decadal variability of the danube river flow in the lower basin and its relation with the north...

INTERNATIONAL JOURNAL OF CLIMATOLOGY

Int. J. Climatol. 22: 1169–1179 (2002)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.788

DECADAL VARIABILITY OF THE DANUBE RIVER FLOW IN THE LOWERBASIN AND ITS RELATION WITH THE NORTH ATLANTIC OSCILLATION

NOREL RIMBU,a CONSTANTA BORONEANT,b,* CARMEN BUTAb and MIHAI DIMAa

a Bucharest University, Faculty of Physics, Department of Atmospherics, Bucharest, Romaniab National Institute of Meteorology and Hydrology, Bucharest, Romania

Received 20 February 2001Revised 5 February 2002

Accepted 12 February 2002

ABSTRACT

The decadal variability (>5 years) of the Danube river flow in the lower basin and its connection with the North AtlanticOscillation (NAO) is analysed for the period 1931–95. Associated linkages with precipitation (PP) in the European sector,global sea surface temperature (SST) and atmospheric circulation for the period 1931–81, and the 500 hPa geopotentialheights (G500) over the Northern Hemisphere for the period 1948–95 are also investigated.

The results show that there is an out-of-phase relationship between the time series of the Danube river flow anomaliesand the NAO index. The time series of a PP index, defined as the average of normalized precipitation anomalies over alarge area including the Danube basin, presents a time evolution similar to that of the river flow anomalies.

The correlation maps between the river flow anomalies and global SST show coherent large-scale patterns. High valuesof the Danube river flow are associated with a tripole-like SST structure in the North Atlantic similar to that appearingduring the negative phase of the NAO, and with negative SST anomalies in the central North Pacific and positive SSTanomalies in the eastern and central tropical Pacific. Physically consistent sea level pressure and 500 hPa geopotentialheight are obtained. Copyright 2002 Royal Meteorological Society.

KEY WORDS: Europe; Danube river lower basin; correlation; composite maps; decadal variability; North Atlantic Oscillation;precipitation

1. INTRODUCTION

Research studies show that climate variations influence many components of the climate system. It hasbeen widely recognized that changes in the cycling of water between land, sea and air can potentially havesignificant impacts on the environment and on many sectors of the economy and society through their effectson water resources and their management. (Arnell, 1995, 1999; Arnell and Reynard, 1996).

Evidence from long hydrological records shows that periods with anomalous hydrological behaviour (Arnellet al., 1993) are associated with persistent climatic anomalies.

Interannual to decadal variability of the atmosphere over the North Atlantic region is characterized bythe North Atlantic Oscillation (NAO) teleconnection pattern (Bjerknes, 1964; Hurrell, 1995). The NAO is afluctuation in the pressure gradient across the North Atlantic with centres of action being the Icelandic lowand the Azores high. It is the dominant mode of atmospheric behaviour in the North Atlantic throughout theyear, mostly pronounced during winter and a primary climatic factor orchestrating hemispheric-scale climaticfluctuations centred over the Atlantic.

Research studies focused on the influence of the NAO on the variability of various climatic elements attime scales ranging from seasonal to interannual and decadal show that the NAO is responsible for generatingsystematic, large-amplitude patterns in the anomalies of temperature, precipitation, wind speed, latent and

* Correspondence to: Constanta Boroneant, National Institute of Meteorology and Hydrology, Sos. Bucuresti-Ploiesti 97, 71552Bucharest, Romania; e-mail: [email protected]

Copyright 2002 Royal Meteorological Society

1170 N. RIMBU ET AL.

sensible heat fluxes, and hence sea surface temperature (SST), over much of the extra-tropical North Atlantic(van Loon and Rogers, 1978; Kushnir, 1994; Hurrell and van Loon, 1997). The NAO has also been associatedwith the path and intensity of Atlantic storm tracks and with evaporation and precipitation patterns in theAtlantic-European region (Hurrell, 1995).

Because the signature of the NAO is strongly regional, a simple index of the NAO has been defined asthe difference of the standardized sea level pressure (SLP) anomaly measured at Lisbon, Portugal, and atStykkisholmur, Iceland (Hurrell, 1995).

