CHANGES OF THE HYDROLOGICAL REGIME OF RIVERS OF NORTHERN AND CENTRAL EUROPE IN VARIOUS PERIODS OF THE NORTH ATLANTIC OSCILLATION

ABSTRACT. The article presents the impact of the north Atlantic atmospheric circulation on the hydrological regimes of rivers of Northern and Central Europe. A river regime is defined as the type and temporal structure of streamflow in an average hydrological cycle. The elements of the structure include so-called hydrological periods, which were the instrument for the study of the regime as well as the basis of its characterisation and an assessment of its variability. Hydrological periods were determined via the grouping of elementary time units of the year (pentads) on the basis of the similarity of one of their features (the frequency distribution of the river discharge). The determination of hydrological periods was carried out on the basis of sets of 20 years with an extremely high and an extremely low NAO index, as well as 20-year periods from the 1901-2000 interval. The varying number and temporal structure of the hydrological periods thus distinguished allowed conclusions about differences in the variability of hydrological regimes of Scandinavian and Central European rivers in the various NAO circulation periods. Dariusz Wrzesiński, Institute of Physical Geography and Environmental Planning, Adam Mickiewicz University, Dzięgielowa 27, 61-680 Poznań, Poland, darwrze@amu.edu.pl

CHANGES OF THE HYDROLOGICAL REGIME OF RIVERS OF NORTHERN AND CENTRAL EUROPE IN VARIOUS PERIODS OF THE NORTH ATLANTIC OSCILLATION DARIUSZ WRZESIŃSKI Adam Mickiewicz University, Institute of Physical Geography and Environmental Planning, Poznań, Poland Manuscript received Revised version Wrzesiński D., 2005 Changes of the hydrological regime of rivers of Northern and Central Europe in various periods of the North Atlantic Oscillation Quaestiones Geographicae 24, Adam Mickiewicz University Press, Poznań, 2005, pp , Figs 7, Table ISBN 83-232-0454-3 ISSN 0137-477X ABSTRACT The article presents the impact of the north Atlantic atmospheric circulation on the hydrological regimes of rivers of Northern and Central Europe A river regime is defined as the type and temporal structure of streamflow in an average hydrological cycle The elements of the structure include so-called hydrological periods, which were the instrument for the study of the regime as well as the basis of its characterisation and an assessment of its variability Hydrological periods were determined via the grouping of elementary time units of the year (pentads) on the basis of the similarity of one of their features (the frequency distribution of the river discharge) The determination of hydrological periods was carried out on the basis of sets of 20 years with an extremely high and an extremely low NAO index, as well as 20-year periods from the 1901-2000 interval The varying number and temporal structure of the hydrological periods thus distinguished allowed conclusions about differences in the variability of hydrological regimes of Scandinavian and Central European rivers in the various NAO circulation periods Dariusz Wrzesiński, Institute of Physical Geography and Environmental Planning, Adam Mickiewicz University, Dzięgielowa 27, 61-680 Poznań, Poland, darwrze@amu.edu.pl Introduction The hydrological regime defines the state and responses of a river system with relation to the climatic system and the catchment's physical-geographic characteristics Variations in the streamflow over the year strongly depend on climatic conditions Deviations of climatic elements from average levels caused by, e.g., changes in the atmospheric circulation modify the conditions in which the streamflow forms, and hence also the characteristics of the river regime It is controlled by both, precipitation and air temperature, whose magnitudes show a significant dependence on the intensity of zonal circulation A simple indicator characterising the atmospheric circulation over the north Atlantic is the North Atlantic Oscillation Index (henceforth, NAO) It describes the pressure gradient between the Icelandic Low and the Azores High Its value, especially in winter, determines the intensity of zonal air circulation over the north Atlantic and the European continent In a positive stage of the NAO (NAO+), the pressure gradient is greater and zonal circulation intensifies, which means a greater inflow of moist maritime polar air over the European continent (Hurrell 1995, Hurrell, van Loon 1997, Rogers 1997, Serrese et al 1997, Trigo et al 2002) Western and Central Europe experience then mild winters with frequent thaws and impermanent snow cover The summer is then moderately warm, cloudy, with low-intensity but frequent rains In the negative stage (NAO-), the westerly flow is restricted or stopped and there appear meridional circulation flows The areas over which air masses flow from the north experience cooling, while those to which air masses come from the south enjoy marked amelioration The occurrence of warm and wet winters as well as frosty and snowy ones in Northern and Central Europe crucially affects the discharge of rivers in those regions In winter, the discharges of north European rivers are positively correlated with the winter NAO index, and those of Southern Europe are correlated negatively (Shorthouse, Arnell 1997) While in Northern Europe this relation can be accounted for by an increase in precipitation in a positive stage of the NAO and in Southern Europe by its decrease, in Central and Western Europe the dependence of winter precipitation on the NAO is