Variability of seasonal and annual precipitation in Slovenia and its correlation with large-scale atmospheric circulation

1 Introduction

Precipitation is a very important meteorological element in the Mediterranean region (MR) as its future change can impact human activities (e.g. water management, agriculture) and ecosystems. Significant progress has been made on climate projections for the MR [1]. Future projections regarding seasonal precipitation show predominant reductions for spring, summer and autumn, however projections for winter are distinctly different [2]. Precipitation is projected to decrease in all parts and all seasons (the most significant percent change occurs in summer) except for the northernmost parts in winter [3]. This is in accordance with the expected drying over the MR as a part of global warming [4].

Precipitation variability and trends in the MR have been analyzed in many studies. For example, Ziv et al. [5] analyzed changes in the precipitation regime of Israel showing a statistically non-significant decreasing trend prevailing in most of the country. The majority of Israel has significant precipitation decrease only during spring. Luković et al. [6] examined spatial patterns of rainfall trends in Serbia suggesting only weak, mostly non-significant trends. The study of de Luis et al. [7] has shown a significant annual precipitation decrease in northwestern and western Slovenia and during all seasons, except autumn.

As local changes in meteorological variables in mid-latitudes are mainly controlled by atmospheric circulation [8, 9], the correlations between circulation indices and precipitation in the MR are very important to analyze. The relationships between atmospheric circulation pattern indices and precipitation in Slovenia have not been well identified: only a few studies [10, 11] have been made on this topic with only a limited number of precipitation stations investigated. One of the goals in this paper is to analyze these relationships in order to obtain a better understanding of the causes of precipitation variability in Slovenia. The study includes data on the large-scale North Atlantic Oscillation (NAO) and regional Mediterranean and Western Mediterranean Oscillations (MO and WeMO, respectively) known to affect the MR.

NAO is associated with a meridional dipole structure in sea level pressure with two centers of action located near Iceland and the Azores [12]. A positive NAO phase leads to more intense precipitation over northern Europe, whereas a negative NAO phase causes a precipitation shift towards southern Europe [8]. The influence of NAO on precipitation in the MR has been investigated by a number of authors [5, 8, 10–14]. A study by Sušelj and Bergant [11] showed significant negative correlation between NAOi and precipitation in Slovenia. However, this interpretation is limited by the fact that only four selected meteorological stations represented the whole country which is geomorphologically and climatologically diverse.

MO was defined in order to explain opposing atmospheric dynamics between the western and eastern part of the Mediterranean basin. The original MOi was defined as normalized pressure difference between Algiers and Cairo [15]. A second version of this index, which was used in this paper, can be calculated as the difference of standardized pressure anomalies at Gibraltar and the Israeli meteorological station of Lod [16]. The influence of MO on precipitation variability has been analyzed in numerous studies [5, 10, 11, 13, 17–19]. A study of Sušelj and Bergant [11] showed significant negative correlation between MOi and precipitation in Slovenia as registered at four selected meteorological stations (Ljubljana, Murska Sobota, RateČe and Postojna).

WeMO was defined by Martin-Vide and Lopez-Bustins [20] by means of the dipole composed by an anticyclone over the Azores and a depression over Liguria. This is the situation with a WeMO positive phase, while in the negative phase the situation is the opposite. We-MOi was defined as the result of the difference between standardized values in surface atmospheric pressure in San Fernando (Spain) and Padua (Italy). WeMOi’s influence on climate variability in the Iberian Peninsula has been analyzed by Martin-Vide and Lopez-Bustins [20] and Martin-Vide et al. [21].