Correlations with hydrological data have shown that, when the NAO index is high, river flow (particularlyin winter) is above average in northern Europe and below average in southern Europe (Shorthouse and Arnell,1997; Dettinger and Diaz, 2000). Because river flows depend directly on precipitation, it is evident that thereis a linkage between precipitation anomalies associated with extreme phases of the NAO and river flowregimes in Europe.

At the decadal time scale (>5 years) the NAO strongly influences the moisture balance (i.e. precipitationminus evaporation) over Europe. During the positive phase of the NAO (deep Icelandic low and strongAzores high), enhanced precipitation over northern Europe associated with less precipitation over central andsouthern Europe occurs (Hurrell, 1995; Hurrell and van Loon, 1997; Rımbu et al., 2001). A reverse situationoccurs during the negative phase of the NAO.

The Danube river has a very large catchment basin extending from central Europe (upper basin) to south-eastern Europe (lower basin). Its flow regime and other hydrologic characteristics are subject to significantinfluences due to climate variability (Bondar and Buta, 1995). Based on existing observational data in theDanube river basin, many research studies have pointed out the effect of precipitation and temperature changeson the Danube flow regime (Starosolszky and Gauzer, 1998), on the possible climate impacts on the waterresources in the Danube river basin (Behr, 1998; Petrovic, 1998), and on changes in hydrological characteristicsfor selected river basins in the case of climate-change scenarios (Dvorak et al., 1997; Stanescu et al., 1998).Because the decadal variability in the NAO has become especially pronounced since the early 1970s andhas determined decade-long regional climatic anomalies (winter dry conditions over southern Europe andMediterranean, and wet anomalies from Iceland eastward through Scandinavia (Zorita et al., 1992; Wilbyet al., 1997; Werrity and Foster, 1998)), we expect that the NAO signal might also be detected in the decadalvariations of the Danube river flow in the lower basin.

The goal of the present study is to investigate the role of the NAO on decadal variability of the Danuberiver flow in the lower basin and its connections with atmospheric circulation and SST at a global scale basedon observed data. It is important to study the linkage between the climate variability and hydrological regimein order to understand the physical mechanisms that determine it and to help in guiding the development ofpolicies for mitigation or prevention of drastic effects of long-term climate variability on various ecologicaland socio-economical sectors.

The paper is organized as follows. After the Introduction a brief description of the principal characteristicsof the Danube river basin follows in Section 2. The data and methods are described briefly in Section 3. Thedecadal variability of river flow and its relation with the NAO and precipitation is presented in Section 4.Section 5 presents the decadal variations of Danube flow in the lower basin in connection with global SST,SLP and 500 hPa geopotential height (G500). A summary of the results and conclusions are presented inSection 6.

2. SOME CHARACTERISTICS OF THE DANUBE RIVER BASIN

The Danube river is the second longest in Europe, cutting across the territories of many countries. It has atotal length of 2870 km and a catchment area of about 817 000 km2. From the Black Forest Mountains inGermany to the Black Sea in Romania, the Danube flows through 13 countries. It is the largest transboundaryriver basin in Europe. The river serves multiple functions as a waterway, natural resource and source ofenergy, and also plays an important role in the ecological balance of the region. The Danube represents anatural resource of water for industry, agriculture, domestic water and groundwater supply.

Copyright 2002 Royal Meteorological Society Int. J. Climatol. 22: 1169–1179 (2002)

CLIMATIC CONTROL OF RIVER DANUBE FLOW 1171

The Danube river basin can be divided into three sub-regions (Figure 1): the upper, the central and thelower basin. The upper basin extends from the source (Germany) to Bratislava, Slovak Republic. The centralbasin is the largest and comprises the section from Bratislava to the Iron Gates dam (the former Yugoslaviaand Romania). The lower basin is formed by the Romanian–Bulgarian lowlands and its upland plateau andmountains.

The climate of the Danube basin is very diverse. There is an influence of Atlantic climate in the westernpart of the upper basin, a Mediterranean influence in the southern part of central and lower basin, and acontinental climate elsewhere.

The Danube river flow is determined principally by the precipitation and evaporation processes from theDanube catchment basin. The mean quantity of precipitation that falls in a certain area of the Danube catchmentbasin is strongly dependent on the orography. Because one-third of the basin is formed by mountains and theremainder consists of hills and plains, the annual precipitation total ranges from about 2000 mm per year inthe high regions to only about 500 mm per year in the plains. Evaporation is also important for the waterbalance in the Danube catchment basin, especially in the lower regions where the mean annual evaporationvaries between 450 and 650 mm per year.

