Theor Appl Climatol DOI 10.1007/s00704-015-1633-5
ORIGINAL PAPER
Circulation factors affecting precipitation over Bulgaria Peter Nojarov 1
Received: 12 November 2014 / Accepted: 28 August 2015 # Springer-Verlag Wien 2015
Abstract The objective of this paper is to determine the influence of circulation factors on precipitation in Bulgaria. The study succeeds investigation on the influence of circulation factors on air temperatures in Bulgaria, as the focus here is directed toward precipitation amounts. Circulation factors are represented through two circulation indices, showing west-east or south-north transport of air masses over Bulgaria and four teleconnection indices (patterns)—North Atlantic Oscillation, East Atlantic, East Atlantic/Western Russia, and Scandinavian. Omega values at 700-hPa level show vertical motions in the atmosphere. Annual precipitation trends are mixed and not statistically significant. A significant decrease of precipitation in Bulgaria is observed in November due to the strengthening of the eastward transport of air masses (strengthening of EA teleconnection pattern) and anticyclonal weather (increase of descending motions in the atmosphere). There is also a precipitation decrease in May and June due to the growing influence of the Azores High. An increase of precipitation happens in September. All this leads to a redistribution of annual precipitation course, but annual precipitation amounts remain the same. However, this redistribution has a negative impact on agriculture and winter ski tourism. Zonal circulation has a larger influence on precipitation in Bulgaria compared to meridional. Eastward transport throughout the year leads to lower than the normal precipitation, and vice versa. With regard to the four teleconnection patterns, winter precipitation in Bulgaria is determined mainly by EA/WR teleconnection pattern, spring and
* Peter Nojarov
[email protected] 1
National Institute of Geophysics, Geodesy and Geography, Bulgarian Academy of Sciences, Akad. G. Bonchev Str., Bl.3, Room 327, 1113 Sofia, Bulgaria
autumn by EA teleconnection pattern, and summer by SCAND teleconnection pattern.
1 Introduction The water cycle is one of the most important factors for the existence of life on Earth. It is a basis for many human activities. One of the key elements of this cycle is precipitation. It has great variability in both time and space. Very often, precipitation is not evenly distributed, which causes periods with dangerous phenomena such as droughts or floods. On the other hand, its uneven spatial distribution requires adequate actions such as transferring certain quantities of water from one area to another. The fundamental importance of this climatic element to people’s lives is the reason to measure and thoroughly investigate precipitation. Serious efforts are also directed toward its prediction. IPCC Assessment Report 5 devotes considerable space to observations and trends in precipitation amounts, both at a global and regional level (Hartmann et al. 2013). The main conclusion is that there is no any common tendency in precipitation at planetary level, as it is for air temperatures. Some regions do not have any statistically significant trends. However, the area of middle latitudes of the northern hemisphere (30° N–60° N), which covers the territory of Bulgaria, shows a statistically significant trend of increase of precipitation during the last century. The period 1951–2008 also has a positive trend, but it is no longer statistically significant. The increase is in the range of 1 to 1.5 mm/decade for the various datasets. Detailed maps of spatial distribution of precipitation included in IPCC AR5 show that the region of Southeastern Europe, which includes Bulgaria, is on the border between positive and negative trends. Positive trends cover the northern part of the Balkan Peninsula, while negative trends are common in the southern (Mediterranean) part.
P. Nojarov
There are enough publications in the last years about precipitation changes in Bulgaria (Alexandrov et al. 2004; Grunewald et al. 2008; Topliisky 2005; Velev 2000; Velev 2006; Nojarov 2006; Nojarov 2010a, etc.). Velev (2000, 2006) investigated mainly low stations. The research period is from 1961 to 2005. An insignificant decrease in precipitation amounts was found in the low parts of the country (2– 5 %) and not at every station. Topliisky (2005) has explored various periods from 1901 to 1999. He revealed mixed, statistically insignificant trends. There are significant negative trends in January precipitation amounts in southwestern Bulgaria and in annual precipitation amounts at half of the investigated stations. Alexandrov and Hoogenboom (2000), Alexandrov and Genev (2003), and Alexandrov et al. (2004) examined precipitation in the low parts of Bulgaria for the period 1901–2000. Statistically insignificant mostly negative trends in annual precipitation amounts were found. Precipitation decrease prevails during summer and autumn seasons while winter and spring show some positive tendencies. Similar results were reported by Koleva and Alexandrov (2008) for the Danubian Plain and the Upper Thracian Plain. Grunewald et al. (2008) investigated southwestern Bulgaria. Significant decrease in precipitation at Bansko station in all seasons except summer has been found. The period of study is from 1955 to 1995. Andreeva et al. (2003) detected a reduction of winter precipitation at high mountain stations Musala, Cherni vrah, and Botev for the period 1961–2002. Petkova et al. (2004) explored mountainous areas in Bulgaria in connection with snow cover and its duration. A significant decrease in winter precipitation for the period 1931–2000 was revealed. Nikolova and Vassilev (2005) and Nikolova and Boroneant (2011)) studied precipitation in the Danube area and in particular in the Lower Danubian Plain. An increase of summer and autumn precipitation was found after the extremely dry period from 1983 to 1993, whereas there is a negative trend in winter. Tran et al. (2002) found negative trends in precipitation amounts in Bulgaria during the periods 1960–1995 and 1976–1995. These trends are most significant in winter months. Nojarov (2010a, 2010b, 2012) investigated precipitation mainly in Bulgarian mountains, where there is a significant decrease in the last 60 years. Quoted sources do not have significant differences in terms of tendencies in the mountains. All of them emphasize the particularly pronounced decrease of precipitation in the cold season. However, there are certain differences between publications in terms of trends at low stations. While the main results of Velev (2000, 2006), Alexandrov and Hoogenboom (2000)), Alexandrov and Genev (2003), Alexandrov et al. (2004), and Koleva and Alexandrov (2008) show that there are no statistically significant trends in annual precipitation amounts in Bulgaria; Grunewald et al. (2008) argued that there is a significant precipitation decrease in southwestern Bulgaria and Topliisky (2005) – at half of the low stations. There are