The Republic of Slovenia is situated in the central-southern part of Europe (Figure 1) abutting four distinct geographical regions: the Mediterranean Sea, the Alps, the Dinaric Alps and the Pannonian Basin [22]. Slovenia extends between 45°25’ and 46°30’ N and 13°23’ and 16°36’ E [23] and covers an area of 20,273 km² [22] with a population of 2.06 million. The Alpine macroregion is in the north of Slovenia, while the Mediterranean macroregion is in the west of Slovenia. Toward the east, the Mediterranean macroregion is replaced by the Dinaric macroregion that stretches in a northwest-southeast direction and covers most of the southern part of Slovenia. The Pannonian macroregion is a densely populated and intensively cultivated area at the east end of Slovenia [24] (Figure 1). Submediterranean, temperate continental and alpine climatic influences intertwine in the territory of Slovenia. However, most of Slovenia has a temperate continental climate. Alpine climate characterizes higher and lower mountain areas to the north and west of the country, while a submediterranean climate is present in the south and southwest of the country at the Adriatic coast (coastal sub-mediterranean climate) and its hinterland (inland sub-mediterranean climate). Continental climate intensifies with the distance increase from the Adriatic Sea and Alps-Dinaric mountain barrier towards the eastern and northeastern Slovenia [25].

In the study of de Luis et al. [7] it was discussed that changes in large-scale atmospheric circulation patterns may have contributed to an observed long-term drying in Slovenia. This study contributes to the investigation of these suggested relationships. The main goals of this paper are to investigate temporal and spatial variability and trends of annual and seasonal precipitation in Slovenia and to correlate them with indices of the large-scale (NAOi) and regional (MOi and WeMOi) atmospheric circulation patterns, which are more representative for MR precipitation.

2 Data and methods

Annual and seasonal precipitation in Slovenia recorded at 45 stations was analyzed (Figure 1,Table 1). Selected precipitation stations are located on the territory of each of the different geographical regions of Slovenia: 16 in the Alpine macroregion, 15 in the Dinaric macroregion, 10 in the Pannonian macroregion and 4 in the Mediterranean macroregion. Selection of adequate stations was based on the data availability and homogeneity as well as macroregions’ average size and altitudes.

Figure 1

Geographical location of the Republic of Slovenia in Europe and the precipitation stations in the Republic of Slovenia.

The investigated period is from 1963 to 2012. Precipitation amounts were obtained from the Environmental Agency of the Republic of Slovenia (EARS). Winter (DJF), spring (MAM), summer (JJA) and autumn (SON) precipitation were calculated for each station using the standard seasons definition. Winter precipitation corresponds to January-February of the calendar year and to December of the previous year, while precipitation for other seasons corresponds to the months from the calendar year.

A standard Normal Homogeneity Test (SNHT) was applied for the detection of abrupt homogeneity breaks [26] in monthly precipitation values. The test is based upon the assumption that the difference between precipitation series at a candidate station (the one being tested) and the reference series is fairly constant in time. Reference series were chosen from 4 to 6 stations, based on distance, similar altitude and squared correlation coefficient from 0.3 to 0.8 with the test station. The critical level of this test was 95% [27]. Inhomogenieties were detected and corrected [28].

Simple linear regression was used for obtaining annual and seasonal precipitation trends, while a Mann-Kendall nonparametric statistical test [29] was used to demonstrate the statistical significance of trends [30]. Statistical significance was defined at the level of 95% and 99%. Rates of precipitation changes are expressed as %/decade, due to the differences in total precipitation amounts between regions [7].

In order to understand Slovenian precipitation and its relationship with atmospheric circulation patterns better, indices were examined using correlation analysis. Pearson’s correlation was used to reflect the degree of linear relationship between the precipitation and the atmospheric circulation pattern indices (NAOi, MOi and WeMOi). Monthly NAOi values were obtained from the National Oceanic and Atmospheric Association (NOAA) Climate Prediction Center (CPC) website [31]. Daily values of MOi were obtained from Climatic Research Unit website [32]. Monthly WeMOi data were obtained from the University of Barcelona website [33]. Annual and seasonal NAOi and WeMOi were calculated using monthly series, while annual and seasonal MOi were calculated using daily series.

The spatial distribution of the results is displayed using spatial interpolation (an ordinary Kriging method) of the observed precipitation and precipitation trends for the 45 stations using the geostatistical software package AR-CGIS 10.1 with the Geostatistical Analyst Extension.

3 Results

3.2 Precipitation and precipitation regimes

The highest annual precipitation amounts (>2400 mm) were registered at stations located in the northwestern, mountainous part of Slovenia, influenced by interaction between alpine and submediteranean air masses. Towards the east, continental climate influence prevails with the lowest precipitation amounts recorded in the northeastern, Pannonian part of the country (<800 mm) (Figure 2a).