rather weak (Kaczmarek 2002) A strong relation does exist, however, between the NAO indices and air temperatures Temperature controls the magnitude of water loss through evaporation in summer and the development and disappearance of snow cover in winter In Central Europe during a warm winter (NAO+) snow cover dwindles, hence meltwater floods are rare and carry small discharge volumes In the negative NAO stage, in turn, winters are severe with a thick snow cover, thus contributing to high and large-volume flood waves In the article a study is made of the effect of the NAO on the temporal structure of hydrological phenomena and their variations in a yearly cycle and on the stability of the characteristics of hydrological regimes The determination of the number and temporal structure of the hydrological periods distinguished in long time series (of about 100 years) is intended to help evaluate changes in the hydrological regimes of the rivers in various circulation periods Study area and source materials The effect of the intensity of zonal circulation on river discharges is especially pronounced in Northern and Central Europe Statistically significant differences in runoffs in the various NAO stages can be observed from the winter months till the summer In a NAO+ stage the runoffs are higher in Northern Europe, e.g the Vosso (Fig 2) and in the NAO- stage, in spring in Central Europe, e.g the Warta (Fig 3) The strength of the discovered links is similar, but their nature is different For this reason these regions seem to be particularly interesting for a study of the effect of the intensity of zonal circulation on the characteristics of a hydrological regime The analysis embraced 17 rivers flowing in a variety of environmental conditions obtaining in those regions (Fig 1) Use was made of the daily discharge figures for the period from 1901 to 2000 They come mostly from the resources of the Global Runoff Data Center The NAO values employed, calculated according to J Hurrell's (1995) method, were obtained from the NOAA through the Internet Figure Study area – the Tana at Polmak, – the Lakselv at Skoganvarre, – the Kalix at Råktfors, – the Fusta at Fustvatn, – the Oulu at Lake Lenfad outlet, – the Öre at Torrböle, – the Kyrön near the mouth, – the Nilakka near Vesanto, – the Gaula at Haga Bru, 10 – the Lagen at Losna, 11 – the Etna at Etna, 12 – the Vosso at Bulken, 13 – the Helge Å at Torsebro, 14 – the Warta at Poznań, 15 – the Oder at Eisenhüttenstadt, 16 – the Labe at Dresden, 17 – the Jizera at Turice VOSSO - BULKEN 100 NAO- 90 NAO+ 80 H [mm] 70 60 50 40 30 20 10 73 69 65 61 57 53 49 45 41 37 33 29 25 21 17 13 pentads Figure Mean runoff of the Vosso river in 20 years with the lowest (NAO-) and 20 years with the highest (NAO+) NAO values WARTA - POZNAŃ NAO- NAO+ H [mm] 73 70 67 64 61 58 55 52 49 46 43 40 37 34 31 28 25 22 19 16 13 10 pentads Figure Mean runoff of the Warta river in 20 years with the lowest (NAO-) and 20 years with the highest (NAO+) NAO values Methods of study The method employed in the article to determine a river regime was one involving the identification of the temporal structure of hydrological phenomena and their variations in a yearly cycle Its theoretical foundations can be found in Rotnicka (1988, 1991) An important point in the research procedure is distinguishing characteristic stages of the cycle that were termed hydrological periods It consists in the grouping of elementary time units of the hydrological year on the basis of the similarity of one of their features The elementary time unit adopted is a five-day period of time, or a pentad, and the grouping criterion, the river discharge Thus, the calendar year consists of 73 pentads, each represented by five discharge figures The nearly secular discharge measurement series under study were divided into 20year intervals In such a 20-year observation period, a 100-element set of discharge values is obtained for each of the pentads The grouping characteristic is described by variable x, which is presented in the form of a discharge frequency distribution The similarity of water discharge distributions was established using the nonparametric Kolmogorov-Smirnov test The result of the testing procedure is a quadratic similarity matrix of the yearly set of pentads in which the rows and columns are designated by the numbers of the pentads in chronological order The matrix is pictured as a diagram presenting relations (links) holding among the pentads in term of the similarity of their feature An illustrative matrix diagram depicting the structure of the elementary time units of the year and the method of distinguishing hydrological periods is shown in Figure Along its diagonal, groups of similar pentads can be noted; it is those groups that form the so-called hydrological periods Thus, a hydrological period is a time segment displaying a uniform structure of links among pentads in terms of the conformity of discharge distributions A river regime is defined here as a kind and temporal structure of river discharges in a normal hydrological cycle The elements of the structure are hydrological periods, which provide a tool for the study of a regime and a basis for its classification In the initial stage of the research, a comparison was made of the characteristics of river regimes in periods with low and high winter values of the NAO Of the 100year study period, two sets of 20 years were selected, one with the lowest and the other with the highest NAO values In the next stage, the 100-year measurement Q [m3s -1] 800 600 SWQ A 400 1% 5% 200 