3. DATA AND METHODS

3.1. Data

The primary quantity analysed in this study is the annual mean of the Danube’s discharge. The timeseries of annual mean flows were calculated from monthly mean flows of the Danube river measured at sixhydrological stations located in the lower basin, on the Romanian border (Figure 1). These data were providedby the National Institute for Meteorology and Hydrology (NIMH), Bucharest, Romania. The names of thestations, their locations (latitude and longitude) and some simple statistical characteristics of the river flowfor the period 1931–95 are presented in Table I.

The decadal variations of the Danube’s flow are strongly related to decadal variations of precipitation overthe river catchment area. The monthly precipitation data set (PP) for global land areas, with a spatial resolutionof 5° latitude × 5° longitude constructed by Hulme (1992, 1994), has been used. From the 1900–98 periodPP data set, the period 1931–91 has been extracted for the present study.

Figure 1. The Danube river catchment basin and the locations of the Romanian hydrological stations from the lower basin used in thisstudy



Table I. The hydrological stations from the Danube river lower basin, their coordinates and some simple statisticalcharacteristics

Station Coordinates Annual mean Standard deviation (m3 s−1) Variancea Coefficient ofdischarge

Original Decadal(%) variabilityb

(m3 s−1)

Orsova 22°22′E, 44°40′N 5441 940 519 30.48 9.54Corabia 24°35′E, 43°46′N 5716 954 573 36.07 10.02Turnu Magurele 24°53′E, 43°42′N 6062 1032 618 35.86 10.19Giurgiu 25°56′E, 43°53′N 5944 974 534 30.06 8.98Calarasi 27°00′E, 44°10′N 6086 1080 658 37.12 10.81Ceatal Izmail 28°48′E, 45°20′N 6372 1160 670 33.36 10.51

a Decadal/original.b (Decadal Standard deviation/annual mean) × 100.

The SST data set is extracted from the global analyses of Kaplan et al. (1997, 1998, 2000) derived fromin situ data using a statistical method known as optimal smoothing (OS). The spatial resolution of the SSTdata set is 5° latitude × 5° longitude.

SLP over the ocean (resolution 4° latitude × 4° longitude) was extracted from the Kaplan data setrepresenting reduced space optimal interpolation of the global SLP anomalies from the ComprehensiveOcean–Atmosphere Data Set (COADS). The anomalies were calculated with respect to the climatologicalannual cycle estimated from the COADS data for the period 1951–80 (Kaplan et al., 2000). From the Kaplandata set for the period 1856–1991, both for SST and SLP, we have selected the period 1931–91.

The 500 hPa geopotential heights (G500) were taken from the National Centers for EnvironmentalProduction–National Center for Atmospheric Research (NCEP/NCAR) reanalyses data set (Kalnay et al.,1996). The horizontal resolution of the G500 field is 2.5° latitude × 2.5° longitude and the period analysed is1948–95.

3.2. Methods

All data were processed in the same way. First, annual means were calculated from monthly means. Then,annual anomalies with respect to the mean and normalized by local standard deviation estimated for the period1931–95 were produced. The annual normalized anomalies were smoothed with a 5 year running mean filterto obtain the decadal component of the series.

To explain the decadal variability of the river’s flow we have drawn composite maps. All mapscorresponding to the times when the normalized flow anomaly was lower (higher) than one standard deviationwere averaged. The map of the difference between high-flow and low-flow averaged maps was used to identifythe large-scale precipitation and atmospheric circulation anomalies associated with decadal variability of theDanube’s flow in its lower basin.

Correlation maps between the time series of decadal anomalies of flow and the global SST and SLP havebeen produced to investigate possible coherent large-scale connections.

4. RELATION WITH PRECIPITATION AND THE NAO

In this section we analyse the relation between decadal variations of the Danube river flow and decadalvariations of precipitation in the river catchment basin.