Figure 2

Spatial distribution of mean precipitation amounts (mm) (a – annual; b – winter; c – spring; d – summer; e – autumn) in Slovenia during the 1963-2012 period.

During winter, the highest precipitation amounts are registered at stations located in the northwestern, mountainous part of Slovenia (550–650 mm) influenced by alpine climate. Southwestern Slovenia is influenced by submediterranean climate and receives between 250 and 350 mm. The largest part of the country receives less than 250 mm and is influenced by temperate continental climate (Figure 2b).

In spring, western mountainous Slovenia is still the wettest part of the country (550–650 mm), while the largest part of the country receives more precipitation (250– 350 mm) compared to winter. Northeastern Slovenia, influenced by continental climate, receives less than 250 mm (Figure 2c).

Summer is characterized by a significant increase of precipitation in the eastern part of the country, influenced by continental climate, and reaching its seasonal precipitation maximum (250–300 mm). The largest part of the country receives between 350 and 450 mm, while northwestern Slovenia receives between 450 and 550 mm (Figure 2d).

In autumn, the seasonal precipitation maximum is evident in the area of alpine (northwestern Slovenia) and submediterranean climate (southwestern Slovenia) with >750 mm and from 450 mm to 550 mm, respectively (Figure 2e).

3.4 The correlations between atmospheric circulation patterns and precipitation

In general, correlations between MOi, NAOi and precipitation in Slovenia are negative, while WeMOi and precipitation have positive correlations on annual and seasonal scales (Table 2). This suggests that precipitation in Slovenia is decreasing during positive NAO and MO phases, while the positive phase of WeMO results in more precipitation in Slovenia.

Annual precipitation in Slovenia was significantly influenced by WeMOi at 21 out of 45 stations (significant correlations from 0.3 to 0.6), MOi at 17 out of 45 stations (significant correlations from −0.3 to −0.4) and NAOi at 10 out of 45 stations (significant correlations from −0.3 to −0.4) (Table 2). The spatial pattern of these relationships showed that WeMOi influence is dominant in central and eastern Slovenia, while MOi and NAOi influence is dominant in western Slovenia with the larger area under the MOi influence (Figure 4).

The strongest correlations between NAOi, MOi and seasonal precipitation in Slovenia are noticed in winter. Significant negative correlations (from −0.3 to −0.7) between MOi and precipitation are present in the majority of the country (43 out of 45 stations) (Table 2). Only stations located in southeastern Slovenia are not significantly influenced by this atmospheric circulation pattern. Similar results are obtained between NAOi and precipitation, but with smaller correlation values (from −0.3 to −0.6) and with a smaller covered area (32 out of 45 stations) (Table 2). Eastern Slovenia is not covered by significant NAOi influence. Significant WeMOi influence is noticed at 31 out of 45 stations with correlation values ranging from 0.3 to 0.6 (Table 2). Central, southeastern Slovenia and the area near the Adriatic Sea are not significantly influenced by this circulation (Figure 4).

Only WeMOi significantly influences spring precipitation in the larger part of Slovenia. Compared to this, correlations between MOi, NAOi and precipitation are rather small and non-significant. WeMOi is positively correlated (from 0.3 to 0.5) with precipitation throughout the country (40 out of 45 stations) (Table 2), except for small area in the north and northeast (Figure 4).

During summer, the precipitation distribution in the majority of the country is not significantly influenced by the investigated atmospheric circulation patterns. It might be that local factors (i.e. orography) shield certain regions from the variability represented by the atmospheric modes. As noticed in previous research, at fine geographical scales the effects of atmospheric circulation are modified by topography, particularly in areas of complex terrain [37, 38]. Significant influence (from −0.3 to −0.4) of MOi is noticed in eastern Slovenia and WeMOi in northwestern Slovenia (from 0.3 to 0.4) (Table 2). Significant NAOi influence (-0.3) (Table 2) is registered at only two stations, located in the northwestern part of the country (Figure 4).