10% GGSQ 25% SSQ 50% 75% 90% 99% DGSQ SNQ B 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 19 10 25 19 18 32 32 32 36 32 25 15 26 26 24 27 27 29 32 32 37 34 32 32 32 37 30 26 24 13 10 10 10 11 1 7 11 10 15 27 28 30 36 43 30 38 40 53 50 49 48 43 38 28 28 24 16 10 25 19 25 25 32 32 32 36 32 25 15 24 29 29 24 27 27 29 32 32 37 34 32 32 32 37 30 26 24 13 6 11 11 11 11 10 13 18 30 32 33 39 46 32 38 43 56 53 52 48 43 38 28 25 18 18 18 14 14 16 27 27 29 30 32 29 25 18 15 26 31 31 25 25 25 25 30 27 25 25 25 30 19 17 15 10 10 7 14 18 12 11 7 13 10 10 16 30 31 33 39 46 37 45 47 56 55 53 55 50 45 35 32 12 27 19 34 34 34 38 34 27 17 25 25 26 29 29 31 34 34 39 36 34 34 34 39 32 28 26 24 15 6 9 13 15 17 28 32 33 37 44 30 36 41 54 51 50 46 41 36 29 29 25 24 24 24 11 13 12 18 28 33 34 37 44 32 40 42 54 51 50 50 45 40 36 36 32 31 31 31 27 13 19 17 15 14 18 30 30 30 34 30 13 15 18 19 25 25 27 30 30 35 32 30 30 30 35 28 24 12 15 16 15 12 11 15 11 11 10 13 12 24 25 33 34 39 43 41 49 51 56 59 57 59 54 49 39 36 29 28 28 28 24 10 10 14 13 17 17 25 14 15 16 16 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24 24 24 26 29 29 34 31 29 29 29 34 27 19 16 13 10 14 71 6 12 14 16 24 33 34 33 40 33 41 43 50 51 49 51 46 41 31 28 16 11 24 18 25 25 31 31 31 35 31 24 14 24 29 29 24 26 26 28 31 31 36 33 31 31 31 36 29 25 18 13 8 12 11 72 11 11 11 10 15 16 19 33 34 29 35 31 39 41 46 49 47 49 44 39 29 26 30 31 30 37 30 38 40 47 48 46 48 43 38 28 25 17 16 16 16 12 10 25 19 25 29 29 32 32 34 36 32 25 17 28 33 33 27 27 27 29 32 32 37 34 32 32 32 37 30 26 24 19 16 11 11 4 73 9 15 12 13 14 I II III IV V I Figure Hydrological periods on the Warta in the Poznań gauging profile A - Hydrogram of water discharges of specified probability of occurrence, with characteristic discharge and boundaries of medium discharge zone: SWQ - average peak discharge, SSQ- average medium discharge, SNQ - average low discharge, GGSQ - upper boundary of medium discharge zone, DGSQ - lower boundary of medium discharge zone B – Diagram of a similarity matrix: – similar distributions, – probably similar distributions, I, II V – successive hydrological periods series were divided into 20-year intervals (1901-1920, 1911-1930, 1921-1940, , 1981-2000) for which hydrological periods were distinguished It allowed an evaluation of long-term changes in the hydrological regimes of selected rivers seen against changes in the NAO index over that time interval The next step was the application of classification methods to reveal the structure of similarity among the hydrological period distinguished, which made it possible to identify types of hydrological periods To this end, use was made of Ward's method of hierarchical grouping Each hydrological period was described by means of four variables characterising the frequency distribution of water discharge: medium discharge (d5), the coefficient of variation (Cv), the coefficient of skewness (s), and the coefficient of discharge (w) defining water abundance of a hydrological period Typology of hydrological periods The grouping procedure allowed 12 types of hydrological periods to be distinguished; their characteristics are presented in Table Table Characteristics of the types of hydrological periods Characteristics of explanatory variables Type of period (N – number) I - Deep stable low water (N- 20) II - Deep unstable low water (N – 7) III – Average low water (N – 16) IV - Shallow low Name of explanatory variable d5 Cv s w duration (days) d5 Cv s w duration (days) d5 Cv s w duration (days) d5 Cv Mean 0.61 0.49 2.14 0.34 118.80 0.44 1.14 4.68 0.30 99.71 0.63 1.08 2.84 0.58 70.81 0.95 0.59 Standard deviation (s) 0.21 0.17 1.72 0.14 53.32 0.19 0.15 1.25 0.11 51.24 0.17 0.57 1.29 0.08 31.89 0.23 0.18 Variation coefficient (Cv) 0.35 0.36 0.80 0.41 0.45 0.43 0.14 0.27 0.35 0.51 0.27 0.53 0.45 0.13 0.45 0.24 0.30 Min Max 0.31 0.24 0.43 0.09 30.0 0.16 0.99 3.25 0.15 30.0 0.41 0.25 0.75 0.41 27.0 0.70 0.35 1.10 0.82 7.54 0.57 200.0 0.72 1.40 6.63 0.46 177.0 0.97 2.18 4.94 0.72 145.0 1.52 0.95 water s w (N – 17) duration (days) d5 Cv V - Normal stable s period w duration (days) (N – 26) d5 VI – Normal Cv unstable period s (N – 18) w duration (days) d5 Cv VII – Low stable s rise w duration (days) (N – 29) d5 VIII – Low unstable Cv s rise w duration (days) (N – 7) d5 Cv IX – Average s stable rise w duration (days) (N – 18) d5 Cv X- Average s unstable rise w duration (days) (N – 5) d5 Cv XI – High stable s rise w duration (days) (N – 11) d5 Cv XII – Disastrous s rise w duration (days) (N – 12) 2.55 0.80 118.76 1.31 0.66 2.23 1.05 62.19 1.48 1.05 2.80 1.04 83.44 1.84 0.53 1.26 1.45 47.10 1.67 1.21 2.10 1.49 31.29 2.96 0.63 1.75 2.00 39.72 2.61 1.05 1.74 1.92 19.60 4.72 0.51 0.92 2.73 32.55 9.25 0.62 1.05 4.19 25.75 2.11 0.08 58.17 0.24 0.12 1.08 0.11 33.97 0.50 0.17 1.72 0.15 50.70 0.40 0.12 0.46 0.17 23.22 0.54 0.15 0.40 0.09 12.98 0.86 0.27 0.85 0.14 20.59 0.89 0.15 0.32 0.11 7.60 1.46 0.18 0.69 0.24 16.49 2.39 0.09 0.50 0.55 9.20 0.83 0.11 0.49 0.18 0.19 0.48 0.10 0.55 0.34 0.16 0.61 0.14 0.61 0.22 0.23 0.37 0.12 0.49 0.32 0.13 0.19 0.06 0.41 0.29 0.43 0.49 0.07 0.52 0.34 0.15 0.18 0.06 0.39 0.31 0.36 0.75 0.09 0.51 0.26 0.14 0.47 0.13 0.36 0.50 0.70 33.00 0.94 0.46 0.76 0.90 15.00 0.68 0.88 1.17 0.85 15.00 1.24 0.38 0.62 1.20 18.00 0.74 1.03 1.52 1.34 20.00 1.16 0.22 -0.01 1.77 20.00 1.77 0.86 1.36 1.78 15.00 2.96 0.27 -0.32 2.40 10.00 6.20 0.49 0.27 3.25 15.00 7.42 1.00 210.0 1.76 0.86 5.58 1.28 142.0 3.04 1.52 8.58 1.30 150.0 2.81 0.88 2.18 1.78 102.0 2.38 1.43 2.55 1.63 52.00 4.62 1.46 3.24 2.28 95.00 3.83 1.29 2.19 2.03 33.00 7.35 0.88 