediterranean coast of Turkeyproduces a secondary lee trough along the Black Sea coast (Brody and Nestor, 1980). Our results alsodemonstrate that the number of cyclones affecting Turkey in a year can range from 20 to 25 duringdrought periods and from 40 to 50 during very wet periods.

4.3. Analysis of precipitation series

Turkish precipitation stations are categorized according to their location on the cyclone trajectories. Aclassified list of four precipitation stations for each path is presented in Table I. This has enabledcomparison of the precipitation data with the cyclone trajectories. Figure 5 depicts the total of fourselected station precipitation time series, thus, a relationship between total number of cyclones for eachpath with the precipitation data can be investigated. It is intuitively clear from the figures that the amountof precipitation is positively correlated with the frequency of low pressure systems. Precipitation data inwinter, 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 convectiveprecipitation in summer.



Table I. The selected list of precipitation stations classified according to their locationon the cyclone paths

Station Latitude (N) Longitude (E) Height (m)

Path 1 Samsun 41.28 36.33 4Sinop 21.01 35.1 32Giresun 40.55 38.23 37Trabzon 41.0 39.43 36

Path 2 Edirne 41.67 26.57 48Luleburgaz 41.24 27.21 46Tekirdag 40.59 27.21 3Goztepe 40.97 29.08 39

Path 3a Canakkale 40.13 26.4 6Bandırma 40.21 27.58 58Balıkesir 39.39 27.52 3Bursa 40.18 29.07 100

Path 3b Akhisar 38.55 27.51 93I: zmir 38.4 27.17 25Kutahya 39.25 29.58 969Usak 38.41 29.24 919

Path 4 Dortyol 36.51 36.13 28Gaziantep 39.05 37.22 840I: skenderun 36.55 36.11 4Mersin 36.48 34.38 3

4.4. The methodology for the determination of latitudinal 6ariation of subtropical jetstream and thegeneration of the jetstream index

A concise picture of the cyclone track climatology over Turkey can only be obtained when subtropicaljetstream and the frequency of the persistence of high pressure centres over the Mediterranean isconsidered. This is because the cyclones affecting the region are mainly due to the interactions betweenpolar and subtropical jetstreams. The meridional oscillation of subtropical and polar jetstreams is veryimportant in bringing warm and cold air together for cyclogenesis. That’s why the authors tried todetermine the latitudinal variation of the subtropical jetstream as a primary step. In order to estimate thevariation of the axis of the subtropical jetstream, ECMWF 250-hPa geopotential heights for the1979–1994 period obtained for the 1769 grid points lying on the latitudinal interval 20°–76°N andlongitudinal interval 40°W–80°E is used. From the data, the geostrophic wind patterns are obtained, andthe reference latitude for the estimations is accepted as 40°N by taking into account the approximatelatitude of Turkey.

The following formula is used:

Dref=%n

i, j=1

Vi, jdi

%n

i, j=1

Vi, j

,

where D is the reference of latitudinal variation of subtropical jetstream, Vi, j is the geostrophic velocity atlevel 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 maininterval window located on 20°–38°N and 16°–60°E. This main interval window is illustrated in Figure6. To detect whether there is a structure disorder interrupting the general variability in the windowingsystem or not, six different windows in the main window are taken. The variability of the subtropicaljetstream in each window is investigated and it is found that the general variability is kept intact.



By using the German Meteorology Catalogue surface and 500-hPa height maps, the number of thecyclones affecting Turkey, the number of blocking cases and their duration were found. In this section thevariation 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 overTurkey, and (iii) the number of observed cyclones. The factors important for the variation of thesubtropical jetstream latitude are also important for the cyclogenesis process, the behaviour of the cycloneand its path. The established equation showing the variability of the jetstream axis on a monthly basis isas follows:

Y=34.234047+0.069088X1−0.012314X2−0.068291X3,

Figure 5. Seasonal variation of total precipitation for all paths



Figure 5 (Continued)

where X1 is the number of observed blocking cases over Turkey, X2 is the period of observed blockingcases over Turkey and X3 is the number of observed cyclones.