Non-significant correlations were found between MOi, NAOi and autumn precipitation in Slovenia. Compared to this, WeMOi is significantly correlated (from 0.3 to 0.5) with precipitation in the majority of the country, except for small areas in central and northern Slovenia (Table 2) (Figure 4).

Figure 3

Spatial distribution of long-term trends (%/decade) of precipitation amounts (a – annual; b – winter; c – spring; d – summer; e –autumn) in Slovenia during the 1963-2012 period. Values significant at the 95% and 99% level (two-tailed). NS – non-significant.

Figure 4

Correlations between annual and seasonal (winter; spring; summer; autumn) precipitation and atmospheric circulation pattern indices in Slovenia during the 1963-2012 period. Values significant at the 95% and 99% level (two-tailed). NS – non-significant.

Table 2

Correlation coefficients between annual and seasonal precipitation and atmospheric circulation pattern indices in Slovenia during the 1963-2012 period. Values significant at the 95% (italic) and 99% level (bold) (two-tailed).

Station		Winter			Spring			Summer			Autumn			Annual
	NAO	MO	WeMO	NAO	MO	WeMO	NAO	MO	WeMO	NAO	MO	WeMO	NAO	MO	WeMO
BG	−0.2	−0.3	0.5	0.0	0.0	0.4	0.3	−0.2	0.0	−0.2	−0.1	0.4	0.0	−0.1	0.2
BLJ	−0.5	−0.7	0.2	−0.2	−0.2	0.4	0.0	−0.1	0.4	−0.3	−0.2	0.3	−0.3	−0.3	0.3
BU	−0.5	−0.6	0.3	−0.2	0.0	0.5	−0.1	−0.1	0.1	−0.1	−0.1	0.3	−0.2	−0.2	0.3
BZ	−0.1	−0.2	0.6	−0.1	0.1	0.5	0.2	−0.2	0.0	−0.2	0.0	0.4	0.0	0.0	0.3
CK	−0.4	−0.5	0.4	−0.1	−0.1	0.4	0.0	−0.1	0.3	−0.1	−0.1	0.4	−0.1	−0.2	0.4
CLJ	−0.3	−0.4	0.5	0.0	0.1	0.4	0.1	−0.3	−0.1	−0.2	0.0	0.3	−0.1	−0.1	0.2
CV	−0.5	−0.6	0.3	−0.3	0.0	0.5	−0.1	−0.1	0.3	−0.2	−0.1	0.3	−0.3	−0.4	0.3
DO	−0.4	−0.4	0.4	0.0	0.1	0.3	0.1	−0.3	−0.1	−0.1	−0.1	0.4	−0.1	−0.2	0.2
DV	−0.3	−0.4	0.4	0.0	0.0	−0.1	0.1	−0.2	0.2	−0.1	−0.1	0.2	−0.1	−0.1	0.2
FZ	−0.4	−0.4	0.4	−0.1	0.0	0.2	0.0	−0.2	−0.1	−0.2	−0.1	0.3	−0.3	−0.3	0.1
GD	−0.5	−0.6	0.2	−0.2	−0.1	0.4	−0.2	0.0	0.2	−0.1	−0.1	0.4	−0.3	−0.4	0.3
GG	−0.3	−0.5	0.3	0.0	0.1	0.4	0.0	−0.1	0.1	−0.3	−0.1	0.3	−0.1	−0.2	0.2
HB	−0.5	−0.5	0.4	−0.1	0.0	0.4	0.0	−0.2	0.1	−0.2	−0.1	0.4	−0.3	−0.2	0.4
JD	−0.5	−0.6	0.3	−0.1	0.0	0.5	0.0	−0.1	0.2	−0.1	−0.1	0.3	−0.1	−0.2	0.5