2.22 3.04 60.00 13.90 0.77 1.86 5.02 40.00 The sequence of the types of hydrological periods in the yearly cycle is presented in Table and Fig Three groups of rivers can be distinguished in them which differ in the sequence and temporal structure of the hydrological periods and their types: the rivers of Norway and Sweden, those of Finland, and those of Central Europe This regional division is clearly the result of the geographical location and specific environmental conditions, mainly climatic, obtaining in the catchments of the rivers in question Scandinavian rivers differ from Central European ones in that they usually have regimes with a greater number of hydrological periods (6 or even 7) and markedly different dates of their occurrence Owing to a higher seasonal variability of their discharges, the types of hydrological periods distinguished are extreme in terms of their water abundance Hence their regimes feature sharp contrasts, with a deep winter low-water stage and a very high late-spring rise In regime terms, they not form a uniform group, since the flow of each river is controlled by its own individual natural conditions, as has been mentioned earlier The Helge Å in southern Sweden and Finnish rivers (mostly the Oulu and Nilakka) stand out among them for the clearly distinct temporal structure of their hydrological periods In comparison with the Scandinavian streams, those in Central Europe have much less contrasting regimes, with a high spring rise and a shallow summer-autumn low-water stage Table Characteristics of the types of hydrological periods at extreme values of the NAO Index Ga ula Fusta 359 98 105 0.483 0.474 0.86 0.090 I 351 95 110 0.658 3.715 1.18 0.123 99 130 32 1.742 2.833 8.95 0.935 VI 96 125 30 1.515 2.890 9.66 1.009 XII 131 150 20 0.621 0.694 50.28 5.253 XII 126 155 30 0.740 1.314 42.71 4.463 XI 151 165 15 0.671 1.189 23.96 2.503 XI 156 165 10 0.883 1.836 26.48 2.767 VI 166 298 133 1.032 2.608 11.42 1.193 VI 166 295 130 1.233 3.655 9.69 1.013 II 299 358 60 0.983 2.965 2.73 0.286 III 296 350 55 1.501 4.941 4.71 0.492 W S Qmean [m3s-1] Cv duration (days) End (day of year) Type of period Start (day of year) W Qmean [m3s-1] I III Type of period S duration (days) End (day of year) NAO + Cv Etna River Start (day of year) NAO - II 359 110 117 1.395 5.577 10.57 0.327 III 331 110 145 1.317 3.157 18.61 0.575 V 111 145 35 0.739 0.986 39.14 1.210 V 111 135 25 0.746 0.761 41.50 1.283 IX 146 180 35 0.492 0.784 67.11 2.074 XI 136 168 33 0.502 0.638 77.74 2.403 VII 181 203 23 0.531 0.966 43.35 1.340 VII 169 223 55 0.455 0.953 52.30 1.617 V 204 250 47 0.668 1.495 29.10 0.899 V 224 305 82 0.823 1.854 37.02 1.144 VII 251 305 55 0.726 1.391 45.28 1.400 VI 306 330 25 1.205 2.470 28.01 0.866 III 306 358 53 1.194 3.479 18.61 0.575 I 361 100 105 0.820 3.471 10.92 0.139 306 103 163 1.322 6.155 20.55 0.264 VI 101 130 30 1.161 1.889 82.42 1.049 VIII 104 125 22 1.219 2.548 119.03 1.515 II 10 Helge Å Jizera Kalix Kyron Lagen Lakselv Labe Nilakka Oder Öre XII 131 170 40 0.625 1.566 255.36 3.251 XII 126 160 35 0.661 1.484 312.55 3.979 X 171 188 18 0.862 1.912 158.85 2.022 IX 161 188 28 0.699 1.535 170.37 2.169 VI 189 320 132 1.214 8.582 67.64 0.861 VI 189 305 117 1.161 3.995 75.54 0.962 II 321 360 40 1.052 6.030 26.57 0.338 V 90 90 0.774 1.902 36.51 1.012 IX 95 95 0.486 1.025 70.41 1.952 VII 91 135 45 0.565 1.284 63.96 1.774 V 96 155 60 0.535 1.219 39.53 1.096 IV 136 168 33 0.542 1.206 32.65 0.905 II 156 255 100 0.996 3.794 16.60 0.460 I 169 300 132 0.791 2.475 15.96 0.443 III 256 300 45 0.767 1.102 23.61 0.655 V 301 343 43 0.815 2.393 38.98 1.081 V 301 325 25 0.706 1.609 39.09 1.084 VII 344 365 22 0.491 1.344 47.53 1.318 VII 326 365 40 0.523 0.716 47.18 1.308 VI 306 80 140 0.937 3.562 22.55 0.891 VI 346 63 83 0.950 3.526 32.62 1.289 VII 81 143 63 0.600 2.062 44.94 1.776 IX 64 135 72 0.652 2.910 46.50 1.838 IV 144 305 162 0.949 7.418 17.82 0.704 IV 136 345 210 0.931 5.482 18.33 0.724 I 301 120 185 0.431 1.940 82.80 0.292 299 113 180 0.376 2.166 89.00 0.314 VI 121 135 15 0.928 1.166 356.40 1.256 VI 114 130 17 1.006 1.443 367.80 1.296 XI 136 160 25 0.448 0.542 813.50 2.867 XI 131 170 40 0.443 0.862 862.20 3.038 IX 161 213 53 0.453 1.761 557.64 1.965 VII 171 253 83 0.400 1.215 462.10 1.629 VII 214 268 55 0.428 1.587 341.45 1.203 V 254 298 45 0.460 1.318 267.64 0.943 V 269 300 32 0.507 3.060 256.20 0.903 0.207 III II 361 98 103 1.225 4.658 8.94 X 99 115 17 1.286 1.623 76.72 XII 116 130 15 0.581 VIII 131 175 45 III 176 255 VI 256 I 306 V I 341 75 100 1.531 3.989 22.71 0.526 X 76 108 33 1.067 1.360 81.67 1.890 0.447 190.94 4.418 XII 109 123 15 0.584 1.274 149.78 3.466 1.187 2.315 58.03 1.343 VIII 124 175 52 1.028 1.783 64.01 1.481 80 1.708 4.238 17.67 0.409 III 176 275 100 1.484 3.156 24.79 0.573 360 105 0.929 1.672 39.33 0.910 VI 276 340 65 1.064 2.032 46.45 1.075 120 180 0.415 0.433 79.32 0.324 289 115 192 0.678 2.289 85.29 0.349 121 145 25 0.856 1.564 284.23 1.162 VI 116 135 20 0.929 1.313 279.90 1.145 XI 146 180 35 0.413 0.986 610.68 2.497 XI 136 195 60 0.468 0.996 696.76 2.850 VII 181 253 73 0.422 1.819 380.05 1.554 IX 196 220 25 0.348 1.235 513.43 2.100 IV 254 305 52 0.465 1.604 187.78 0.768 VII 221 255 35 0.487 2.184 346.39 1.417 IV 256 288 33 0.400 1.626 206.89 0.846 1.775 I I 286 123 203 0.553 1.178 5.29 0.347 I 304 130 192 0.680 6.418 5.58 0.366 VIII 124 143 20 1.432 2.496 23.52 1.541 X 131 145 15 1.039 1.604 31.04 2.035 XII 144 178 35 0.659 0.928 53.47 3.505 XII 146 180 35 0.583 1.278 68.12 4.465 IX 179 198 20 0.705 1.963 29.71 1.947 IX 181 213 33 0.729 2.171 28.83 1.890 V 199 285 87 0.520 1.866 14.11 0.925 