also certain differences in terms of trends in seasonal amounts. Alexandrov and Hoogenboom (2000), Alexandrov and Genev (2003), Alexandrov et al. (2004), and Koleva and Alexandrov (2008) reveal that there is a precipitation decrease in summer and autumn, while the main conclusions of Nikolova and Vassilev (2005) and Nikolova and Boroneant (2011) are that precipitation in the Danube plane (which covers the northern part of Bulgaria) has increased in summer and autumn. Alexandrov and Hoogenboom (2000), Alexandrov and Genev (2003), Alexandrov et al. (2004), and Koleva and Alexandrov (2008) reported an increase of winter precipitation, while Nikolova and Vassilev (2005), Nikolova and Boroneant (2011), Tran et al. (2002), and Grunewald et al. (2008) (southwestern Bulgaria) showed that winter precipitation has decreased. It can be seen that there are certain differences between publications on the subject, which gives grounds to make a clarification of the situation on the basis of data from recent years. Also, in most of these publications, factors that affect precipitation are covered only partially. For its part, the explanation of the observed tendencies is associated with observed changes in (mainly) circulation factors. This was the main reason to conduct this study. The aim of the article is to determine the influence of circulation factors on precipitation in Bulgaria. Several tasks were completed in order to achieve this aim. The relationship between the two indices, which represent the main types (zonal and meridional) of circulation and precipitation in Bulgaria, was revealed. Trends in both circulation indices and precipitation were calculated. Influence of some main circulation patterns (North Atlantic Oscillation, East Atlantic, East Atlantic/ Western Russia, Scandinavia) on circulation and precipitation in Bulgaria was investigated. Vertical motions in the atmosphere and their influence on precipitation were also studied using omega equation. The spatial distribution of precipitation anomalies under conditions of different circulation types is shown.
2 Data and method Data from 20 meteorological stations, distributed evenly throughout and close to Bulgaria (Fig. 1) were used in this study. Two of these stations are mountainous—peaks Botev and Musala. The remaining 18 are located in the low parts with an altitude of up to 600 m a.s.l. The stations are as follows: Vidin, Lom, Vratsa, Pleven, Veliko Tarnovo, Ruse, Kalarasi, Varna, Burgas, Sliven, Haskovo, Kardzhali, Edirne, Kazanlak, Plovdiv, Sofia, Kyustendil, and Sandanski. The period of study is from 1950 to 2012. Data were obtained from the annual meteorological books of Bulgaria (Annual Meteorological Books), as well as from the internet. Homogeneity of the data was checked in three ways—by method of ratios, by nonparametric criterion of Mann-Whitney (Wilks 2006), and by examining the history of each station. Details on the
Circulation factors affecting precipitation over Bulgaria
Fig. 1 Location of the stations, which measure precipitation, in and around Bulgaria
homogeneity of the data of the two mountain stations could be found in Nojarov (2012). The study of history of the other stations showed that some of them have been relocated, which has led to certain changes in their precipitation amounts. Such a relocation of the instrument (rain gauge) occurred at stations: Veliko Tarnovo (01/02.1993), Varna (1958/59), Sofia (1966/67), Kyustendil (1968/69), and Kardzhali (1977/78). Homogenization of the series with different precipitation amounts was performed using the method of ratios, based on nearby stations, which are homogeneous over the investigated period. The same method was used to fill data gaps in the series of some stations, such as Kardzhali (02, 06-09.1982; 01.1995), Lom (05.1991), Plovdiv (01.2001), Ruse (06, 12.1991; 03, 11.1993; 07-08.1994; 01.1995; 11.1996; 08, 11.1997), Sliven (07.1982; 09.1990; 01.1995; 02, 05, 1112.1998; 01, 07.1999) and Edirne (09.2004). Some of the larger gaps were not filled – Vratsa (01-09.2001), Burgas (03,05, 08-10.1970), Lom (1951-52; 01.1999–09.2001), Ruse (1951-52; 1999-09.2000), Sliven (1951). Two circulation indices were used in the study—zonal and meridional. Methods for their calculation are described in Nojarov (2013). They are after Tomingas (2002), who uses essentially the way of calculation of the NAO index. Monthly SLP data for the period 1950–2012 for 12 grid cells (2.5°× 2.5° resolution) that surround the four grid cells, covering the territory of Bulgaria were used. In other words, the north side
is covered by cells with coordinates 45–47.5° N and 20–30° E (4 in total); the south side is covered by cells with coordinates 37.5–40° N and 20–30° E (4 in total); the west side is covered by cells with coordinates 37.5–47.5° N and 20–22.5° E (4 in total); the east side is covered by cells with coordinates 37.5– 47.5° N and 27.5–30° E (4 in total). The values of the four northern cells were averaged to obtain the value for the north side. The same procedure was performed for the south, west, and east sides. The pressure data of each of the four sides were standardized by division of monthly pressure anomalies by the standard deviation for the investigated period. Zonal index was calculated as a difference between the standardized SLP values of the south and north sides. This means that positive values of the index indicate higher than normal eastward (zonal) transport of air masses and vice versa. Meridional index was calculated as a difference between the standardized SLP values of the east and west sides. Positive values here are associated with higher than normal northward (meridional) transport of air masses and vice versa. These indices make it possible to objectively assess atmospheric circulation and its influence on air temperatures in different months of the year. The necessary data for sea level atmospheric pressure used to calculate the two indices for the period 1950–2012 were taken from reanalysis monthly data of the National Centers for Environmental Prediction/National Center for Atmospheric Research (Kalnay et al. 1996).