JR	−0.4	−0.6	0.2	−0.1	−0.1	0.4	−0.1	−0.1	0.1	−0.1	−0.1	0.3	−0.2	−0.4	0.2
JU	−0.5	−0.5	0.3	−0.2	0.0	0.4	−0.1	−0.2	0.2	−0.1	−0.1	0.4	−0.1	−0.3	0.5
KA	−0.6	−0.7	0.2	−0.1	0.0	0.1	0.0	−0.1	0.1	−0.2	−0.1	0.0	−0.3	−0.4	0.2
KC	−0.2	−0.4	0.4	0.1	0.0	0.3	0.1	−0.3	0.1	−0.2	0.0	0.4	−0.1	−0.2	0.2
KG	−0.5	−0.6	0.3	−0.1	0.0	0.5	−0.3	0.1	0.2	−0.2	−0.1	0.3	−0.3	−0.3	0.3
KO	−0.4	−0.4	0.4	0.0	0.0	0.4	0.0	−0.3	0.0	−0.2	0.0	0.4	−0.3	−0.1	0.4
KT	−0.2	−0.4	0.4	0.2	0.0	0.4	0.0	−0.2	−0.1	−0.1	−0.1	0.3	0.0 −	0.2	0.1
KZ	−0.5	−0.6	0.3	−0.3	−0.1	0.3	0.0	−0.1	0.2	−0.1	0.0	0.5	−0.2	−0.3	0.5
LD	−0.3	−0.4	0.4	0.1	0.1	0.3	0.0	−0.4	0.1	−0.1	0.0	0.4	−0.1	−0.3	0.2
LJB	−0.5	−0.5	0.4	−0.1	0.0	0.4	0.0	−0.1	−0.1	−0.3	−0.1	0.3	−0.2	−0.3	0.3
LO	−0.5	−0.6	0.2	−0.2	−0.1	0.4	−0.2	−0.1	0.3	−0.2	−0.2	0.4	−0.4	−0.4	0.3
LS	−0.3	−0.4	0.4	0.0	0.0	0.4	0.2	−0.3	−0.1	−0.2	−0.1	0.3	0.1	−0.2	0.1
MB	−0.2	−0.3	0.4	0.1	0.0	0.4	0.1	−0.3	0.1	−0.2	0.1	0.5	0.2	−0.1	0.3
MI	−0.2	−0.4	0.4	0.0	0.1	0.4	0.0	−0.2	0.0	−0.2	0.0	0.3	0.0	−0.2	0.1
MK	−0.5	−0.6	0.2	−0.2	0.0	0.5	−0.2	−0.1	0.4	−0.1	−0.1	0.3	−0.3	−0.3	0.4
MO	−0.3	−0.4	0.4	−0.1	0.1	0.3	0.0	−0.2	−0.1	−0.1	−0.1	0.4	−0.2	−0.2	0.2
MS	−0.2	−0.4	0.4	0.1	0.1	0.3	0.0	−0.3	0.0	−0.2	0.1	0.4	0.0	−0.2	0.1
NM	−0.2	−0.3	0.4	0.0	0.0	0.3	0.2	−0.2	−0.2	−0.1	−0.1	0.3	0.1	−0.1	0.2
PD	−0.5	−0.6	0.3	−0.1	0.0	0.4	−0.1	−0.1	0.2	−0.1	−0.2	0.3	−0.1	−0.4	0.2
PG	−0.5	−0.6	0.2	−0.1	−0.1	0.4	−0.2	−0.2	0.0	−0.1	−0.1	0.3	−0.3	−0.4	0.2
PI	−0.5	−0.6	0.3	−0.3	−0.1	0.4	0.0	−0.3	0.1	−0.1	−0.1	0.4	−0.3	−0.4	0.4
PO	−0.5	−0.5	0.3	−0.1	0.0	0.4	0.0	0.0	0.4	−0.1	−0.1	0.4	−0.1	−0.3	0.6
PT	−0.2	−0.4	0.4	0.0	0.0	0.4	0.2	−0.4	0.1	−0.2	0.1	0.4	0.0	−0.2	0.3
RT	−0.5	−0.6	0.3	−0.2	0.0	0.5	−0.3	0.1	0.2	−0.2	−0.1	0.3	−0.3	−0.2	0.3
RU	−0.4	−0.5	0.3	−0.2	0.0	0.5	−0.2	−0.1	0.3	−0.2	−0.2	0.3	−0.3	−0.4	0.3
SK	−0.3	−0.4	0.5	0.1	0.0	0.4	0.1	−0.3	0.1	−0.2	0.0	0.3	0.0	−0.1	0.2
SL	−0.5	−0.6	0.3	−0.2	0.0	0.5	−0.1	−0.1	0.2	−0.1	−0.1	0.3	−0.2	−0.3	0.3
SM	−0.4	−0.5	0.4	−0.1	−0.1	0.3	0.0	−0.2	0.1	−0.2	−0.1	0.4	−0.3	−0.4	0.3
SO	−0.4	−0.4	0.4	−0.1	0.0	0.3	0.1	−0.2	0.1	−0.1	−0.1	0.4	−0.3	−0.2	0.4
VD	−0.3	−0.4	0.4	0.1	0.0	0.4	0.0	−0.3	−0.1	−0.1	0.1	0.4	0.0	−0.1	0.1
ZR	−0.4	−0.6	0.2	−0.1	−0.1	0.4	−0.2	−0.1	0.1	−0.2	−0.1	0.3	−0.3	−0.4	0.2

