Atmospheric Circulation Influence on Heavy Showers Over Black Sea Coasts

1. Influence of the atmospheric circulation changes in the Northern Hemisphere on the formation of heavy showers over the Black Sea western and eastern coasts VELEV St.*, KONONOVA N.K.** *Institute of Geography BAN, st. Akad. G. Bonchev, bl. 3, Sofia - 1113, Bulgaria. **Institute of Geography RAS, Staromonetny, 29, Moscow, 119017. Russia. e-mail: st.velev@mbox.bol.bg; NinaKononova@yandex.ru

2. Introduction • Heavy showers are one of the extreme weather processes. They are responsible for such hazardous processes as floods, mudflows and landslides. The aim of this work is to reveal the types of circulations that form heavy showers over the western and eastern coasts of the Black Sea and to identify the changes of their frequency. Authors have collected data and information for 32 years from 1972 to 2004 using all recordings from the very beginning. This work is the continuation of research, which the authors began in 1972 (Velev, Kononova, 1979, 1979a, 1980, 1982).

3. Data • 1.In order to reveal links and differences between circulation conditions of heavy showers that fall over the western and the eastern coasts of the Black Sea, we have scrutinized the classification of the atmosphere circulation in the Northern Hemisphere according to B.L .Dzerdzeevskii at al (1946). This classification is carried out for the atmospheric processes over the extra tropical latitudes of the Northern Hemisphere. There are 13 types of atmospheric processes. Each of them is divided into subtypes, known as ‘elementary circulation mechanisms’ (ECM) and has its own letters. 41 ECM reflect all circulation processes over the Hemisphere. The average duration of each ECM is about 3-5 days. ECM vary with direction and amount of the arctic intrusions and the southern cyclone outlets. Letters a, b, c, d mean different directions of blocking and southern cyclone outlet, for example 2a, 2b, 2c. Letters 's' (summer) and ‘w' (winter) mean seasonal differences of the ECM, e.g. 13s, 13w. The ECM patterns have been grouped in four clusters. The first one is “zonal” (ECM types from 1 to 2). It is characterized by high pressure on the North Pole, low pressure on the sub-Pole and the middle latitudes. The second one is “disturbance of zonal circulation” (ECM types from 3 to 7). It is characterized by high pressure on the North Pole and one artic intrusion (blocking) in some sectors of the Hemisphere. The third type is “northern meridional”(ESM types from 8 to 12). It has high pressure on the North Pole and 2-4 artic intrusions in different sectors. The last, fourth one is “southern meridional” (ECM type 13). It has low pressure on the North Pole and cyclone outlets (from Mediterranean, sub-tropical Atlantic and Pacific districts). This classification gives opportunity to analyze the circulation conditions in a certain region of the Northern Hemisphere. There is History of ECM alternation for 1899 – 2004. • 2. We use daily precipitation sums from both coasts of the Black Sea that were recorded during 1951 - 1969 because the available information for this period is almost complete. This period we compare with last years – 1998 – 2004. • On the western coast we operate with the following meteorological stations: Kaliakra, Balchik, Varna, Necebor, Sozopol, Pomorie, Burgas, Obzor, Tzarevo, Rezovo and on the eastern coast - with Sochi, Krasnaja Poliana, Pskhu, Sukhumi, Poti, Chakva, Batumi.

4. Method • - Because the annual precipitation sum over the eastern coast is three times as heavy as the one over the western coast we, decided to take into consideration Fukui’s criteria (1970) in order to define catastrophic and heavy showers. According to these criteria the showers are heavy if the daily precipitation sum is not less than 5% of the annual sum of the current year and the showers are catastrophic if the daily precipitation sum is equal to or exceeds 10% of the annual sum of the current year. • - ECM for every case of extreme daily precipitation sum was show. • - The data, obtained on the basis of the aforesaid criteria, were divided into circulation seasons (Dzerdzeevskii, 1962; Savina, Khmelevskaya, 1984) which enables us to reveal the extreme precipitation sum for different years in the same circulation conditions.

5. Discussion • In the course of research the both coasts were divided into three regions according to the degree of influence of the northwestern and the southern cyclones. They are: • northern (Kaliakra, Balchik, Varna, Sochi, Krasnaja Poliana), • middle (Necebor, Sozopol, Pomorie, Burgas, Obzor, Pskhu, Sukhumi), • southern (Tzarevo, Rezovo, Poti, Chakva, Batumi). • We have found out that the extraordinarily heavy showers on these two coasts fall in different circulation seasons. The great differences between western and eastern coast in time of extraordinarily heavy rainfalls, quantity of extreme and connection between heavy shower and catastrophic precipitation have been revealed (table 1).