P. Nojarov
Four teleconnection indices (patterns), directly related to the territory of Bulgaria, were used in this study. The indices were calculated by the NOAA National Weather Service Climate Prediction Center. The procedure and methods of calculation are shown on their website. The four indices (patterns) are NAO, EA, EA/WR, and SCAND. The North Atlantic Oscillation (NAO) consists of a north-south dipole of anomalies, with one center located over Greenland and the other center of opposite sign spanning the central latitudes of the North Atlantic between 35° N and 40° N. The positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern USA, and western Europe. It is also associated with above-average precipitation over northern Europe and Scandinavia in winter, and below-average precipitation over southern and central Europe. The East Atlantic (EA) pattern is the second prominent mode of low-frequency variability over the North Atlantic and appears as a leading mode in all months. The EA pattern is structurally similar to the NAO and consists of a north-south dipole of anomaly centers spanning the North Atlantic from east to west. The anomaly centers of the EA pattern are displaced southeastward to the approximate nodal lines of the NAO pattern. The positive phase of the EA pattern is associated with aboveaverage precipitation over northern Europe and Scandinavia and with below-average precipitation across southern Europe. The East Atlantic/West Russia (EA/WR) pattern consists of Table 1
four main anomaly centers. The positive phase is associated with positive height anomalies located over Europe and northern China and negative height anomalies located over the central North Atlantic and north of the Caspian Sea. The main precipitation anomalies reflect generally above-average precipitation in eastern China and below-average precipitation across central Europe. The Scandinavia pattern (SCAND) consists of a primary circulation center over Scandinavia, with weaker centers of opposite sign over western Europe and eastern Russia/ western Mongolia. The positive phase of this pattern is associated with positive height anomalies, sometimes reflecting major blocking anticyclones, over Scandinavia and western Russia, while the negative phase of the pattern is associated with negative height anomalies in these regions. The positive phase of the Scandinavia pattern is associated with above-average precipitation across central and southern Europe and belowaverage precipitation across Scandinavia. Values from the solution of omega equation at 700-hPa level were used in order to represent vertical motions in the atmosphere. The omega equation is
σ∇2H ω þ f 2
∂2 ω ∂p2
2 ∂ ∂φ V g ⋅∇H ζg þ f −∇H V g ⋅∇H ¼f ∂p ∂p
Trends (mm/decade) of precipitation at studied stations for the period 1950–2012 January
February
March
April
May
June
July
August
September
October
November
December
Vidin
−0.3
−0.5
−1.5
−2.6
−2.3
−4
1.3
3.4
1.4
0.9
−5.7
0.8
Pleven Vratsa
0.7 0.4
−0.2 −0.4
1.2 −0.2
0.5 −1.2
−0.3 −3.2
−5.3 −2.1
3 −1.6
3.2 1.6
3.3 0.6
−0.2 0.7
−3.9 −5.8
0.5 1
Lom V.Tarnovo Ruse Varna Burgas Sliven Kardzhali Plovdiv Kyustendil Sandanski Haskovo Kazanlak Sofia Kalarasi Edirne p. Musala p. Botev
0.3 0 1 −0.4 0 −1.7 −3.1 −2 −2.9 −3.2 −3.4 −1.4 0.7 1.3 −3.4 0.1 −8.2
−1.1 0.1 −1.3 −0.4 −0.5 −1.1 3.5 0.3 −2 −3.2 2.2 −2 1.4 −1.9 0.7 3.3 −8.1
−0.7 2.3 0.8 1.7 1.5 0.6 2.6 −0.2 −1.8 −2.1 0.2 0.1 0.9 1.8 −0.3 4.9 −5.2
−2.2 −0.9 −3.3 0.2 −0.2 −3.2 −8.4 −2.1 −1.3 −1.2 −3.2 −2.3 −1.1 −1.4 −1.6 −2.8 −4.4
−0.8 −2.6 −1.5 −2.3 −2.3 −0.6 −1.7 −4.5 −2.3 −1.9 −3.3 −5.1 −0.5 −1.9 0.7 −1.7 −3.5
−5.1 −4.7 −4.9 −3.6 −2.1 −4.2 −3 −4.4 0.2 −2.8 −3.6 −1 −2.6 0 −3.4 −5.8 −8.9
2.3 3.2 −2.1 4.8 2.2 −1.4 2.4 0.5 −0.7 1.6 3 0.8 −0.6 −2.7 2.8 −1.8 −0.9
2.5 0.1 2.8 −2 0 0.1 −1 3.7 2.7 2.5 0 −1.8 6.4 2.3 0.4 1.6 0.5
1.6 6.3 2.4 5.2 3 4.7 8 −0.2 2.8 1.9 2.9 3.5 2.6 5.3 3.6 1.4 −2.9
2.5 1.8 3.9 5.3 5 2.7 10 −0.8 1 0.8 −1.8 0.7 2.3 3.7 2.4 −6.3 −4
−6 −3.5 −2.4 −2.6 −4.9 −6.6 −16.9 −5.3 −4 −6.3 −3.8 −5.8 −3.4 −0.9 −2.7 0.3 −12
0.6 1.2 0.8 2.2 1.3 1.2 −1.8 0.1 1.2 2.3 0.4 0.6 2.1 1.6 1.6 0.6 −3
Statistically significant numbers are in italics
Circulation factors affecting precipitation over Bulgaria
Fig. 2 Difference of mean November sea level pressure (hPa) between the periods 1987–2012 and 1950–1986
where ƒ is the Coriolis parameter, σ is the static stability, Vg is the geostrophic velocity vector, ζg is the geostrophic relative vorticity, φ is the geopotential, ∇2H is the horizontal Laplacian operator, and ∇H is the horizontal del operator. This equation combines the effects of vertical differential of geostrophic absolute vorticity advection and three-dimensional Laplacian of thickness thermal advection and determines the resulting vertical motion. Positive vorticity advection and no thermal advection results in a negative omega (ω) that is, ascending