4 Discussion and conclusions

The statistical analysis of annual and seasonal data from a network of precipitation stations in Slovenia from 1963 to 2012 has allowed for quantitatively characterizing precipitation variability in the study area and for assessing similarities and differences with respect to precipitation fluctuations in other areas of the MR.

The highest precipitation amounts are registered at the mountains in western Slovenia with a decrease towards the northeast of the country. This is due to the fact that western Slovenia is exposed to the inflow of moisture from the Adriatic Sea and to topographically-induced precipitation by the Alps and the Dinaric Alps.

Negative annual and seasonal (except autumn) precipitation trends are observed. A significant precipitation decrease was noticed in spring (from −6% to −10% per decade) and summer (from −4% to −10% per decade) and at an annual timescale (from −3% to −6% per decade) in southwestern and western Slovenia. Non-significant decreasing trends (up to −7% per decade) during winter and non-significant increasing trends during autumn (up to 4% per decade) were registered in the majority of the country. Compared to the study of de Luis et al. [7] that analyzed precipitation in Slovenia for the 1950-2007 period, this study obtained higher negative trends during spring and summer and smaller during winter, while annual trends had the same values. Annual and seasonal trends from our study agree with trends derived from E-OBS gridded observations [39]. ERA-Interim meteorological reanalysis [40] and results from our study showed increasing trend of precipitation during autumn and decreasing trend during spring, while they disagree on the winter and summer trends. Trends from NCEP/NCAR R1 meteorological reanalysis [41] and trends from our study disagree during all seasons, except for the autumn.

By the end of the 21^st century it is expected that precipitation in Slovenia will continue to decrease in summer months (up to 20 %), increase in winter (up to 30%) and show no significant changes in spring and autumn [42]. Furthermore, results from high-resolution climate change simulations over the MR showed substantial precipitation increases in winter (>25%) and spring (from 10% to 25%) and a decrease in summer (from −10% to −25%) and a small change in autumn precipitation (from −5% to 5%) in Slovenia by the end of the 21^st century [43]. Compared to these studies, our results showed significant drying during summer and non-significant changes during autumn, and this is compatible with model projections for the future. Observed winter and spring trends are not compatible with climate model projections.

Annual and seasonal precipitation in the MR shows no uniform trend. Weak or non-significant trends for precipitation in the past century have been found at the Mediterranean scale [44], in Serbia [6], Israel [5], northwestern Italy [45], in the Alpine region [46], etc. Seasonal trends indicate a slight decrease in winter and spring and an increase in autumn precipitation in Serbia [6]. Lopez-Bustins et al. [14] noticed a significant decrease of winter rainfall at the Iberian Peninsula in its western and central areas throughout the second half of the 20^th century, whereas over the eastern fringe it showed little variation. Obtained trends in this study together with a projected future precipitation decrease in the MR [2, 4] that is going to be most pronounced in summer [3] could imply that these changes have already started in Slovenia, which could lead to an increased economic and social vulnerability in the country due to reduced water availability in the future. Results from this study show drying in the spring and summer seasons as in the Coupled Model Intercomparison Project phase 5 (CMIP5) models, but the observed trends are opposite for winter and autumn seasons compared to the modeled trends. Observed trends also have higher values when compared to the modeled trends [47].

