Table 1. Climatic stations in Bulgaria under the present study. Meteorological stations. Latitude. Longitude. Eleevation. (m). Pleven. 43o 25'. 24o 35'. 133.
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Rainfall Variability In Bulgaria And Its Relation With North Atlantic Oscillation Nina Nikolova Sofia University “Saint Kliment Ohridski” Sofia, Bulgaria
Abstract The paper gives information on monthly and annual variations of precipitation in Bulgaria and their relation to North Atlantic Oscillation (NAO). The main research questions are 1) whether the trend observed in Bulgaria corresponds to the global climate change and 2) what is the influence of the NOA of rainfall variability in Bulgaria. Linear regression equations are calculated individually for each station for two periods 1931-2000 and 1961-2000 in order to characterize the long-term and more recent trends. The trend is negative and better determined for wintertime months and annual precipitation totals. Correlation analysis is used to define the relationship between precipitation and NAO. The correlation coefficients are negative. Statistical significant coefficients are calculated for January, February and March. Key words: precipitation totals; linear trend; North Atlantic Oscillation
Introduction The variations of climate exert strong influence on the productivity of agriculture and on various aspects of human activity. The precipitation is a key element of climate, which determines the availability of drinking water and the level of the soil moisture. Changes in precipitation could have a significant impact on society. That is why the variability of precipitation has drawn increasing attention in the recent years. Recent trends of precipitation in Europe are analyzed by many authors. Studies on variability show that precipitation is decreasing in Central and Southern Europe. According to Brazdil et al. (1994) significant linear trends of precipitation in Central and Southeastern Europe occur rarely. Recent climate variability in Bulgaria has been investigated by Topliiski (2002), Zlatunova & Penkov (2002) etc. However, in-depth statistical and geographical analysis needs to be done in order to throw light on some still open questions on recent climate change in regional scale. The paper is intended to provide information about monthly and annual variation of precipitation in Bulgaria and its relation with North Atlantic Oscillation (NAO). The aim of this study to answer the questions: 1) what is the influence of the NOA of rainfall variability in Bulgaria and 2) whether the observed trend of rainfall variability in Bulgaria corresponds to the global climate change? The research will primarily focus on NAO which is one of the major modes of variability of the Northern Hemisphere atmosphere. The NAO is an index of the difference in mean-sea-level pressure recorded at the Azores and Iceland. It is the object of the most climate studies (Hurrell, 1995; Osborn, 1998) which are based on the analysis of monthly and seasonal means.
Study area Data and methods The present analysis is based on the monthly and annual precipitation totals observed at 10 meteorological stations in Bulgaria. The stations are representative of regions with different geographical and climatic conditions. The main investigated period is 1931 – 2000 and the reference period is 1961-2000. The precipitation data are taken from the meteorological yearbook published by the National Institute of Meteorology and Hydrology, Sofia, Bulgaria and from the Statistical Yearbook of the National Statistical Institute, Bulgaria. The stations are listed in Table 1. In order to discuss the relationship between rainfall variability and NAO, the data on NAO indices (NAOI) (Phill Jones, based on the method given by Ropelewski & Jones, 1987) are also used in this research. The NAOI is defined as the normalized pressure difference between a station on the Azores and one on Iceland.