motion. Similarly, warm advection also results in a negative omega (ω) corresponding to ascending motion. Negative vorticity advection or cold advection both result in a positive omega (ω) corresponding to descending motion. Data were obtained from the reanalysis of the National Centers for Environmental Prediction/ National Center for Atmospheric Research (Kalnay et al. 1996). Statistical methods were used in the study (Wilks 2006). The level of significance everywhere is p=0.05.
Fig. 3 Difference of mean June sea level pressure (hPa) between the periods 1987–2012 and 1950–1986
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Fig. 4 Spatial distribution of trends (mm/decade) of annual precipitation amounts in Bulgaria for the period 1950–2012
Table 2 Spearman correlations between Bulgarian circulation indices, teleconnection indices, omega, and first unrotated principal component (PC1) scores of precipitation at low stations for the period 1950–2012 PC1 of precipitation amount in:
Zonal index
January February
−0.35 −0.48
March - Western Bulgaria March - Eastern Bulgaria April May June - Western Bulgaria June - Eastern Bulgaria July - Western Bulgaria July - Eastern Bulgaria August - Western Bulgaria August - Eastern Bulgaria September October November - Western Bulgaria November - Eastern Bulgaria December - Western Bulgaria December - Eastern Bulgaria
Meridional index
EA
EA/WR
SCAND
NAO
Omega 700 hPa
0.13 0.26
−0.33 −0.33
−0.39 −0.42
0.03 0.3
−0.34 −0.4
−0.69 −0.76
−0.34 −0.22 −0.28 −0.34 −0.34 −0.06 −0.32
0.14 −0.23 −0.06 −0.13 0.12 0.11 −0.1
−0.13 −0.04 −0.32 −0.42 −0.31 −0.14 −0.09
0.02 −0.2 −0.09 −0.18 0.12 −0.08 0.22
0.25 0.01 0 −0.14 0.27 0.32 0.15
−0.42 −0.25 0.11 −0.08 0.3 0.2 0.18
−0.77 −0.64 −0.51 −0.73 −0.66 −0.58 −0.53
−0.14 −0.14 0.06 −0.37 −0.53 −0.49 −0.52 −0.36 −0.38
−0.25 −0.48 −0.44 −0.32 −0.04 0.21 0.14 0.24 0.04
−0.23 −0.2 −0.38 −0.19 −0.18 −0.38 −0.31 −0.13 −0.08
0.09 −0.22 −0.06 −0.08 −0.26 −0.15 −0.15 −0.44 −0.29
0.29 0.18 0.33 0.18 −0.24 0.13 0.11 0.1 0.18
0.21 0.09 0.22 −0.05 −0.15 −0.18 −0.2 −0.26 0
−0.39 −0.53 −0.42 −0.53 −0.59 −0.77 −0.7 −0.67 −0.68
Statistically significant numbers are in italics. In March, November, and December, BWestern Bulgaria^ includes stations Vidin, Vratsa, Sofia, Kyustendil, Sandanski, and Lom. In June and July, BWestern Bulgaria^ includes stations Vidin, Pleven, Vratsa, Sofia, Kyustendil, Sandanski, and Lom. In August, BWestern Bulgaria^ includes stations Vidin, Pleven, Vratsa, Sofia, Kyustendil, Sandanski, Lom, and Plovdiv
Circulation factors affecting precipitation over Bulgaria Table 3
Some trends per decade in omega and four teleconnection indices for the period 1950–2012 January February March
Omega at 700 hPa (Pa*s−1) EA EA/WR SCAND NAO
0.0039
0.0049
0.28 −0.037
0.096 0.12
0.17
0.006 0.18
April
May
June
July
August September October November December
0.0051 0.004 0.0033 0.006 0.004 0.0021 0.16
0.11
0.033
0.25
−0.14 0.039 −0.13
−0.11
0.0004
0.0016
0.0047
0.0028
0.2 0.021
0.24
0.056
Statistically significant numbers are in italics
Trend analysis is the main tool to detect tendencies in precipitation. It was done using linear regression. Linear regression attempts to model the relationship between explanatory variable (time) and a response variable (precipitation) by fitting a linear equation to observed data. Every value of the independent variable (time) is associated with a value of the dependent variable (precipitation). In the least-squares model, the best-fitting line for the observed data is calculated by minimizing the sum of the squares of the vertical deviations from each data point to the line. Correlation coefficients were calculated in order to reveal relationships between precipitation, atmospheric circulation over Bulgaria, the
four main Northern Hemisphere teleconnection patterns, and vertical motions (omega equation). Spearman nonparametric rank statistic was used to perform a reliable assessment of the significance of relations between studied climatic elements. Principal component analysis was the main tool to reduce the dimension of precipitation field over Bulgaria. Residuals of linear regression were used in order to detect precipitation anomalies under various typical patterns of transport of air masses over Bulgaria. Thus, the influence of possible trends in precipitation was removed. Spatial presentation of results was made by mapping. Kriging is the method of interpolation of the data.