To obtain a quantitative measure of the strength of decadal (>5 year period) variations of the flow, thevariances for both the original (annual means) and the decadal time series of the annual mean flow measuredat selected hydrological stations have been calculated. The ratio between the decadal and original variance



for each station is presented in Table I. Consistent with partition of variance between interannual and decadaltime scales of river flows in Europe (Dettinger and Diaz, 2000), more than 30% of the mean annual flowvariability is contained in the decadal component at all selected stations in the Danube lower basin.

The time series of annual means of the Danube’s discharge recorded at selected hydrological stations duringthe period 1931–95 are presented in Figure 2 (thin line). The solid line represents the decadal componentobtained by smoothing the series with a 5 year running mean filter. A simple visual inspection of these timeseries shows that the decadal variations at all stations are quite similar. A decreasing trend is evident at allstations during the last two decades.

The correlation coefficients between the decadal flow time series at Ceatal Izmail and the remaining fivestations from the Danube lower basin have been calculated and are presented in Table II. Because the six time

Figure 2. The time series of annual means of the Danube river flow in the lower basin (thin line) measured at hydrological stationspresented in Table I for the period 1931–95. The solid line represents the decadal component obtained by smoothing the series with a

5 year running mean filter



Table II. The linear correlation coefficients between the time series of annual flow atCeatal Izmail station and five hydrological stations from the Danube river lower basin

Correlation coefficient

Orsova Corabia Tr. Magurele Giurgiu Calarasi

Ceatal Izmail 0.978 0.971 0.914 0.938 0.975

series of the Danube decadal streamflow are highly correlated with each other (the correlation coefficients aregreater than 0.91), for simplicity, in the following we consider only the time series of flow at Ceatal Izmail asrepresentative for the Danube river lower basin and have referred it as the Danube flow. The flow behaviourhas been analysed in connection with the precipitation field and the NAO.

The decadal flow variations have been analysed in connection with decadal variations of precipitation(PP) in the Danube catchment basin. A composite PP map has been constructed to identify the PP patternsassociated with decadal flow variations (Figure 3). This map shows that high values of precipitation over theDanube basin catchment occur in association with high values of Danube flow. It is interesting to note thathigher precipitation anomalies are located in the upper and central basins, which are the regions with themain tributaries to the total Danube flow in the lower basin. Accordingly, the decadal variability of flow inthe lower basin reflects mainly the decadal variability of precipitation from the upper and central catchmentbasins.

Many research studies have shown that decadal precipitation variability over Europe is related to the decadalvariability of the NAO (Hurrell, 1995; Rımbu et al., 2001). Therefore, the NAO signal should be present inthe European river flow time series. Our goal was to search for the NAO influence on the decadal variabilityof the Danube river flow in its lower basin.

Based on the PP pattern presented in Figure 3, a PP index has been defined by averaging the normalizedPP anomalies from the region (40–55 °N; 5–35 °E). This domain includes the whole Danube river catchmentbasin. The correlation coefficient between the time series of normalized river flow anomalies and the time

Figure 3. The composite map of precipitation in the European sector based on the decadal component of the Danube river flow. The mapwas obtained as the difference between the averaged maps of positive and negative composites. The composites were constructed fromPP maps for which the decadal normalized anomalies of river flow were higher/lower than one standard deviation. Contour interval is

30 mm



series of PP index is 0.80. The high value of the correlation coefficient between PP index and flow suggests thatdecadal PP variability is dominant compared with evaporation variability over this region at the decadal timescale. This is also consistent with the values of coefficient of variability (i.e. standard deviation/mean × 100)of river flow (about 10%) and of precipitation over the Danube river catchment basin (about 7%).

In Figure 4 we present the decadal time series of the NAO index (thin line), the normalized Danuberiver flow anomalies (solid line) and the PP index (dotted line). It is evident from Figure 4 that there is anout-of-phase relationship between the Danube flow and the NAO index. The correlation coefficient betweenthese time series is −0.75, consistent with other studies (Shorthouse and Arnell, 1999; Dettinger and Diaz,2000) that showed that the river flows tend to be lower than normal in central and southern Europe andhigher than normal in northernmost Europe when the NAO is in its positive phase. As Figure 4 shows, since1970 the NAO index has been on an upward trend while the Danube’s flow was continuously decreasing. Asimple visual inspection shows that the time series of Danube flow and the PP index present similar decadalvariations, with precipitation leading the river flow.