V 214 303 90 0.698 5.580 14.37 0.942 V 299 75 142 0.810 3.458 333.75 1.051 V 329 58 95 0.746 3.137 371.71 1.171 IX 76 145 70 0.682 1.828 597.04 1.881 VII 59 123 65 0.495 2.087 472.28 1.488 V 146 193 48 0.804 3.170 318.39 1.003 V 124 163 40 0.547 1.875 314.76 0.991 IV 194 298 105 0.734 2.546 241.96 0.762 IV 164 328 165 0.747 3.584 232.75 0.733 IV 224 25 167 0.472 0.738 15.65 0.794 IV 246 45 165 0.432 1.086 16.98 0.861 III 26 115 90 0.388 1.516 12.04 0.611 III 46 105 60 0.251 1.044 12.99 0.659 V 116 130 15 0.540 0.987 18.23 0.925 VII 106 130 25 0.509 0.619 24.13 1.224 VII 131 223 93 0.425 0.670 26.75 1.357 IX 131 170 40 0.218 -0.012 41.10 2.084 VII 171 245 75 0.383 0.738 27.11 1.375 V 321 60 105 0.504 2.424 319.54 1.044 V 349 33 50 0.623 1.960 318.34 1.040 VII 61 78 18 0.668 1.207 474.98 1.551 VII 34 135 102 0.433 1.472 381.60 1.246 IX 79 115 37 0.518 1.600 585.83 1.913 IV 136 348 213 0.655 3.085 225.51 0.736 VII 116 170 55 0.560 1.518 416.60 1.361 IV 171 320 150 0.561 3.200 305.73 0.870 I 331 105 140 0.323 0.501 7.86 0.244 321 100 145 0.454 3.026 10.29 0.319 VIII 106 130 25 1.093 1.519 52.53 1.630 VIII 101 120 20 1.139 1.795 46.49 1.443 XII 131 148 18 0.530 0.461 141.92 4.405 XII 121 140 20 0.486 0.272 152.98 4.748 IX 149 180 32 0.753 1.872 1.767 IX 141 170 30 0.890 2.342 2.281 56.93 I 73.51 11 Oulu Tana Vosso Warta VI 181 330 150 0.939 3.957 27.31 0.848 VI 171 320 150 0.884 1.805 29.40 0.913 III 356 40 50 0.343 1.848 15.98 0.636 III 361 50 55 0.336 0.750 16.16 0.643 I 41 115 75 0.332 4.386 10.16 0.404 51 110 60 0.240 0.434 11.56 0.460 VII 116 135 20 0.704 0.790 31.13 1.238 VII 111 135 25 0.773 0.943 40.72 1.620 XI 136 168 33 0.400 0.366 60.83 2.420 XI 136 155 20 0.269 -0.323 75.84 3.017 VII 169 210 42 0.384 1.002 35.67 1.419 VII 156 220 65 0.455 0.655 43.07 1.714 III 211 275 65 0.552 2.032 18.02 0.717 IV 221 360 140 0.487 0.782 22.96 0.913 IV 276 355 80 0.507 0.775 21.35 0.849 I 276 110 200 0.300 1.160 66.77 0.397 304 110 172 0.290 2.431 59.92 0.356 VIII 111 145 35 1.407 2.235 244.42 1.454 IX 111 145 35 1.455 3.017 324.19 1.929 XII 146 168 23 0.772 1.861 668.15 3.975 XII 146 160 15 0.510 0.805 844.17 5.022 IX 169 188 20 0.689 2.264 331.42 1.972 IX 161 205 45 0.692 3.242 355.19 2.113 V 189 275 87 0.543 2.036 166.18 0.989 V 206 303 98 0.634 4.315 160.26 0.953 II 361 110 115 1.076 3.700 9.92 0.150 III 311 103 158 1.202 3.494 34.71 0.526 VII 111 145 35 0.653 0.809 82.97 1.258 VII 104 135 32 0.884 1.483 85.12 1.290 IX 146 170 25 0.378 0.774 143.53 2.176 XI 136 195 60 0.387 0.791 184.78 2.801 VII 171 215 45 0.547 1.145 84.78 1.285 VII 196 245 50 0.511 1.684 95.86 1.453 VI 216 310 95 0.877 1.886 68.17 1.034 VI 246 310 65 0.963 1.919 70.51 1.069 III 311 360 50 1.256 3.248 34.73 0.527 V 321 65 110 0.658 2.466 101.81 1.048 V 321 40 85 0.741 2.533 118.79 1.223 X 66 80 15 0.990 2.188 180.31 1.856 VII 41 65 25 0.442 1.155 158.06 1.627 XI 81 110 30 0.773 2.223 282.23 2.905 IX 66 85 20 0.468 1.129 183.53 1.889 VII 111 135 25 0.513 1.947 153.68 1.582 VII 86 105 20 0.454 0.953 146.68 1.510 V 136 153 18 0.527 1.654 107.75 1.109 V 106 143 38 0.538 2.436 113.56 1.169 IV 154 320 167 0.573 2.424 0.722 IV 144 320 177 0.672 6.404 0.734 70.13 I I 71.28 Changes in the features of the hydrological regime at extreme values of the NAO When analysing the temporal structure and sequence of hydrological periods at various levels of NAO activity, special attention was paid to the dates and patterns of low-water and high-water stages The sequence and number of the hydrological periods distinguished are similar, but there are differences in their dates and types In the years with high winter (December-March) values of the NAO (NAODJFM+), rises usually start earlier Meltwater floods are forward by a week (the Fusta, Kalix, Oulu, Öre, Lagen, Etna, Vosso), or even a month (the remaining rivers) Only the rivers of northern Norway (the Tana, Lakselv) not display significant variations The date of rise termination in a NAO+ stage is usually later than or similar to that in the negative NAO stage Markedly earlier, also by a month, is the termination date of 12 rises on Central European rivers – Fig On most Scandinavian rivers, in a NAO+ stage there is a noticeable lengthening of the duration of high rises, while in Central Europe they tend to be shortened or not appear at all During the negative stage of the NAO, on most Scandinavian rivers the low-water season starts slightly earlier, and because it usually ends later, low-water flows are longer and deeper In Central Europe it is the other way round In a positive NAO stage low-water flows are decidedly longer, even by two months, but the mean low-flow discharge is similar to that in the NAO- stage 13 VOSSO - BULKEN NAO+ 500 500 400 400 Q [m3s -1] 600 300 300 200 200 100 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 5 4 8 8 8 36 54 68 78 83 88 92 94 95 95 97 95 93 92 90 80 77 80 67 71 67 60 56 50 52 53 49 57 64 69 65 61 65 58 62 62 73 65 50 42 41 22 19 18 20 19 21 16 27 20 1 2 3 3 2 3 2 14 41 59 73 83 88 93 97 97 97 97 97 97 97 97 95 85 82 85 72 76 72 65 61 55 57 58 54 62 69 74 70 66 70 63 67 67 78 70 55 47 46 27 24 19 25 23 26 21 32 25 14 1 2 3 3 2 3 2 14 41 59 73 83 88 93 97 97 97 97 97 97 97 97 95 85 82 85 72 76 72 65 61 55 57 58 54 62 69 74 70 66 70 63 67 67 78 70 55 47 46 27 24 19 25 23 26 21 32 25 14 2 3 3 2 3 3 14 41 59 73 83 88 93 97 97 97 97 97 97 97 97 95 85 82 85 72 76 72 65 61 55 57 58 54 62 69 74 70 66 70 63 67 67 78 70 55 47 46 27 24 19 25 23 26 21 32 25 14 3 1 4 0 4 4 4 2 17 13 44 40 62 58 76 72 86 82 91 87 96 92 ## 96 ## 96 ## 96 ## 96 ## 97 ## 96 ## 96 ## 96 98 94 88 84 85 