Fig. 5 Spatial distribution of precipitation anomalies (mm) under conditions of eastward transport of air masses over Bulgaria in January. The thick black arrow shows the direction of the atmospheric flow
P. Nojarov
3 Results and discussion Trends by months at studied stations for the period 1950–2012 are shown in Table 1. There is a decrease of precipitation at almost all studied stations in April, May, June, and November. However, most of the statistically significant values are observed in November. They are common for stations mainly in the western half of Bulgaria, which used to have two precipitation maxima—May/June and November. In recent years, November is no longer a precipitation maximum. Causes for this are circulation changes. Nowadays, anticyclones with centers to the east of Bulgaria are more frequent in November and they replace the prevailing cyclonic circulation typical for the beginning of the investigated period. The net result is an overall increase of mean sea level pressure (Fig. 2—SLP data from NCEP/NCAR reanalysis) leading to the observed significant decrease in precipitation. The influence of the Azores High increases in June on the expense of the Icelandic Low (Fig. 3—SLP data from NCEP/NCAR reanalysis). SLP is higher, and air masses are more stable, which also leads to a decrease of precipitation amounts. However, in general, the values are not statistically significant in most of the months, which means that there is not any definitive tendency in Bulgarian precipitation amounts.
Spatial distribution of trends of annual precipitation amounts in Bulgaria for the period 1950–2012 is shown in Fig. 4. The figure reveals a pattern of trends in the direction from southwest to northeast. There is a decrease of annual precipitation in the southwestern half of Bulgaria, most significant in the western part of the Upper Thracian Plain (the area around Pazardzhik). The trend turns around and there is an increase of annual precipitation in northeastern Bulgaria. This distribution is consistent with the results in the IPCC AR5. In terms of water supply these trends are contradictory. Areas with the smallest precipitation in Bulgaria (below 500 mm) are the extreme south west (station Sandanski), the western part of the Upper Thracian Plain (station Pazardzhik), and the extreme northeast (northern coast of the Black Sea). This spatial distribution of trends of annual precipitation will intensify problems in southwestern Bulgaria and the western part of the Upper Thracian Plain, but will improve the situation in the northeast. Correlation coefficients between the two Bulgarian circulation indices, the four teleconnection indices, omega values, and precipitation over Bulgaria are presented in Table 2. Precipitation field over Bulgaria was reduced using principal component analysis. Unrotated first principal component (PC1) scores derived on the basis of 18 low (located at an
Fig. 6 Spatial distribution of precipitation anomalies (mm) under conditions of northward transport of air masses over Bulgaria in January. The thick black arrow shows the direction of the atmospheric flow
Circulation factors affecting precipitation over Bulgaria
altitude of up to 600 m) stations were used for correlation. Varimax and oblimin rotation methods were used to rotate monthly PC1s. Varimax rotation is an orthogonal rotation of the factor axes to maximize the variance of the squared loadings of a factor (column) on all the variables (rows) in a factor matrix, which has the effect of differentiating the original variables by extracted factor. Direct oblimin rotation is the standard method for a nonorthogonal (oblique) solution—that is, one in which the factors are allowed to be correlated. Thus, in some months (March, June, July, August, November, and December), significant differences in precipitation field were found, which allowed splitting Bulgaria generally into two parts—eastern and western. These parts are shown separately in Table 2 in order to detect any regional differences in relationships. Table 2 shows that in all months of the year (with the exception of August in Eastern Bulgaria), the correlation between zonal index and precipitation is negative. This means that eastward transport of air masses over Bulgaria is associated with lower than the normal precipitation, and vice versa. The correlation is statistically significant in almost all months of the year with the exception of August in Western Bulgaria and March, June, July, and August in Eastern Bulgaria. The eastward transport over Bulgaria is usually caused by an anticyclone to the south and cyclones passing north of the country,
which pattern is generally associated with lower than the normal precipitation. The strengthening influence of the Iranian Low in summer (affecting more strongly eastern part of the country) somewhat disturbs this circulation pattern. Westward transport is connected with an anticyclone to the north of Bulgaria and cyclones passing to the south, which pattern is associated with high precipitation, especially in the cold half of the year. Correlations between meridional index and precipitation show different, mostly insignificant, relationships. Northward transport of air masses is connected with higher precipitation in the months from November to February. This is a typical for Bulgaria pattern when Mediterranean cyclones are the main precipitation suppliers. Southward transport is associated with higher precipitation in the other months (except June). This pattern is typical when cyclones, generated in the Icelandic Low, pass through Bulgaria. Statistically significant correlations between this index and precipitation are observed only in August and September. It should be pointed out that in these months this index has a leading role in defining precipitation amounts. Generally, east-west transport of air masses has greater influence on precipitation in Bulgaria compared to north-south transport. Relationships between precipitation and the four teleconnection patterns are also shown in Table 2. EA pattern