The cross-correlation functions between the monthly means of the Danube river flow anomalies and thePP and NAO indices (not shown) reveals that precipitation variations are in phase with the NAO indexvariations and lead the river flow variations by 2–3 months. This time delay between precipitation (or NAO)and streamflow variations is small compared with the time scale considered in our analysis.

5. GLOBAL CONNECTIONS OF RIVER FLOW

5.1. Relations with global SST

According to the results presented in the previous sections, the decadal variations of Danube’s dischargecan be related to the NAO to the extent in which the NAO controls the precipitation regime in the AtlanticEuropean sector (Hurrell and van Loon, 1997; Rımbu et al., 2001). Although our analysis was focused onthe NAO influence on the Danube’s flow in the lower basin, we looked at some atmospheric and ocean fieldassociations at the global scale.

Several studies have established that large-scale SST fluctuations can be linked to atmospheric circulationsthat produce precipitation fluctuations (Dai et al., 1997; Latif et al., 2000). One of the most widely studied

Figure 4. The time series of normalized anomalies of the Danube’s discharge at Ceatal Izmail station (solid line), NAO index (thinline), PP index (dashed line). All time series were normalized and smoothed with a 5 year running mean filter



phenomenon is El Nino–Southern Oscillation (ENSO), which generates coherent anomaly patterns oftemperature and precipitation in regions all over the globe (Dai and Wigley, 2000). However, the directimpact of ENSO on the North Atlantic and Europe appears to be weak.

Rajagopalan et al. (1998) have reported observational evidence of significant coherence at decadal timescales, with no phase lag, between tropical South Atlantic SSTs and the NAO, with warm SSTs associatedwith the positive phase of the NAO.

The correlation map between the time series of the Danube river flow and global SST represented inFigure 5(a) emphasizes coherent large-scale patterns. In the North Atlantic a tripole-like SST pattern similarto the SST pattern associated with the negative phase of the NAO (Hurrell, 1995) appears in connectionwith positive anomalies of the Danube river flow. The centres of the tripole anomaly SST pattern appearto be displaced towards the eastern coast of North America compared with the SST pattern in the NorthAtlantic associated with the NAO. Higher than average values of the Danube river flow tend to be associatedwith cooler than average SST anomalies in the central North Pacific and warmer than average SSTs alongthe western coast of North America, eastern and central tropical Pacific. Our results are compatible withthe negative correlation between North Pacific SSTs and river flows throughout North America, tropicalAfrica, central and southern Europe reported by Dettinger and Diaz (2000). Although the Pacific SST patternassociated with the Danube flow decadal variability bears some resemblance to the SST pattern characterizingthe Pacific Decadal Oscillation (PDO) (Mantua et al., 1997), the shift from positive (negative) to negative(positive) phase of PDO from 1948 (1977) is not clearly evident in the variability of the Danube river flow(Figure 2). This suggests that although the North Pacific SSTs exhibit coherent patterns similar to the PDO,their direct influence on decadal variations of the Danube river flow are smaller than the North Atlanticprocesses (i.e. the NAO) that control European hydrological behaviour.

a)

b)

Figure 5. (a) The correlation map between the time series of decadal component of the Danube river flow measured at Ceatal Izmailstation and SST. (b) As in (a), but for SLP



5.2. Connection with global SLP

The correlation map between the time series of the Danube river flow and SLP over the North Atlantic(Figure 5(b)) emphasizes coherent large-scale SLP patterns consistent with the corresponding SST patterns(Figure 5(a)). Over the North Atlantic, high values of flow are related to a dipole-like pattern of SLP anomaliessimilar to that corresponding to negative phase of the NAO, and are consistent with the negative correlationbetween the NAO index and the Danube flow. In agreement with SST anomalies, the central North Pacific isdominated by negative SLP anomalies. It is evident from Figure 5(b) that the SLP anomaly pattern over thecentral and eastern Pacific does not present an out-of-phase relation with the SLP anomalies over the westerntropical Pacific, as in the case of the interannual ENSO phenomenon. This pattern might suggest that theSST and SLP anomalies from the tropical Pacific are not generated through the atmosphere–ocean interactionprocesses that produce the typical interannual ENSO, and that their impact on the decadal variability ofthe Danube river is small. However, the coherent patterns presented in Figure 5 might be the result of asuperposition of different decadal modes of climate variability, such as the NAO (Hurrell, 1995) or PDO(Mantua et al., 1997). The mechanisms by which the correlations are established are still uncertain, becausethe way in which such decadal modes interact is not yet clarified (Dettinger and Diaz, 2000).