Correlations between NAOi, MOi and annual and seasonal precipitation in Slovenia are negative. When NAO is in its positive phase, low-pressure anomalies over the Icelandic region and throughout the Arctic combine with high-pressure anomalies across the subtropical Atlantic to produce stronger than average westerlies across the midlatitudes. During a positive NAO, conditions are drier over the MR [48], including Slovenia, as noticed in our study. The negative NAO index phase shows a weak subtropical high and a weak Icelandic low. The reduced pressure gradient results in fewer and weaker winter storms crossing on a more west-east pathway and bringing moist air into the Mediterranean [49] and increasing precipitation amounts into Slovenia. The positive mode of MO is related to anticyclonic conditions in the western Mediterranean and a trough in the east, and with below-average rainfall rates in the entire Mediterranean basin [50], including Slovenia. In its negative mode, a low pressure region is located near the British Isles or north of the Iberian Peninsula while anticyclonic conditions prevail in the Mediterranean. This situation is related with rainfall events in the western part of the Mediterranean basin [19]. Results from our study indicate more precipitation in Slovenia during the positive MO phase. NAOi and MOi significantly influence winter precipitation. In spring and autumn their significant influence is not registered, while in summer it is limited to a small area. Statistically significant correlations are stronger for MOi than for NAOi.

Correlations between WeMOi and annual and seasonal precipitation in Slovenia are positive. The positive phase of the WeMO corresponds to an anticyclone over the Azores and low-pressures in the Liguria Gulf [20], resulting in higher precipitation amounts in the larger part of Slovenia in all seasons except for summer when its influence is spatially limited. The WeMO negative phase coincides with a central European anticyclone located north of Italy and a low-pressure centre in the framework of the Iberian southwest [20] leading to a precipitation decrease in Slovenia.

NAOi shows a positive and significant (at 95% level) trend in the second half of the 20^th century [14]. The trend of this atmospheric circulation index is consistent with the reduction in winter precipitation throughout Slovenia registered in this paper and in the study of de Luis et al. [7], and over most of the Iberian Peninsula at the end of 20^th century [14]. Also, NAOi displays a strong, negative correlation with winter [45] and spring precipitation in Italy [51] and at high altitudes in the Alps [46, 52]. Except with mean precipitation amounts, NAOi is negatively correlated with the frequency of extreme precipitation days over the northwestern MR in the period 1961-2000. The correlations are rather small during autumn (October-November) but are quite considerable during the rest of the rainy season (December-January, February-March) [12].

MO is strongly linked in winter to NAO [19]. An observed increase in MOi during the second half of the 20^th century [17, 19] led to smaller precipitation amounts south from 50°N [17] which is in accordance with a winter precipitation decrease in Slovenia when its influence is the strongest.

WeMOi is significantly reduced in winter throughout the second half of the 20^th century [20] with WeMO entering an extreme negative phase in the 1990s. This recent WeMOi decay leads to a rainfall reduction over the northern fringe of the Iberian Peninsula [14] and torrential rainfall reduction in the northeastern Iberian Peninsula [21]. A significant increase in sea-level pressure in northern Italy has been detected during the twentieth century [53] that could contribute to the precipitation decrease in Slovenia in all seasons except autumn. In autumn, a large increase in cyclone activity was identified in the Gulf of Genoa and the southern part of the Adriatic Sea that could be responsible for the positive trend in autumn precipitation [54–56].

Results from this study contribute to a better understanding of relationships between atmospheric circulation patterns and precipitation variability in this transitional area. It is shown that the influence of NAO on precipitation variability over Slovenia is smaller than the effects of the atmospheric circulations belonging to the Mediterranean area (namely WeMO and MO). WeMO appeared as the most dominant atmospheric circulation pattern influencing annual and seasonal precipitation variability and distribution in the Republic of Slovenia, except for winter when MO is the most significant atmospheric circulation pattern. Relationships between other weather variables (e.g. temperature) and atmospheric circulation patterns could be investigated in future papers in order to acquire a more detailed picture of climatic change in this country.