The formula obtained above is calibrated by comparing it with the equation of the variation of thejetstream axis as explained in Section 2. Thus, by putting the related variables into the equation, themonthly jetstream index can be generated (Table II). In the variability of jetstream axis, this formulashows us that among the three factors the most important one is the number of observed blocking casesover Turkey. It is clear that the effect of the number of blocking cases is more important than theirduration and the number of observed cyclones (Table III). Another value in the equation of approxi-mately 34.2 depicts that there are other factors playing a role in the variability of the jetstream axis andin turn cyclone formation and motion.



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 Number ofThe axis ofblocking (day) subtropicalblocking cyclones

jet (°N)

January 16 100 28.73 80February 21 160 28.64 84March 23 151 28.72 83April 32 208 29.12 82May 24 143 29.56 74June 12 87 31.24 54July 18 112 31.84 42August 17 89 32.04 40September 19 117 31.52 45October 24 158 29.52 61November 28 186 29.15 68December 23 150 28.95 94

5. SUMMARY AND CONCLUSIONS

In this study a data set of cyclone frequency statistics is produced and the prevailing tracks of cyclonesare derived. The results revealed that Turkey is under the effect of five dominant trajectories. Investigationof the seasonal variability of the cyclone frequencies revealed that the highest number of cyclones detectedin Turkey occur in winter. Although northern parts of Turkey are accepted as having a transition climatetype between the Mediterranean and temperate regions, they are under the effect of low pressure systemsoriginating in the Mediterranean more, and the above result of more cyclones in winter proves this. It isalso illustrated that there is a positive impact of the number of cyclones on the precipitation amounts ofthe stations located on the trajectories of cyclones.

It can be concluded that Paths 1 and 2 are typical summer-time trajectories that bring summer stormsover the northern parts of Turkey. They bring abundant rain with them and in some cases flooding is nota surprise. In winter, cold air masses from the Balkans and northern Europe are associated with these



Table III. Comparison of El Nino and non El Nino years for 20 precipitation stations in Turkey

Duration of Number of Number of Precipitation (mm)blockingblocking (day) cyclones (winter)

(winter)

1982–1983 (ENSO year) 80 (minimum) 13 (minimum) 23 (higher) 4623.2 (higher)1988–1989 (non-ENSO year) 138 (maximum) 21 (maximum) 21 (lower) 3016.1 (lower)

Table IV. Total numbers of cyclones according to the paths

Years Path 1 Path 2 Path 3a Path 3b Path 4

1979 1 12 9 12 141980 2 15 12 24 131981 4 9 10 23 131982 5 20 7 14 141983 13 11 12 17 171984 3 9 16 21 111985 5 17 10 14 201986 5 5 8 24 111987 1 12 4 29 101988 9 11 10 21 41989 4 9 8 15 101990 8 10 2 26 71991 7 12 9 20 101992 7 11 4 14 10

trajectories. They are characterized by a substantial amplification of the planetary-scale long waves duringthe development phase of the cyclone that brings cold air to very low latitudes. Thus, it is not uncommonto 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 ofcyclones are generally associated with above normal temperatures in their warm sector and normal or alittle below normal temperatures at the back of the cold front. Air–sea interaction during the cyclogenesisprocess has a contribution to the moisture content of the cyclones, generally causing them to becomeunstable leading to thunderstorms.

The analysis of the variability of the jetstream axis revealed that the most important factor among threechosen factors is the number of observed blocking cases over Turkey. It is also shown that the effect ofthe 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 adaptedto a number of climatic and environmental applications. Assuming a longer period of homogeneous data,perhaps 30–40 years, the inventory of mean weather type properties and frequencies could serve as aclimatic baseline inventory for Turkey and the surrounding region.

This inventory would permit evaluation of monthly or seasonal regimes of favourable or unfavourableweather for industries and activities that are strongly weather dependent. The inventory could also serveas a regional yardstick for the evaluation of local micro-scale climates in connection with meso-scaleclimates and their effects on standard data. If a synoptic analysis could be standardized, the inventoriescould 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 activityfrom place to place as related to weather types and the general circulation and also illustrate themodification of properties associated with the various regional settings.

In order to understand the cyclogenesis processes in the Mediterranean Basin to a greater extent andestablish their effects on Turkey and the relationship between precipitation and cyclone tracks, similarstudies with longer data periods are needed.



ACKNOWLEDGEMENTS

The authors thank H. Nuzhet Dalfes for his valuable suggestions on calculating the subtropical jetstreamcore. This study is supported by the State Planning Office (DPT) and ITU Research Fund.

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cyclone track variability over turkey in association with regional climate

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