About some anomalies in precipitation regime in Bulgaria

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1 About some anomalies in precipitation regime in Bulgaria Ivan Drenovski, Krasimir Stoyanov SWU Neofit Rilski, Blagoevgrad, Bulgaria Abstract: An attempt for analysis on the precipitation regime in the part of territory of Bulgaria in last 14 years is made. As a base a data published in monthly hydro-meteorological bulletins from 1995 till now are used. Some anomalies in annual trade of precipitation are revealed. Some suggestions about their causes are expressed. Keywords: Precipitations, Anomalies, September, Period of 14 years. 1. INITIAL DATA Data about month s precipitations for 16 stations in Bulgaria for the period of 14 years ( ) are used. Most of them (for 14 stations) are gathered from national month s hydro-meteorological bulletins[3]. For 2 stations data are obtained through internet. Unfortunately there are no more available and reliable comparative data for other stations in Bulgaria. The entire period of published data in national month s hydro-meteorological bulletins cover 17 years from 1992 to But the first three years (1992, 1993, 1994) are omitted in analysis, because they are abnormally dry. According [6][7] these three years are ones of driest from the beginning of meteorological observation in Bulgaria. The probability of occurrence of two contiguous extremely dry years as 1993 and 1994 is estimated to 0.1%. 2. REGIME OF PRECIPITATIONS IN BULGARIA All climatologists, dealing with annual distribution of precipitation in Bulgaria [1],[4],[5],[6],[7] record that two main regime types are observed. First of them is called tempered-continental with maximum in May-June and minimum in February (rarely in September). The second one continental-mediterranean is known by maximum in November-December (rarely in January) and minimum in August (rarely in September). Considerable part of Bulgarian territory has transitional regime with two maximums and two minimums. The main maximum and minimum are respectively in May- June and August-September and the secondary ones in November- December and February-March [4].

2 3. RESULTS Some different and considerable changes occurred in precipitation regime in the most of examined stations. 1. In all 16 stations amazingly big increase of rainfall in September is observed (Tab. 1). Tab. 1: Absolute and relative increase of September rainfalls for period in comparison with period Station Precipitation Precipitation Increase of September Change in mm in mm rainfalls of an nual mm (%) rainfalls (%) Varna Dobrich 32.0 ( Razgrad Sliven Russe V.Tarnovo Blagoevgrad 26.4 ( Pleven Vidin Kardzhali Burgas Plovdiv Sandanski Vratsa Sofia Kyustendil (1 The data are for period (2 The data are for period The most considerable increase of September precipitations, between 2,2 and 2,75 times, is observed in six station located mainly in eastern and northeastern part of the country. Absolute values of rainfalls growth is between 43 and 51 mm.

3 In the central and western part of Northern Bulgaria, as in the Blagoevgrad the increase is from 83 to 108%. In absolute values growth mm is recorded. In these stations (excluding Pleven and Blagoevgrad) the annual rainfall maximum is moved in September (Fig.1). To this group of stations should be included Vratsa station with more than 26 mm absolute growth. The relative increase in this case is deceitful small, due to the very high background value of September rainfalls 59 mm. mm Month's rainfall distribution for the most part of Northern Bulgaria ( ) months Varna Dobrich Russe V.Tarnovo Vidin Sliven Fig. 1: Month s rainfall distribution for the most part of Northern Bulgaria ( ) Least of all is the increase of September precipitations in South Bulgaria between 31 and 71%. Absolute growth of rainfalls here is from 12 to 23 mm. But in some stations the amount of September rainfalls becomes the second big in the year (Fig.2). Based on these two diagrams a considerable growth of rainfall in July and August can be revealed too. It regards to some stations in Northern Bulgaria. Relative increase is about 15-25%, but in some cases can reach up to 30%. This increase leads to tendency the sum of precipitation in July to become maximum or the second big in the year in tree stations - Pleven, Vidin, Dobrich. In Russe it is due to the decrease of May and June rainfalls.

4 mm 90 Month's rainfalls in some stations for period months Burgas Pleven Vratsa Blagoevgrad Plovdiv Fig. 2: Month s rainfall in some stations for period 2. As just have been mentioned, conversely in some months of the year significant decrease of precipitations take place (Table 2). Tab. 2: Relatively decrease (in %) of precipitations in chosen months for period Station/ January April May June November months Kurdzhali Sandanski Kyustendil Sliven Burgas Vidin Vratsa Pleven 34.3 V. Tarnovo Russe 34.6 Varna Plovdiv Unfortunately it happened mainly when the maximum of rainfalls to the specific climate zone in Bulgaria supposed to be. In continental-

Mediterranean zone in South Bulgaria these are the months from November to January. Average value of rainfall reduction in November and January is about 25%.

5 Mediterranean zone in South Bulgaria these are the months from November to January. Average value of rainfall reduction in November and January is about 25%. In fact in December in some stations relative increase is recorded, but it can t balance winter decrease of precipitations. In tempered-continental climate zone the biggest decrease is recorded in June (average 25-30%). If the decrease in April and May (average 15-20%) would be given in account, a very disturbing reduction of spring rainfalls promises to become. In transitional-continental climate zone both of pointed tendencies are revealed. Cartographic interpretation is done on Fig.3. Fig.3. Relative change of precipitation monthly sums for period

6 4. COMMENTS Most likely the main reason for pointed changes in precipitation regime in Bulgaria is connected with changes in atmosphere circulation in our region, as [7] and [4] presume. There is no statistical evidence for increase of rainfall synoptic situations in September. Years with intense cyclonic activity alternate ones with almost no occasions of cyclones in September. And what is more the number of passed cyclones is not in a direct ratio to rainfall amount. In some cases in one and only situation in September fallen rain amount can be more than a monthly norm. Typically such situations arise when a movement of Mediterranean depression from south-west is prevented by an anticyclone lying northerly or north-easterly from Bulgaria. Usually such combination is most common in the winter. A numerous other reasons are enumerated in specific paper [2]. As matter to precipitation decrease in November and January, most probably it is related with the reduced number of Mediterranean cyclones in winter, as earlier was observed by [7]. A rainfall reduction in June and May probably may be explained with smaller number of Atlantic depressions in the spring. This assumption need to be proved further. 5. CONCLUSIONS Taking in account the relatively short period of observation (14 years) no definite conclusions can be done. Very likely all these anomalies in precipitation regime in Bulgaria are associated with global climate change. The question whether is this stable tendency or casual climate fluctuation remains opened. 6. REFERENCES [1] Dimitrov, D. (1979). Climatology of Bulgaria, Science & Art, Sofia, 1979, 253p. [2] Drenovski, I., Kr. Stoyanov. (2009) Increase of September rainfalls in Bulgaria for period, (in Bulgarian) Problems of Geography (in press). [3] National month s hydro-meteorological bulletins ( ), NIMH, BAS, Bulgaria. [4] Toplijski, D. (2006) Climate of Bulgaria, Amstels, S.355p., (in Bulgarian). [5] Velev, St. (1990) The Climate of Bulgaria, Narodna prosveta, Sofia, (in Bulgarian).

7 [6] Velev, St. (1997). Contemporary variations of air temperature and precipitations In "Geography of Bulgaria", Academic publishing house "Marin Drinov", Sofia, 1997, с (in Bulgarian). [7] Velev, St. (2002). Contemporary variations of air temperature and precipitations In "Geography of Bulgaria", publishing house "ForCom", Sofia., 2002, (in Bulgarian).

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