81 88 84 75 71 79 75 75 71 68 64 64 60 58 54 60 56 61 57 57 53 65 61 72 68 77 73 73 69 69 65 73 69 66 62 70 66 70 66 81 77 73 69 58 54 50 46 49 45 30 26 27 23 22 18 28 24 26 22 29 25 24 20 35 31 28 24 17 13 2 1 5 5 5 12 39 57 71 81 86 91 95 95 95 95 97 95 95 95 93 83 80 83 70 74 70 63 59 53 55 56 52 60 67 72 68 64 68 61 65 65 76 68 53 45 44 25 22 18 23 21 24 19 30 23 12 2 1 4 4 4 1 13 40 58 72 82 87 92 96 96 96 96 97 96 96 96 94 84 81 84 71 75 71 64 60 54 56 57 53 61 68 73 69 65 69 62 66 66 77 69 54 46 45 26 23 18 24 22 25 20 31 24 13 3 0 0 17 44 62 76 86 91 96 ## ## ## ## ## ## ## ## 98 88 85 88 75 79 75 68 64 58 60 61 57 65 72 77 73 69 73 66 70 70 81 73 58 50 49 30 27 22 28 26 29 24 35 28 17 8 3 3 3 3 0 4 5 4 4 0 1 0 1 0 1 1 1 5 0 0 0 2 5 3 6 17 17 17 16 44 44 44 43 62 62 62 61 76 76 76 75 86 86 86 85 91 91 91 90 96 96 96 95 ## ## ## 99 ## ## ## 99 ## ## ## 99 ## ## ## 99 ## ## ## 99 ## ## ## 99 ## ## ## 99 ## ## ## 99 98 98 98 97 88 88 88 87 85 85 85 84 88 88 88 87 75 75 75 74 79 79 79 78 75 75 75 74 68 68 68 67 64 64 64 63 58 58 58 57 60 60 60 59 61 61 61 60 57 57 57 56 65 65 65 64 72 72 72 71 77 77 77 76 73 73 73 72 69 69 69 68 73 73 73 72 66 66 66 65 70 70 70 69 70 70 70 69 81 81 81 80 73 73 73 72 58 58 58 57 50 50 50 49 49 49 49 48 30 30 30 29 27 27 27 26 22 22 22 21 28 28 28 27 26 26 26 25 29 29 29 28 24 24 24 23 35 35 35 34 28 28 28 27 17 17 17 16 2 1 5 5 5 12 39 57 71 81 86 91 95 95 95 95 97 95 95 95 93 83 80 83 70 74 70 63 59 53 55 56 52 60 67 72 68 64 68 61 65 65 76 68 53 45 44 25 22 18 23 21 24 19 30 23 12 8 3 3 3 3 0 4 5 4 0 0 0 0 1 1 5 0 2 0 2 2 5 3 3 6 17 17 17 15 44 44 44 42 62 62 62 60 76 76 76 74 86 86 86 84 91 91 91 89 96 96 96 94 ## ## ## 98 ## ## ## 98 ## ## ## 98 ## ## ## 98 ## ## ## 98 ## ## ## 98 ## ## ## 98 ## ## ## 98 98 98 98 96 88 88 88 86 85 85 85 83 88 88 88 86 75 75 75 73 79 79 79 77 75 75 75 73 68 68 68 66 64 64 64 62 58 58 58 56 60 60 60 58 61 61 61 59 57 57 57 55 65 65 65 63 72 72 72 70 77 77 77 75 73 73 73 71 69 69 69 67 73 73 73 71 66 66 66 64 70 70 70 68 70 70 70 68 81 81 81 79 73 73 73 71 58 58 58 56 50 50 50 48 49 49 49 47 30 30 30 28 27 27 27 25 22 22 22 20 28 28 28 26 26 26 26 24 29 29 29 27 24 24 24 22 35 35 35 33 28 28 28 26 17 17 17 15 2 1 5 5 5 12 39 57 71 81 86 91 95 95 95 95 97 95 95 95 93 83 80 83 70 74 70 63 59 53 55 56 52 60 67 72 68 64 68 61 65 65 76 68 53 45 44 25 22 18 23 21 24 19 30 23 12 2 3 3 2 3 3 14 41 59 73 83 88 93 97 97 97 97 97 97 97 97 95 85 82 85 72 76 72 65 61 55 57 58 54 62 69 74 70 66 70 63 67 67 78 70 55 47 46 27 24 19 25 23 26 21 32 25 14 3 2 6 6 6 11 38 56 70 80 85 90 94 94 95 95 97 95 94 94 92 82 79 82 69 73 69 62 58 52 54 55 51 59 66 71 67 63 67 60 64 64 75 67 52 44 43 24 21 18 22 20 23 18 29 22 11 14 14 14 17 13 12 13 17 17 17 17 16 12 17 17 17 15 12 14 11 27 45 59 69 74 82 86 90 91 91 93 91 89 83 81 72 68 71 58 62 58 51 47 41 43 44 40 48 55 60 56 52 56 49 53 53 64 56 41 33 32 18 10 14 13 15 12 18 11 36 41 41 41 44 40 39 40 44 44 44 44 43 39 44 44 44 42 39 41 38 27 18 32 59 63 72 76 80 81 81 83 81 79 65 54 62 43 44 48 37 37 24 20 14 16 17 21 32 32 46 37 30 31 34 34 32 50 34 29 16 14 17 22 16 18 15 20 16 27 54 59 59 59 62 58 57 58 62 62 62 62 61 57 62 62 62 60 57 59 56 45 18 14 48 52 61 65 76 70 70 72 71 68 54 37 51 32 31 37 26 26 9 11 10 21 21 35 26 21 28 23 23 21 39 23 18 12 13 32 35 40 34 36 33 38 27 34 45 78 83 83 83 86 82 81 82 86 86 86 86 85 81 86 86 86 84 81 83 80 69 59 48 40 28 25 33 53 46 43 42 48 34 17 12 16 17 13 22 22 46 39 47 49 42 38 27 27 13 22 29 28 25 25 27 25 30 43 52 56 63 64 58 60 62 62 51 58 69 88 93 93 93 96 92 91 92 96 96 96 96 95 91 96 96 96 94 91 93 90 82 72 61 53 25 31 21 21 17 23 10 24 22 29 33 38 41 45 59 52 60 62 55 51 40 40 26 35 42 41 38 38 40 26 38 43 56 65 66 76 74 69 70 75 74 64 71 82 92 94 95 95 97 95 97 97 97 97 97 97 97 97 97 97 97 97 97 97 97 97 97 97 ## ## ## ## ## ## 96 96 96 96 97 96 95 95 95 95 97 95 96 96 96 96 97 96 ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## 99 99 99 99 99 99 95 95 95 95 97 95 ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## 98 98 98 98 98 98 95 95 95 95 97 95 97 97 97 97 97 97 94 94 95 95 97 95 86 90 91 91 93 91 76 80 81 81 83 81 65 76 70 70 72 71 57 74 67 64 64 69 33 53 46 43 42 48 13 30 18 24 20 20 31 21 21 17 23 31 16 25 16 15 31 15 11 16 19 16 15 12 25 11 12 11 18 16 16 11 15 19 18 10 41 26 30 25 22 16 39 29 28 25 31 32 52 45 42 41 47 30 51 43 40 39 45 36 56 49 46 45 51 41 61 54 51 50 56 46 66 59 56 55 61 49 69 62 59 58 64 53 73 66 63 62 68 63 83 76 73 72 78 56 73 66 63 63 68 64 68 69 69 71 69 66 75 71 71 73 71 59 65 64 64 66 64 55 68 61 60 62 63 44 57 50 49 51 52 44 60 53 50 51 55 30 50 35 39 37 37 39 53 46 44 46 48 46 55 51 51 53 51 45 49 50 50 52 50 42 58 51 48 49 53 42 54 47 47 49 49 44 55 49 49 51 50 34 55 47 44 43 49 42 56 49 47 49 51 47 60 53 52 54 55 60 68 65 65 67 65 69 74 74 74 76 74 70 72 73 73 75 73 80 84 85 85 87 85 78 78 78 78 79 78 73 77 78 78 80 78 74 75 76 76 78 76 79 83 84 84 86 84 78 82 83 83 85 83 68 75 73 73 75 73 75 79 80 80 82 80 86 90 91 91 93 91 93 92 90 97 97 95 97 97 95 97 97 95 ## ## 98 96 96 94 95 95 93 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16 14 13 18 26 24 30 32 24 26 31 30 41 33 35 41 40 37 33 34 41 61 61 60 66 70 60 60 62 60 59 57 51 47 46 46 46 46 46 43 41 35 26 26 27 21 16 15 10 8 15 16 13 14 10 10 13 10 14 19 20 22 20 16 16 14 13 18 26 24 30 32 24 26 31 30 41 35 41 45 46 46 46 42 54 74 74 73 79 83 71 73 75 73 72 70 64 60 47 44 43 41 41 38 36 30 21 21 22 16 11 13 15 21 19 14 17 14 14 11 11 11 13 5 17 15 11 11 10 11 13 32 31 28 28 30 35 26 31 45 35 38 42 43 43 43 39 51 71 71 70 76 80 68 70 72 70 69 67 61 