Fig. 7 Spatial distribution of precipitation anomalies (mm) under conditions of eastward transport of air masses over Bulgaria in April. The thick black arrow shows the direction of the atmospheric flow
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has negative values in all months, which means that its positive phase (strong anticyclone to the west or to the southwest of Bulgaria) is associated with lower than the normal precipitation and vice versa. As already mentioned, the positive phase leads to enhanced eastward transport of air masses, shift of cyclones in northern direction and a corresponding decrease of precipitation amounts. EA pattern has the greatest influence on precipitation in Bulgaria in January, February, April, May, and November. It is a leading circulation pattern in April, May, and November. These are generally the months of transition from winter to summer precipitation patterns and vice versa. EA/WR pattern has negative correlation values that are statistically significant in winter months (December, January, and February). During these months, it is also the leading circulation pattern, with anticyclone located to the west of Bulgaria, which influences the most important for precipitation direction of transport—east-west. The influence is the same as EA during transitional seasons. Scandinavian pattern shows predominantly positive correlation with precipitation, which means that a strong anticyclone over Scandinavia is associated with more precipitation over Bulgaria. In such conditions, the country is located along its southern periphery, where the transport is from east to west leading to an increased precipitation. Statistically significant values could be
observed mainly in summer months (June, July, and August) and in eastern Bulgaria. In June and July, this is the leading circulation pattern in the eastern part of the country. NAO pattern has the greatest influence on precipitation in Bulgaria in winter when correlation values are negative and statistically significant. This means that in enhanced zonal transport over the Atlantic-European sector precipitation in Bulgaria is less and vice versa. The mechanism of this interaction coincides with those typical for EA and EA/WR patterns. However, only in March NAO is the leading circulation pattern in western Bulgaria. It should be mentioned that in summer correlation values become positive but not statistically significant. The main conclusion is that in winter precipitation in Bulgaria is determined mainly by the EA/WR teleconnection pattern, in spring and autumn—by EA teleconnection pattern and in summer—by SCAND teleconnection pattern. Table 2 shows clearly that the most important factor for precipitation in Bulgaria is vertical motions in the atmosphere, expressed by omega equation. In all months, the relationship is inverse and statistically significant. Positive values of omega are connected with descending motions in the atmosphere, which either prevents formation of clouds, or only low stratus clouds can form. Under these conditions, there is virtually no precipitation. Otherwise, negative values of omega are connected with
Fig. 8 Spatial distribution of precipitation anomalies (mm) under conditions of northward transport of air masses over Bulgaria in April. The thick black arrow shows the direction of the atmospheric flow
Circulation factors affecting precipitation over Bulgaria
ascending vertical motions in the atmosphere that contribute to the formation of clouds which can produce precipitation. Table 3 shows some trends in teleconnection indices and omega at 700 hPa in order to reveal whether there is some correspondence with precipitation trends (Table 1). Also, Table 3 shows trends only for months in which there is a statistically significant correlation between the respective elements according to Table 2. Ubiquitous and statistically significant decrease in precipitation in Bulgaria is observed in November. During this month both factors that have major influence on precipitation (omega and EA index) show significantly increasing trend, i.e., the decrease of precipitation is due to changes in these two factors. A decrease of precipitation is also common for Bulgaria in April, May, and June, although in most cases the trends are not statistically significant. During these months, there is a statistically significant increase of the values of omega. Also, there is an increase of the values of EA index in April, and there is a significant decrease of the values of SCAND index in June, i.e., these three factors determine precipitation trends in the three months as the leading is the role of omega. A prevailing increase of precipitation is observed in the period August–October and December. These are exactly the months which show no significant tendencies in omega.