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In order to characterize the long-term and more recent trends linear regression equations are calculated. The type of regression equations is: y = a.t + b where y is the value of precipitation totals and t is the serially consecutive year within the period for which the equation is calculated. The parameter a indicates the sign of the trend. The coefficient of determination (Byt ) is used to estimate the influence of independent variable (t) on the precipitation (y). To determine the significance of coefficient of determination the F-statistic is used (Foerster & Roenz, 1983). Correlation analysis is used to define the relationship between precipitation and NAO. Table 1. Climatic stations in Bulgaria under the present study Meteorological Latitude Longitude Eleevation stations (m) Pleven 43o 25' 24o 35' 133 Obrazcov chiflik 43o 48' 26o 21' 159 o Vidin 43 39' 22o 53' 37 Sofia 42o 42' 23o 20' 550 o Kazanlyk 42 37' 52o 24' 380 Kyustendil 42o 17' 22o 41' 520 Varna 43o 12' 27o 55' 12 Haskovo 41o 58' 25o 35' 194 o Sandanski 41 33' 23o 16' 191 Cherni vryh 42o 35' 23o 16' 2286 The series of the mean monthly precipitation have normal distributions according to the parametric assessment made by the method of the central moments. The initial series are homogeneous (Topliiski, 2002, Topliiski, personal communication). Climate Bulgaria is situated on the Balkan Peninsula in Southeastern Europe. The climate is moderate continental with Mediterranean ascendancy. Geographical situation, relief and atmospheric circulation are the main factors, which determined the diversity of Bulgarian climate. The weather processes are determined by the transformed oceanic air masses from West and North-West and by the continental air masses of temperate latitudes pushing in from North-East. Dominant circulation factors for Bulgaria are described by Martinov (1991). During the summer the Subtropical (Azores) high spread out to North-east and Balkan peninsula and Bulgaria are on its influence. This determines dry weather in Bulgaria. The summer precipitation is determined by Atlantic cyclones. During the cold half of the year the climate is formed to some extend by Mediterranean cyclones which are more frequent in January and December. An analysis of spatial distribution of precipitation in Bulgaria is made by Koleva (1991). The author shows that annual precipitation totals in Bulgaria are between 500 mm for lowlands and 1000-1400 mm for high mountain. The precipitation maxima in most parts of the country are during May – June, and in South-west and South-east part the maxima are during December – January.
Results Linear trend The linear trend method is used by many authors for defining the sign of the trend of the climate change on global and regional scales. Over the 20-th century, annual zonal averaged precipitation increase by 7 to 12% for the zones 30oN to 85oN (Folland et al., 2001). During the 1900 – 1995 there were relatively small increases in global land areas experiencing severe drought or severe wetness. On the background of global changes many papers show different tendencies of precipitation change in regional scale. Piervitali et al. (1998) have found dry wintertime conditions over southern Europe and Mediterranean. The trends of monthly and annual precipitation totals in Bulgaria are discussed from Koleva (1995), Vasilev (1996), Vekilska &. Rathcev (2000), Velev (1996, 2000), Zlatunova & Penkov (2002). The authors indicate decreasing of precipitation in Bulgaria. During the 1981 – 1990 the precipitation are 10 % lower than precipitation during the 1961 – 1990 and during 1985 – 1995 the decrease is bigger (Koleva, 1995). Velev (1996) shows similar results. Annual precipitation amounts during 1982 - 1993 decreased by 7.5 – 18 % as