57 44 44 44 44 44 41 39 33 24 24 25 19 14 10 10 6 13 18 16 16 14 11 12 12 8 10 3 10 20 18 14 14 12 10 16 29 28 28 30 27 32 29 28 42 38 37 41 42 42 42 38 50 70 70 69 75 79 67 69 71 69 68 66 60 56 45 45 45 45 45 42 40 34 25 25 26 20 15 11 14 17 18 19 13 11 15 15 7 5 10 11 21 19 15 15 13 10 17 28 27 29 31 26 31 30 29 41 38 42 46 47 47 47 43 55 75 75 74 80 84 72 74 76 74 73 71 65 61 48 45 44 41 41 39 36 30 21 21 22 16 11 13 10 16 22 20 19 18 15 15 15 12 12 14 5 17 15 11 11 13 33 32 29 29 31 36 26 32 46 41 47 51 52 52 52 48 60 80 80 79 85 89 77 79 81 79 78 76 70 66 53 50 49 40 42 44 39 28 19 19 20 14 9 15 15 12 21 27 25 19 23 20 20 17 14 17 17 19 10 1 15 13 10 15 12 11 38 37 34 34 36 41 31 37 51 42 48 52 53 53 53 49 61 81 81 80 86 90 78 80 82 80 79 77 71 67 54 51 50 41 43 45 40 27 18 18 19 13 10 16 16 13 22 28 26 19 24 21 21 18 15 18 18 20 10 11 1 14 12 11 16 13 10 39 38 35 35 37 42 32 38 52 41 47 51 52 52 52 48 60 80 80 79 85 89 77 79 81 79 78 76 70 66 53 50 49 40 42 44 39 23 14 10 22 30 24 14 21 27 25 20 25 26 25 21 23 22 22 22 17 20 21 17 15 14 10 15 12 11 38 37 34 34 36 41 31 37 51 38 42 46 47 47 47 43 55 75 75 74 80 84 72 74 76 74 73 71 65 61 48 45 44 35 37 39 34 18 9 20 28 22 12 16 22 20 19 23 24 23 19 21 20 20 20 15 18 19 15 13 12 5 10 33 32 29 29 31 36 26 32 46 35 37 41 42 42 42 38 50 70 70 69 75 79 67 69 71 69 68 66 60 56 43 40 39 30 32 34 29 19 10 10 14 16 24 18 11 17 15 16 19 20 19 15 17 16 16 16 11 14 15 11 10 11 10 5 5 28 27 24 24 26 31 22 27 41 33 32 36 37 37 37 33 45 65 65 64 70 74 62 64 66 64 63 61 55 51 38 35 34 30 30 29 25 19 10 13 19 12 11 16 24 18 12 13 14 19 20 19 15 17 16 16 16 11 14 15 11 15 16 15 10 5 23 22 19 19 21 26 20 22 36 Figure Diagrams of a similarity matrix and hydrograms of water discharges of specified probability at extreme values of the NAO; legend as in Figure 15 31 35 39 40 40 40 36 48 68 68 67 73 77 65 67 69 67 66 64 58 54 41 38 37 32 32 32 27 21 12 12 16 14 22 16 15 13 12 17 18 17 13 15 14 14 14 10 12 13 12 13 12 5 26 25 22 22 24 29 19 25 39 35 41 45 46 46 46 42 54 74 74 73 79 83 71 73 75 73 72 70 64 60 47 44 43 35 36 38 33 24 15 15 16 10 5 11 19 13 15 21 19 14 17 15 14 11 15 14 13 13 11 10 10 7 11 9 32 31 28 28 30 35 25 31 45 33 38 42 43 43 43 39 51 71 71 70 76 80 68 70 72 70 69 67 61 57 44 41 40 31 33 35 30 17 8 13 6 18 26 20 10 12 18 16 16 21 22 21 17 19 18 18 18 13 16 17 13 11 10 5 29 28 25 25 27 32 22 28 42 31 26 23 16 22 17 23 31 42 52 44 47 53 58 49 52 47 45 40 36 28 22 20 20 20 20 17 15 15 24 36 42 35 32 34 29 34 28 26 17 11 20 24 29 30 29 25 27 26 26 26 32 29 28 33 38 39 38 33 28 23 26 32 29 18 18 20 20 28 23 20 16 19 17 20 28 43 49 42 48 52 55 47 49 44 42 39 33 29 22 22 22 22 22 19 17 14 23 35 41 34 31 33 28 32 26 25 16 10 18 22 27 28 27 23 25 24 24 24 31 28 27 32 37 38 37 32 27 22 25 31 28 15 15 17 17 26 21 18 18 18 18 18 26 46 47 45 51 55 53 47 47 45 44 42 36 32 19 16 16 16 16 13 11 11 20 32 38 31 28 30 30 38 32 22 15 14 24 28 33 34 33 29 31 30 30 30 28 28 29 29 34 35 34 29 24 19 22 28 25 6 13 14 15 17 28 23 20 18 19 18 20 28 46 49 45 51 55 55 46 49 45 44 42 36 32 19 16 15 14 14 11 11 20 32 38 31 28 30 32 40 34 24 17 16 26 30 35 36 35 31 33 32 32 32 28 30 31 29 34 35 34 29 24 19 22 28 25 8 15 15 17 17 23 18 17 16 16 17 15 24 44 44 43 49 53 50 43 45 43 42 40 34 30 22 22 22 22 22 19 17 13 22 34 40 33 30 32 27 32 26 24 15 18 22 27 28 27 23 25 24 24 24 30 27 26 31 36 37 36 31 26 21 24 30 27 8 10 10 12 17 13 15 15 16 17 19 39 39 38 44 48 40 39 40 38 37 35 29 25 20 20 20 20 20 17 15 20 27 39 45 38 35 37 32 34 28 29 20 14 20 26 29 30 29 25 27 26 26 26 35 32 31 36 41 42 41 36 31 26 29 35 32 18 15 13 15 10 10 15 13 16 20 21 21 21 17 29 49 49 48 54 58 46 48 50 48 47 45 39 35 22 19 18 15 15 13 13 20 23 29 35 28 25 27 31 39 33 25 19 15 25 29 34 35 34 30 32 31 31 31 26 29 30 26 31 32 31 26 22 20 19 25 22 18 15 14 15 10 10 20 73 71 69 67 65 63 61 59 57 55 53 51 49 47 45 43 41 39 37 35 33 31 29 27 25 23 21 19 17 15 19 13 15 10 15 12 20 21 19 25 29 27 29 22 19 18 16 10 12 18 20 22 20 19 18 23 37 46 58 64 57 54 56 51 48 45 48 39 33 35 45 43 44 43 43 46 43 43 41 54 51 50 55 60 61 60 55 50 45 48 54 51 31 28 26 28 24 19 29 23 11 13 15 12 13 12 11 15 28 36 28 33 37 42 33 36 31 29 24 20 18 18 18 18 18 18 16 17 29 38 50 56 49 46 48 43 39 37 40 31 25 27 37 31 32 32 35 38 35 35 33 46 43 42 47 52 53 52 47 42 37 40 46 43 17 17 18 18 17 17 21 17 17 11 73 71 69 67 65 63 61 59 57 55 53 51 49 47 45 43 41 39 37 35 33 31 29 27 25 23 21 19 17 15 22 21 23 20 19 15 14 14 17 15 21 29 29 56 76 77 71 62 54 54 41 30 23 24 19 17 11 17 21 24 30 32 28 35 40 44 46 50 50 43 29 31 36 41 36 36 36 36 40 31 36 41 41 37 35 41 37 32 27 21 18 19 14 12 19 19 18 13 7 15 12 11 15 15 15 21 29 35 56 76 77 71 62 54 54 49 45 38 24 19 16 9 12 15 17 16 20 25 29 31 35 35 28 26 30 37 42 31 22 25 21 25 21 23 27 27 22 20 26 22 17 12 6 18 18 15 19 19 18 11 3 800 Q [m3s-1] Q [m3s -1 ] NAO1600 11 10 14 15 16 17 11 23 43 43 42 48 52 40 42 44 42 41 39 33 29 16 16 16 16 16 13 13 22 25 35 41 34 31 33 30 38 32 27 21 14 24 28 33 34 33 29 31 30 30 30 31 28 29 32 37 38 37 32 27 22 25 31 28 20 17 15 17 12 6 14 15 10 15 16 17 11 29 32 28 34 38 38 36 32 28 27 25 19 15 11 11 11 11 12 28 37 49 55 48 45 47 42 49 43 39 30 25 35 39 44 45 44 40 42 41 41 41 45 42 41 46 51 52 51 46 41 36 39 45 42 20 17 17 17 17 15 20 14 350 300 Days of year 250 200 150 100 NAO+ VOSSO NAO- NAO+ NAO+ JIZERA NAO- NAO+ NAOETNA NAO+ LAGEN NAO- NAO+ GAULA NAO- NAO+ NAO- NAO+ NAO+ ÖRE KALIX NAO- NAOKYRÖN NAO- NAO- NAO+ NAO+ FUSTA LAKSELV NAONILAKKA NAO- NAO+ TANA NAO - NAO+ 50 350 300 Days of year 250 200 150 100 I XI II X III IX IV VIII V VII VI VI VII V VIII IV IX V LABE