6. Table 1.Frequency (cases) of heavy showers (1) and catastrophic showers (2) from 1951 to 1969

7. Stations: 1 - Kaliakra, 2 - Balchik, 3 - Varna, 4 - Sochi, 5 - Krasnaja Poliana; 6 - Obzor, 7 - Necebor, , 8 - Pomorie, 9 - Burgas, 10 - Sozopol, 11 - Pskhu, 12 – Sukhumi; 13 - Tzarevo, 14 - Rezovo, 15 - Poti, 16 - Chakva, 17 – Batumi.

8. Heavy showers in the northern and middle regions of the western coast are more frequent as compared to those on the eastern coast in spite of the fact that the annual precipitation sum of the eastern coast is three times larger than that on the western coast. Heavy showers in the middle region of the western coast are more common than in the northern or the southern regions. • The damage from the catastrophic showers is more severe in the northern region of Bulgarian coast and the landslide activity is so great that people have to leave this part of the Black Sea coast (for example, Balchik). In this region construction of buildings is forbidden. • Over the eastern coastthe frequency of heavy showers increasesfrom north to south. So the catastrophic showers fall only in the southern region. • Over the eastern and the western coaststhe heavy showers maximum and minimum occur in different seasons. In the southern region of the western coast the showers have their autumn – winter maximum and summer minimum. Autumn maximum and spring minimum is observed in the southern region of the eastern coast. The winter normal precipitation sum exceeds the summer one.

9. The maximum of the heavy showers does not always conform to the maximum of the annual precipitation (Velev, Kononova, 1980, 1982). For example, in summer there is one maximum of heavy showers in the northern region of the western coast and there is another maximum in pre-winter and winter. Though the maximum of precipitation sum in the northern region of the eastern coast is observed in autumn and winter, the maximum of heavy showers is in summer. In the middle region of the western coast the maximum of frequency of the heavy showers is in pre-winter and winter ( the maximum sum of the precipitation is observed in the autumn - winter period). In the middle region of the eastern coast the maximum of the heavy showers is observed in summer. In the southern region of the western coast the maximum of the heavy showers is observed in pre-winter. On the eastern coast it is in summer. • On the western coast the maximum of frequency of the heavy showers conforms to the maximum of the annual precipitation sum. On the eastern coast, in spite of the annual march of precipitation sum, the maximum of the frequency of the heavy showers occurs in summer. • In order to reveal the circulation processes, which cause heavy showers, we have chosen all cases when heavy showers spread over the whole region of one of the coasts. ECM that form heavy showers we give in table 2.

10. Table 2ECM that form heavy showers (the daily sum is 5% of the annual sum)

12. Table 2, the continuation

14. 1 – ECM; 2 – number of cases

15. During the cold period in winter and pre-spring in allregions except for the middle one of the eastern coast the heavy showers are associated with ECM 11a: Mediterranean cyclones move over the Bulgarian coast and the Black Sea. The centers of these cyclones are over the western coast. In every case the trajectory of cyclones may be different (Pisarsky, 1955). • In some cases the Mediterranean cyclones bring heavy showers to the northern and the middle regions of the western coast in spring (ECM 13w, 10a). In other cases the heavy showers fall over the western coast on the cold front of the Atlantic cyclones (ECM 7as, 7bs, 9a). The heavy showers fall over the eastern coast on the fronts of the Mediterranean cyclones (ECM 8bs, 12a, 12bs).

16. In summer the Mediterranean cyclones bring heavy showers to both coasts with ECM 2a and 13s. In the northern and the middle regions of the western coast the heavy showers are brought with ECM 12a and 7as; in the northern and the middle regions of the eastern coast – with ECM 4b, 6, 8a; in the southern region – with more than 10 ECM (Table 2). • In autumn the Mediterranean cyclones bring heavy showers to both coasts of the northern region with ECM 13s, to both coasts of the middle region – with ECM 11a and 11c, to both coasts of the southern region – with ECM 1a, 6, 7as, 10a and 11a. • In pre-winter the Mediterranean cyclones bring heavy showers to both coasts of the northern region with ECM 8cw and to both coasts of southern region with ECM 5d.

17. So the heavy showers over both coasts are related to the Mediterranean cyclones. The heavy showers depend on the concrete trajectory and depth of the cyclone. • Catastrophic showers (Table 3) fall predominantly over the western coast. Over the eastern coast only 3 cases have been established for 19 years. The precipitation is in the form of heavy showers with thunderstorms on the fronts of the Mediterranean cyclones in the southern region during the summer-autumn period. • Over the western coast the catastrophic precipitation falls mainly in summer and autumn on the fronts of the Mediterranean cyclones. In summer ECM 13s and 4c bring catastrophic precipitation, in autumn – ECM 7as and 12bw, in pre-winter – ECM 5c, 8bw, 11a and 13w.