5.3. Connection with Northern Hemisphere G500

To assess better and confirm the links between the decadal Danube flow and large-scale SSTs andatmospheric circulation patterns described in the previous sections, we have constructed a composite map ofannual 500 hPa geopotential heights. The large-scale atmospheric circulation patterns associated with decadalvariability of precipitation over Europe, which influence the decadal variability of the Danube river flow areclearly emphasized in this field.

The composite map of G500 (Figure 6) emphasizes a dipole-like pattern in the North Atlantic that ischaracteristic of the negative phase of the NAO, and a large area of negative G500 anomalies in the NorthPacific that are consistent with the corresponding SLP patterns presented in Figure 5(b). The G500 variationscorresponding to the difference between the high values and low values of the Danube river flow are higherin the North Atlantic than the corresponding variations over the North Pacific. This confirms our suppositionthat the North Atlantic processes (i.e. the NAO) play the principal role in generating the Danube flow decadalvariations. Low values of G500 over central and southern Europe associated with high values of the Danuberiver flow are consistent with the increasing of precipitation over a large area that includes the entire Danuberiver catchment basin (Rımbu et al., 2001).

6. SUMMARY

In this study we have analysed the decadal variability of the Danube river flow in its lower basin using theflow records from six hydrological stations on the Romanian border. The decadal variations dominate theyear-to-year Danube flow variations at all stations analysed. Because all six time series of the annual meanof the Danube flow are highly correlated and present a similar time evolution, we considered only the riverflow at Ceatal Izmail station in our analysis.

The decadal flow variability was analysed in connection with land precipitation over Europe and the NAO.The composite PP map showed that high values of PP anomalies throughout the river catchment occur in

association with high values of the Danube river flow anomalies. As expected, the decadal variations of riverflow are in good agreement with the decadal variations of precipitation in the Danube catchment basin, beinglargely controlled by North Atlantic processes, particularly by the NAO.

Decadal NAO variability is reflected in flow variability as an out-of-phase relationship, in the sense thatthe Danube river flow in the lower basin tends to be lower than normal when the NAO is in its positivephase, and vice versa.

The correlation map between the Danube river flow and global SSTs shows that higher than average valuesof river flow tend to be associated with a tripole-like SST pattern in the North Atlantic, similar to the SST



Figure 6. The composite map of Northern Hemisphere geopotential height at 500 mbar (G500) based on the time series of the decadalDanube river flow. Solid line corresponds to high flow and dashed line to low flow. The regions where the difference between thecomposite G500 maps corresponding to high flow and low flow are higher (lower) than 10 gpm are shaded (light shaded). The contour

interval is 10 dam

pattern associated with the negative phase of the NAO, and with cooler than average SST anomalies in thecentral North Pacific and warmer than average SSTs along the western coast of North America, eastern andcentral tropical Pacific. The corresponding SLP correlation map emphasizes a dipole-like pattern of SLPanomalies over the North Atlantic, similar to that characterizing the negative phase of the NAO, and a largearea of negative SLP anomalies in the North Pacific.

Consistent with the global coherent patterns in the fields of SST and SLP, the composite map of geopotentialheights at 500 hPa confirms the dominant role of the NAO in generating large-scale atmospheric circulationpatterns and its associated precipitation modes that influence the hydrologic fluxes in Europe.

The composite and correlation patterns presented in this paper demonstrate that decadal variations of theDanube’s flow respond to climate forcings on nearly global scales, just as does the decadal precipitationvariability over Europe.

ACKNOWLEDGEMENTS

The authors would like to thank Dr Glenn R. McGregor and the anonymous referees for their helpful commentsand suggestions. N. Rımbu and M. Dima thank Dr Gerrit Lohmann and Dr Ute Merkel for fruitful discussionsduring their visit at the Max Plank Institute in Hamburg that helped to improve this paper.

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decadal variability of the danube river flow in the lower basin and its relation with the north...

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