The influence of atmospheric circulation on precipitation in Bulgaria is best seen in precipitation anomalies in typical for the four seasons’ months—January, April, July, and October. Months with predominantly eastward transport of air masses (positive values of the zonal index), westward transport of air masses (negative values of the zonal index), northward transport of air masses (positive values of the meridional index), and southward transport of air masses (negative values of the meridional index) were consecutively selected in the period 1950–2012. After that, monthly anomalies of precipitation were calculated using residuals. Then, these anomalies were averaged in order to obtain mean precipitation anomaly for the different directions of transport of air masses. Finally, these anomalies were interpolated and mapped. Only the low stations were used because they are sufficient in quantity to represent the horizontal distribution. High mountainous stations are too few in number in order to obtain a relatively complete picture in vertical direction. The influence of atmospheric circulation on precipitation in January is shown in Figs. 5 and 6. Figure 5 shows precipitation anomalies under conditions of eastward transport. It could be seen that in the entire country amounts are 5 to 10 mm
Fig. 9 Spatial distribution of precipitation anomalies (mm) under conditions of eastward transport of air masses over Bulgaria in July. The thick black arrow shows the direction of the atmospheric flow
P. Nojarov
lower than the norm. There is no any clear spatial distribution. Overall, anomalies are slightly larger in southern Bulgaria. This is probably due to the fact that in such a situation, cyclones pass to the north of Bulgaria and bring more precipitation in the northern part of the country. Under westward transport of air masses, the picture is similar, but with positive anomalies of precipitation amounts. These positive anomalies are slightly larger in southern Bulgaria, which is related to the fact that in such a situation cyclones pass to the south of the country and bring more precipitation there. Under conditions of northward transport of air masses in January (Fig. 6), there is a clear spatial differentiation. Precipitation anomalies are positive in southwestern Bulgaria and negative in the northeastern part. This could be explained by the fact that northward transport of air masses is associated with anticyclone located to the east or to the northeast of Bulgaria, which explains the low precipitation in that direction. Cyclones pass from south to north, affecting mostly southwestern Bulgaria. Under conditions of southward transport of air masses, the picture is just opposite—above the normal precipitation in northeastern Bulgaria, while in the other part it is below the norm. In this situation the anticyclone is generally
situated to the west of Bulgaria and respectively cyclones pass over the Black Sea, bringing more precipitation there. Overall, it can be concluded that in January the west-east transport of air masses leads to larger variations in precipitation compared to north-south transport. However, there are more spatial differences under conditions of meridional transport. Zonal transport in April is associated with the same precipitation anomalies as in January. Eastward transport is characterized by below the normal precipitation (Fig. 7). Values range from −2 to −6 mm without significant spatial differentiation. Westward transport shows positive anomalies from +2 to +6 mm. There is again a clear spatial differentiation under conditions of northward transport of air masses (Fig. 8). Unlike January, negative anomalies in April cover almost the entire Bulgaria. Only the western part (approximately west of the line Sofia - Montana) has a positive deviation of precipitation amounts. This means that the anticyclone, which in such a situation in winter is situated to the east-northeast of Bulgaria, has a more western position in spring. The picture is just opposite under conditions of southward transport as in the western part of Bulgaria negative anomalies in precipitation are observed
Fig. 10 Spatial distribution of precipitation anomalies (mm) under conditions of northward transport of air masses over Bulgaria in July. The thick black arrow shows the direction of the atmospheric flow
Circulation factors affecting precipitation over Bulgaria
while in the east they are positive. The main conclusion is that in this month the influence of meridional circulation on precipitation in Bulgaria is approximately equal to that of the zonal circulation. However, precipitation anomalies are not as large as in winter. The influence of atmospheric circulation on precipitation in July is shown in Figs. 9 and 10. Figure 9 shows precipitation anomalies under conditions of eastward transport of air masses. There are negative anomalies throughout Bulgaria, with the smallest values in the east—about −1 mm. In other areas (northern Bulgaria) they reach up to −7 mm. The spatial distribution is just opposite under conditions of westward transport. The cause for very small variations in precipitation in the eastern part, especially along the Black Sea coast, is that in summer there is a characteristic breeze circulation, which has a stabilizing effect on precipitation. Cyclones in this month usually pass to the north of Bulgaria, and therefore, these northern areas show the largest anomalies. Under conditions of northward transport in July (Fig. 10), the country is dominated by negative anomalies of precipitation that reach up to −5 mm in northern Bulgaria. An exception is the central-western part of Bulgaria where weak positive
anomalies are observed. It is possible that there is an orographic enhancement of precipitation carried by cyclones that pass west of Bulgaria, as this part of the country is very mountainous. The picture is just the opposite under conditions of southward transport of air masses. In this case, the anticyclone, which is situated to the west of Bulgaria, covers only the central western part, thereby reducing precipitation. This month again shows that the influence of zonal circulation on precipitation in Bulgaria is much more important than the influence of meridional circulation. However, there are more spatial differences under conditions of meridional circulation. Zonal transport in October shows the same precipitation anomalies as in other studied months. The distinction of this month is that the anomalies are larger in value. Under conditions of eastward transport (Fig. 11), negative anomalies reach −16 mm, while the westward transport is characterized by positive anomalies of up to +18 mm. This is an amplitude of 34 mm, which is quite significant. October is similar to April in meridional circulation. Under conditions of northward transport (Fig. 12), negative anomalies cover most of the country and positive occur only in southwestern
Fig. 11 Spatial distribution of precipitation anomalies (mm) under conditions of eastward transport of air masses over Bulgaria in October. The thick black arrow shows the direction of the atmospheric flow
P. Nojarov
Fig. 12 Spatial distribution of precipitation anomalies (mm) under conditions of northward transport of air masses over Bulgaria in October. The thick black arrow shows the direction of the atmospheric flow
Bulgaria. The picture is just the opposite under conditions of southward transport. Anomalies in meridional circulation are from +5 to −3 mm (northward transport) and +4 to −2 mm (southward transport). The influence of zonal circulation on precipitation in Bulgaria in this month is much stronger than the influence of meridional circulation.
4 Conclusions A ubiquitous decrease of precipitation is observed in November, due to the strengthening of the eastward transport (expressed by strengthening of EA teleconnection pattern) and anticyclonal weather (expressed by an increase of descending motions in the atmosphere). There is also a precipitation decrease in May and June due to the growing influence of the Azores High. An increase of precipitation happens in September. All this leads to a redistribution of annual precipitation course, but annual precipitation amounts remain the same. However, the decrease of late spring precipitation is extremely unfavorable for the plant growing, as it leads to a drought during critical for the growth of the plants, period of the year. This will require measures for provision of additional
irrigation, which in turn will increase the cost of the final production. The increase of precipitation in September is not particularly favorable for plant growing, because this is the period of harvesting and possible rain can spoil the yield. Less precipitation in November will lead to a later formation of stable snow cover in the mountains, which in turn will shorten the ski season and generally will reduce water reserves accumulated in the mountains during the cold season. This, together with the reduced spring precipitation, will adversely affect the spring freshet of Bulgarian rivers, which is the main source of water in the country. The most important factor for precipitation in Bulgaria is vertical motions in the atmosphere. Zonal circulation has larger influence on precipitation in Bulgaria compared to meridional. Eastward transport throughout the year leads to lower than the normal precipitation, and vice versa. This is valid for the entire territory of Bulgaria. On the other hand, meridional circulation shows mixed weaker relationships with precipitation in space as well as in time. With regard to the four teleconnection patterns the main conclusion is that in winter precipitation in Bulgaria is determined mainly by EA/WR teleconnection pattern, in spring and autumn—by EA teleconnection pattern and in summer—by SCAND teleconnection pattern.