18. Table 3ECM that form catastrophic rainfalls (10% of the annual sum a day)for the period 1951 - 1969

19. A – ECM; B – cases. The number of stations is the same as in table 1.

20. On 20 August 1951 catastrophic precipitation (258 mm) occurred in Varna, due to ECM 4c (Khristov, Pisarsky, 1956). It is the largest daily precipitation sum that has ever been recorded in the whole Bulgaria region. The cyclone, formed on 19 August 1951 over Bulgaria with its center near Varna, was the cause for this phenomenon. The cold front was not active. The vertical and horizontal thermal gradients were very great. The nearness of the sea – coast border and the convection of the tropical air on the edge of the cold Atlantic air were very important. They caused the catastrophic precipitation. • The same causes and the same ECM produced heavy showers and catastrophic precipitation in the years 1998 - 2004 (Table 4).

21. Table 4.Heavy showers and catastrophic precipitation over the western coast during 1998 - 2004

23. Changes in the atmospheric circulation processes in the last decades (Kononova, 2003) have led to the increase in the southern outlets of cyclones from the Mediterranean Sea. Now the heavy and the catastrophic showers on western coast are chiefly associated with the ECM 11a, 11b, 12a, 12bw, 13w, 13s (table 4). Landslides were activated near Varna as a result of heavy showers which continued two days: on 29 September 2002 the recorded amount was 27 mm and on 30 September 2002 - 42 mm. • On the eastern coast catastrophic showers fell in January, June and August of 2002.

24. Table 5Catastrophic rainfalls in the northern region of the Black Sea eastern coast in the beginning of August 2002 They caused the catastrophic floods because the daily precipitation sums were more than the average monthly sum (Table 5). In August they initiated mudflow activity in the mountainous regions as well. The catastrophic showers fall on the fronts of the Mediterranean (ECM 12a and 12cs, table 5) and Atlantic (ECM 9a) cyclones.

25. Figure 1. The deviations from long-term average of the annual ECM duration and annual precipitation sums: a) ECM that bring heavy and catastrophic showers primarily tо the western coast; b) ECM that bring heavy and catastrophic showers primarily tо the eastern coast; c) the deviations of annual precipitation sums over the western coast; d) The deviations of annual precipitation sums over the eastern coast.

26. Within the ECM, that recently have been forming heavy and catastrophic showers over both coasts, most distinct are ECM 12a, 13w and 13s. The annual duration of each of the ECM for the last decade is longer than the average for the period from 1899 to 2004. Their total duration is 171 days per year for the last decade, i.e. 47% of the year. The tendency towards changes of the annual ECM duration and annual precipitation sums is shown in Fig. 1. The deviation of annual duration of ECM that bring heavy and catastrophic showers primarily to the western coast from their average for 1899-2004 (Fig 1a) at present are more than in 1951-1969. Their annual duration is more than average. The annual duration of ECM 12a, 13w and 13s, producing heavy and catastrophic showers over both coasts, increased and has positive deviations (Fig. 1b). The duration of ECM 9a that brings heavy and catastrophic showers mainly to the eastern coast (Fig 1b) was also extended. As a result, the amplitude of annual precipitation sums over the western coast grew (Fig 1c), but the frequency of the heavy and catastrophic showers decreased. Over the eastern coast the annual precipitation sums and frequency of the heavy and catastrophic showers have increased in recent years.

27. Conclusion • There are two circulation systems in the middle part of the atmosphere, necessary for the formation of heavy or catastrophic showers over the Black Sea coasts. on that • 1. Truncated cyclone locates to the northeast of the Scandinavian Peninsula. barometric trough occupies all Europe up to the Mediterranean Sea. On the southeast periphery of this barometric trough the Mediterranean cyclones move towards the Black Sea. In this case the heavy and the catastrophic showers fall on the fronts of the Mediterranean cyclones. This phenomenon is typical for the cold period (ECM 11a, 13w, 11c, 12bw, 12cw, 12d) and the transitional seasons (12a, 12bs). In summer similar circulation systems form heavy and catastrophic showers with ECM 13s. • 2. Truncated cyclone locates near Iceland. Its barometric trough stretches southeastward and occupies south Europe. In the southern part of the Russian Plain and over the Balkan Peninsula a truncated cyclone is likely to form. In this case the heavy and the catastrophic showers fall on the fronts of the Atlantic cyclones. This phenomenon is typical for the warm period (ECM 4c, 7as, 9a). • 3.The high frequency of the extreme rainfall will remain on the eastern coast of the Black Sea because the total duration of ECM that brings large amount of heavy or catastrophic showers coast of the Black Sea is greater than the average one.