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compared to the average amounts during 1954 – 1993 and the decrease is larger in South Bulgaria than in North Bulgaria. Droughts occur primarily in January, May, September and October. The results from the present research show that for both periods under review (1931-2000 and 1961 – 2000) most part of Bulgaria has negative trend of monthly and annual precipitation totals. In more recent years, (1961-1990), the negative trend of annual precipitation totals is clearly determined (Table 2.). The decrease of precipitation in wintertime is most significant (Table 3.). The decrease of precipitation in January is well expressed in Southwest part of the country. Winter drought in Bulgaria is under the influence of Siberian anticyclone and arctic air pushed in Bulgaria from the north (Slavov et al., 2000). Other causes for dry conditions in Bulgaria in the last decades are the reduced number of Mediterranean cyclones (Velev, 1996). Significant negative trend of winter and spring months and annual precipitation totals occur in high mountain (Table 3, station Cherni vryh). Topliiski (2002) also shows the negative trend of annual precipitation totals in South-west part of Bulgaria and in high mountain. Table 2: Trend of annual precipitation totals in Bulgaria (1931-2000 and 1961-2000) Meteorological station
1931-2000
1961-2000
F B F Trend B Trend Pleven 0.00 0.01 1.14 1.18 -0.08 -1.74 Obrazcov chiflik 0.00 0.07 0.08 0.08 -0.16 0.44 Vidin 0.05 3.58 3.04 3.30 -1.35 -2.48 Sofia 0.07 5.12 1.52 1.58 -1.56 -1.91 Kazanlyk 0.05 3.58 4.94 5.68 -1.28 -3.36 Kyustendil 0.13 10.16 5.32 6.19 -2.37 -3.17 Varna 0.00 0.20 0.01 0.01 0.29 -0.57 Haskovo 0.01 0.69 3.80 4.22 -0.65 -3.56 Sandanski 0.15 12.00 0.38 0.38 -2.24 -2.18 Cherni vryh 49.24 22.80 57.00 -10.76 0.42 -17.44 B – coefficient of determination; F – statistic, value above 4 indicate linear and significant trend Table 3: Trend of monthly precipitation in Bulgaria (1961-2000); B – coefficient of determination; F – statistic, value above 4.1 indicate linear and significant trend Jan
Feb
Mar
April
May
Jun
Station
Trend B
F
Trend B
F
Trend B
F
Trend B
F
Trend B
F
Trend B
Pleven
-0.45
0.04
1.58
-0.50
0.10
4.22
-0.01
0.00
0.00
0.26
0.01
0.38
-0.30
0.01
0.61
-0.31
0.01 0.38
Obrazcov chiflik -0.39
0.03
1.18
-0.05
0.00
0.00
0.15
0.00
0.11
0.17
0.00
0.15
-0.27
0.01
0.61
0.07
0.00 0.04
Vidin
-0.29
0.02
0.78
-0.45
0.03
1.18
-0.54
0.07
2.86
-0.08
0.00
0.04
-0.29
0.01
0.52
-0.99
0.09 3.76
Sofia
-0.16
0.01
0.38
0.05
0.00
0.04
-0.16
0.01
0.38
0.08
0.00
0.04
-0.82
0.08
-1.49 -0.34
0.01 0.38
Kazanlyk
-0.65
0.10
4.22
-0.17
0.01
0.27
-0.43
0.05
2.00
-0.28
0.02
0.78
-0.31
0.01
0.61
0.02 0.78
Kyustendil
-0.84
0.13
5.68
-0.20
0.00
0.04
-0.58
0.07
2.86
-0.20
0.01
0.38
-0.71
0.09
-1.41 -0.33
0.02 0.78
Varna
-0.51
0.03
1.18
-0.61
0.06
2.43
-0.01
0.00
0.00
0.54
0.06
2.43
-0.67
0.12
-1.28 0.01
0.00 0.00
Haskovo
-0.74
0.04
1.58
-0.08
0.00
0.04
0.36
0.01
0.38
-0.12
0.04
1.58
-0.62
0.03
-8.14 -0.83
0.09 3.76
Sandanski
-0.73
0.10
4.22
-0.20
0.00
0.04
-0.67
0.09
3.76
-0.11
0.00
0.15
-0.23
0.01
0.61
-0.23
0.01 0.38
Cherni vryh
-2.23
0.30
16.29 -2.00
0.30
16.29 -1.54
0.20
9.50
-1.15
0.10
4.22
-1.63
0.20
-1.15 -1.37
0.09 3.76
Jul
Aug
Sept
Oct
Nov
0.42
F
Dec
Station
Trend B
F
Trend B
F
Trend B
F
Trend B
F
Trend B
F
Trend B
Pleven
0.28
0.01
0.38
-0.19
0.00
0.15
0.10
0.01
0.38
-0.17
0.01
0.38
-0.13
0.00
0.15
-0.30
0.02 0.78
Obrazcov chiflik -0.11
0.00
0.04
-0.50
0.02
0.78
0.18
0.00
0.04
0.82
0.10
4.22
0.10
0.00
0.04
0.28
0.01 0.38
Vidin
0.39
0.01
0.38
0.12
0.00
0.08
0.11
0.00
0.04
-0.17
0.00
0.11
-0.30
0.01
0.35
0.01
0.00 0.00
Sofia
-0.39
0.02
0.78
0.08
0.00
0.04
0.21
0.01
0.19
0.09
0.00
0.04
-0.40
0.03
1.18
-0.13
0.00 0.15
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Kazanlyk
-1.14
0.13
5.68
-0.34
0.02
0.78
0.28
0.01
0.38
-0.36
0.01
0.38
-0.48
0.03
1.18
0.11
0.00 0.04
Kyustendil
-0.36
0.01
0.38
0.00
0.00
0.00
0.24
0.00
0.04
0.22
0.01
0.38
-0.53
0.03
1.18
0.12
0.00 0.00
Varna
0.20
0.01
0.38
-0.05
0.00
0.04
0.57
0.03
1.18
0.40
0.02
0.78
0.01
0.00
0.00
-0.45
0.03 1.18
Haskovo
-0.76
0.08
3.30
0.21
0.00
0.15
-0.74
0.05
2.00
-0.26
0.00
0.15
0.28
0.01
0.38
-0.28
0.00 0.15
Sandanski
0.00
0.00
0.00
0.09
0.00
0.08
0.06
0.00
0.00
0.28
0.01
0.38
-0.67
0.04
1.58
0.21
0.01 0.38
Cherni vryh
-0.83
0.03
1.18
-1.13
0.06
2.43
-0.45
0.01
0.38
-0.62
0.03
1.18
-1.81
0.40
25.33 -2.69
0.40 25.3 3