Circulation factors affecting precipitation over Bulgaria
References Alexandrov V, Genev M (2003) Climate variability and change impact on water resources in Bulgaria. European Water 1(2):25–30 Alexandrov VA, Hoogenboom G (2000) The impact of climate variability and change on crop yield in Bulgaria. Agric For Meteorol 104:315– 327 Alexandrov V, Schneider M, Koleva E, Moisselin JM (2004) Climate variability and change in Bulgaria during the 20th century. Theor Appl Climatol 79(3–4):133–149. doi:10.1007/s00704-004-0073-4 Andreeva T, Martinov M, Momcheva S (2003) Mild winters and the precipitation in the mountain regions in Bulgaria. ICAM/MAP. Available at http://www.map.meteoswiss.ch, access date – 15.05. 2014. Annual Meteorological books. National Institute of Meteorology and Hydrology, Bulgarian Academy of Sciences, Sofia (in Bulgarian). Grunewald K, Scheithauer J, Monget JM, Brown D (2008) Characterization of contemporary local climate change in the mountains of southwest Bulgaria. Clim Chang 95(3–4):535–549. doi:10. 1007/s10584-008-9508-8 Hartmann DL, Klein Tank AMG, Rusticucci M, Alexander LV, Brönnimann S, Charabi Y, Dentener FJ, Dlugokencky EJ, Easterling DR, Kaplan A, Soden BJ, Thorne PW, Wild M, Zhai PM (2013) Observations: atmosphere and surface. In: Stocker TF, Qin D, Plattner G-K, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM (eds) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA Kalnay E, Kanamitsu M, Kistler R, Collins W, Deaven D, Gandin L, Iredell M, Saha S, White G, Woollen J, Zhu Y, Leetmaa A, Reynolds R, Chelliah M, Ebisuzaki W, Higgins W, Janowiak J, Mo KC, Ropelewski C, Wang J, Jenne R, Joseph D (1996) The NCEP/NCAR 40-year reanalysis project. Bull Am Meteorol Soc 77:437–470 Koleva E, Alexandrov V (2008) Drought in the Bulgarian low regions during the 20th century. Theor Appl Climatol 92(1–2):113–120 Nikolova N, Boroneant C (2011) Observed changes in precipitation in the Danube river lower basin in the context of climate change. Forum
geografic Studii şi cercetări de geografie şi protecţia mediului 10(1): 117–128 Nikolova N, Vassilev S (2005) Variability of summer-time precipitation in Danube plain, Bulgaria. Zbornik radova Geografskog instituta BJovan Cvijić^. SANU 54:19–32 Nojarov P (2006) An assessment of the contemporary tendencies and flood risks on the basis of average monthly precipitation sums in different regions of Bulgaria. In: Proceedings of First national research conference on emergency management and protection of the population. National Security and Defence Research Center, Bulgarian Academy of Sciences, Sofia, pp 157–163 (in Bulgarian) Nojarov P (2010a) Changes in precipitation regime at peak Moussala for the period 1933–2008. Problems of Geography 3–4:145–165 Nojarov P (2010) Precipitation changes at peak Botev for the period 1947 – 2008. In Proceedings of International Conference BGeography and Regional Development^, Sofia: 243–247 (in Bulgarian). Nojarov P (2012) Variations in precipitation amounts, atmosphere circulation, and relative humidity in high mountainous parts of Bulgaria for the period 1947–2008. Theor Appl Climatol 107:175–187. doi: 10.1007/s00704-011-0473-1 Nojarov P (2013) Circulation over Bulgaria and its connection with NAO indices and Sea Surface Temperatures. Theor Appl Climatol 114(1): 329–348. doi:10.1007/s00704-013-0846-8 Petkova N, Koleva E, Alexandrov V (2004) Snow cover variability and change in mountainous regions of Bulgaria, 1931–2000. Meteorol Z 13(1):19–23 Tomingas O (2002) Relationship between atmospheric circulation indices and climate variability in Estonia. Boreal Environ Res 7:463–469 Topliisky D (2005) Chronological fluctuations of climate in Bulgaria during the 20th century. Sofia University, Sofia, Autopaper of dissertation (in Bulgarian) Tran L, Knight CG, Wesner V (2002) Drought in Bulgaria and atmospheric synoptic conditions over Europe. GeoJournal 57(3):149– 157 Velev S (2000) The global changes and the climate of Bulgaria. In: Proceedings of International Scientific Session B50 years Institute of Geography, BAS^, Sofia: 99–106 (in Bulgarian). Velev S (2006) Global climatic change and Bulgarian climate. Geography’21, 2: 4–8 (in Bulgarian). Wilks DS (2006) Statistical methods in the atmospheric sciences, volume 91. Elsevier, Second Edition (International Geophysics)