Correlation between monthly and annual precipitation totals and north atlantic oscillation The NAO is one of the major modes of the Northern Hemisphere atmosphere. Rimbu & Boroneant (2000) show that 32% of the total variance of the decadal winter precipitation in Europe is explained by spatial structure associated to the NAO. NAO occurs in all seasons but during winter it has a dominant role. During the positive phase of NAO negative precipitation anomalies occur over the southern Europe (Wibig, 1999). This fact is confirmed by the results of the present research. The correlation coefficients between monthly mean precipitation for selected meteorological station from Bulgaria and NAO indices were calculate in order to determine the role on NAO in the precipitation variation. The correlation coefficients are negative for most of months. Positive coefficients are established for some stations during the summer for period 1961-2000 and for period 1931-2000 – for April, May and August (Fig. 1). Statistical significant coefficients are found for January, February and March. The influence of the NAO increases from north to south. a)
0.3 0.2 Pleven
correlation coeficients
0.1
Obrazcov chiflik
0
Vidin Sofia
-0.1
Kazanlyk
-0.2
Kyustendil
-0.3
Varna Haskovo
-0.4
Sandanski
-0.5
Cherni vryh
-0.6 -0.7 Jan
Feb
Mar
Apr
May Jun
Jul
Aug Sept Oct
Nov Dec
b) 0.4
correlation coeficients
0.2
Pleven Obrazcov chiflik Vidin
0
Sofia Kazanlyk
-0.2
Kyustendil Varna
-0.4
Haskovo Sandanski
-0.6
Cherni vryh
-0.8 Jan
Feb
Mar
Apr
May
Jun
Jul
Aug Sept
Oct
Nov
Dec
Figure 1 Correlation coefficient between monthly precipitation and NAO: a) for period 1931–2000; b) for period 1961-2000 Climate
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The correlation analysis between the time series of the NAO and annual precipitation totals in Bulgaria shows negative correlation for both periods – 1931 – 2000 and 1961 – 2000. The relationship is stronger during the long period (1931 – 2000). Table 4. presents the correlation coefficients between NAO and annual precipitation totals for different periods for selected meteorological stations. The results of the present research show that NAO has important impact on rainfall variability in Bulgaria for wintertime. The climatic conditions in Bulgaria are influenced by the transformed oceanic air masses from West and North-West and by the continental air masses of temperate latitudes pushing in from North-East. During the cold half of the year the climate is formed to some extend by Mediterranean cyclones. Activation of zonal air transfer (positive phase of NAO) makes the formation of Mediterranean cyclones very rare. This is a cause for decrease of precipitation (Velev, 2000).
Conclusion On the background of the global climate change the results of present research show the negative trends for monthly and annual precipitation totals. This tendency is statistical significant for wintertime. It is clearly determined for high mountain regions. Decreasing of precipitation in Bulgaria may lead to significant problems with drinking water supply and reduced streamflow volume in the country. The results of the research will have importance for better understanding the relationship between rainfall and NAO. The correlation is negative for most of months and annual values and is well established for wintertime. The knowledge on rainfall variability and physical mechanisms responsible for this variability could be applied in risk management and land use planing in investigated regions. Acknowledgements The author would like to thank Assoc. Prof. D. Topliiski for providing some part of precipitation data and for useful discussion on investigated problem. Reference Brazdil, R., P. Dobrovolny, J. Mika, T. Niedzwiedz & N. Dalezios (1994). Contemporary climate tendencies in selected regions of Central and Southern Europe. In Contemporary Climatology, (eds. By R. Brazdil & M. Kolar), (Proc. of the meeting of the Commission on climatology of the International Geographical Union,August, 1994, Brno, Czech Republic, 73-82 Ferster, E. & B. Renc. Metodi koreliacionnogo i regersionnogo analiza, Moskva, Finansi I statistika, 1983. (The methods for correlation and regression analysis, Moscow, Finance and statistic, 1983) Folland, C. K., T. R. Karl, J. R. Christy, R. A. Clarke, G. V. Gruza, J. Jouzel, M. E. Mann, J. Oerlrmans, M. J. Salinger & S. W. Wang, (2001): Observed climate variability and change. In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (eds . by Houghton, J. T., Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell and C. A. Johnson). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Hurrell, J. M.(1995). Decadal trend in the North Atlantic oscillation: regional temperature and precipitation, Science, 269, 676–679. Koleva, E. (1991). Razpredelenie na walevite. In: Klimatyt na Bulgaria (ed. by. Sv. Stanev, M. Kyuchukova & St. Lingova), Sofia, Bulgarian Academy of science, 225-237 (Percipitatiopn ditribution. In: Bulgarian climate (ed. by. Sv. Stanev, M. Kyuchukova & St. Lingova), Sofia, Bulgarian Academy of science, 225-237, in Bulgarian) Koleva, E. (1995). Drought in the lower Danube basin. Drought network news. Vol. 1, N 1, Feb. Nebraska, USA, 6-7 Martinov, M. (1991). Atmosfernata cirkulacia nad Balkanskia poluostrov i nashata strana. Klimatyt na Bulgaria (ed. by. Sv. Stanev, M. Kyuchukova & St. Lingova), Sofia, Bulgarian Academy of science, 4053. (Atmospheric circulation on Balkan peninsula and our country. In : Klimatyt na Bulgaria (ed. by. Sv. Stanev, M. Kyuchukova & St. Lingova), Sofia, Bulgarian Academy of science, 40-53, in Bulgarian) Osborn, T. J. (1998). The North Atlantic Oscillation: observations, models and paleodata _ 2nd European Conference on Applied Climatology, 19 to 23 October, Vienna, Austria; Central Institute for Meteorology and Geodynamics, Vienna, Austria. Climate
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Piervitali, E., M. Colacino & M. Conte, (1998). Rainfall over the Central – Western Mediterranean Basin in the Period 1951 – 1995. Part I: Precipitation trends. Geophysics and Space Physics, 21 (3), 331–334 Rimbu, N. & C. Boroneant, (2000). Decadal climate variability over the Europe during winter and its relation with the Atlantic sea surface temperature. (ed. by T. Mikami), (Proc. of the International Conference on Climate Change and Variability, Tokyo, Japan, 1999), 225-230. Ropelewski, C. F. & Jones, P. D.(1987). An extention of the Tahiti – Darwin Southern Oscillation Index. Mon. Wea. Rev, 115, 2161–2165 Slavov, N., E. Koleva & V. Alexandrov (2000). Klimatichni osobenosti na zasushavaneto v Bulgaria. Bulgarian Journal of Meteorology & Hydrology, vol. 11, N 3-4, 100-113. (Climatic characteristics of drought in Bulgaria. Bulgarian Journal of Meteorology & Hydrology, vol. 11, N 3-4, 100-113) Topliiski, D., (2002) Klimatichni promeni v Bulgaria za perioda 1901-1990. Nauchna konferencia s mejdunarodno uchastie v pamet na prof. Dimitar Yaranov, Varna, Bulgaria, 98-106. (Climatic changes in Bulgaria in the period 1901 – 1990. Proceedings of the International Scientific Conference in Memory of Prof. Dimitar Yaranov, Varna, Bulgaria, 98-106. in Bulgarian). Topliiski, D., (Personal communication),. Climate change in Bulgaria. Vasilev, I. (1996). River runoff changes and the recent climatic fluctuations in Bulgaria. Geojournal, vol. 40.4. Vekilska, B. & G. Rathcev, (2000). Current changes in the precipitation in Bulgaria. Sofia University Year Book, Vol. 90, Geography., 31-37. Velev, St. (1996). Is Bulgaria become warmer and drier. Geojournal, 40.4, 363-370. Velev, St. (2000). Globalnite promeni i klimatyt na Bulgaria. Sbornik ot dokladi. Mejdunarodna nauchna sesia 50 godini Geografski institut, Sofia, Bulgaria, 99-107. (Velev, St. Global changes and climate of Bulgaria. Proceedings of the International scientific session 50 years Institute of Geography, Sofia, Bulgaria, 99-107, in Bulgarian). Wibig, J., (1999). Precipitation in Europe in relation to circulation patterns at 500 hPa level. Int. J. Clim., 19: 253–269 Zlatunova, D. & I. Penkov. River runoff and climate change in Bulgaria – Proceedings of the International Conference “Preventing and Fighting Hydrological Disasters”, 2002, Timisoara, Romania. 413-416.
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