NAO- NAO+ NAOODER NAO+ NAOWARTA HELGE A NAO+ NAO+ OULU NAO- 50 X III XI II XII I Figure Sequence of the types of hydrological periods at extreme values of the NAO Types of periods: I - Deep stable low water, II - Deep unstable low water, III - Average low water, IV - Shallow low water, V- Normal stable period, VI - Normal unstable period, VII - Low stable rise, VIII - Low unstable rise, IX - Average stable rise, X - Average unstable rise, XI - High stable rise, XII - Disastrous rise 16 Stability of the hydrological regime In the years 1901-2000, one can observe fluctuations of various duration in the pattern of the NAO index Presumably they may reflect the pattern of so-called circulation periods Marsz (1999) distinguishes the boundaries of 'NAO circulation epochs' in the past century which bear a strong resemblance to the boundaries of intensity periods of circulation over Europe discovered by KoŜuchowski (1995) The first and third periods (the years 1900-1929 and 1971-1995) have positive values of the mean NAO index The dividing period, 1930-1970, is the so-called second circulation period with a negative value of the mean NAO index A 350 Days of year 300 250 200 150 100 50 1901-1920 1911-1930 1921-1940 1931-1950 1941-1960 1951-1970 1961-1980 1971-1990 1981-2000 B 350 Days of year 300 250 200 150 100 50 1901-1920 1911-1930 1921-1940 1931-1950 1941-1960 1951-1970 1961-1980 1971-1990 1981-2000 17 NAOIDJFM (HURRELL) -2 -4 -6 1901-1920 1911-1930 1921-1940 1931-1950 max 1941-1960 1951-1970 1961-1980 1971-1990 1981-2000 mean Figure Changes in regime characteristics in the multi-year period 1901-2000: A - Vosso river, B - Labe river; legend as in Figure 5., C – mean values of Hurrell’s NAOIDJFM Changes in the sequence of hydrological periods distinguished on the basis of chronological time series are harder to analyse, largely because of great variations in the NAO index from year to year The changes were especially pronounced in the last two circulation periods when mean NAO figures were distinctly lower (19311970) or distinctly higher (1971-1995) The results for those periods are corroborated by observations made on the basis of an earlier analysis of the sequence of hydrological periods derived from material from the years of the highest and the lowest winter NAO values The pattern of the 1901-2000 hydrological periods on Central European rivers (Fig 6B) indicates their sequence to be slightly different in the positive and the negative stage of the NAO In the NAO-, the year starts with a normal or low flow followed by a high rise In a NAO+, a low rise opening the year is followed by an average rise In the positive NAO periods high-water seasons are twice as long, but higher rises could be observed in the years 1931-1950 (a NAOperiod) During those periods the rivers of the region also show longer and deeper low flows The temporal structure of hydrological periods in the yearly cycle is different on Scandinavian rivers, too (Fig 6A) In the NAO- period 1931-1970, highwater seasons could be much shorter, even by two months, than after 1971 (a NAO+ period), with the exception of the Tana and the Lakselv The high-water flows 18 observed then were not only longer, but also of greater magnitude, while low flows in a positive NAO period tend to be shorter and shallower Conclusions The observed strong effect of atmospheric circulation on climatic conditions, mainly precipitation and temperature, is also responsible for the fact that river discharges in the analysed regions of Europe are correlated, to a greater or lesser extent, with the winter value of the NAO index As follows from the analysis of the temporal structure of hydrological phenomena and its variations over the year, a hydrological regime, too, can be destabilised by the changing intensity of zonal circulation What characterises the rivers of Northern Europe in a positive stage of the NAO is an earlier start of the high-water season, which is also longer and shows higher peak discharges Low flows, in turn, tend to be shorter and shallower In Central Europe a reverse tendency can be observed: higher and longer-lasting rises and shorter low-flow periods occur in the negative NAO stage The sequence of the hydrological periods distinguished is usually the same; what changes are their dates, duration, and the intensity of the hydrological phenomenon occurring at that time The few exceptions from these regularities result from the idiosyncratic properties of each river The location, environmental conditions, and man's activity affect the characteristics of the hydrological regimes of rivers of the same region That is why in a research carried out at the regional scale it might be difficult to assess how the intensity of zonal circulation controls the hydrological regimes of rivers The presented method of analysis of the hydrological regime can also be employed in the typology and regionalisation of current stream regimes in various geographical zones It can assist in the study of the stability of a regime under 19 changing climatic conditions due to both, natural factors and human impact It can also be used in a model forecasting research intended to define features of a hydrological regime under various scenarios of global climate change References Hurrell J.W., 1995 Decadal trends in the North Atlantic Oscillation and relationship to regional temperature and precipitation Science, 269, 676-679 Hurrell J.W., van Loon H., 1997 Decadal variations in climate associated with on North Atlantic Oscillation, Climatic Change, 36, 301-326 Kaczmarek Z., 2002 Wpływ Oscylacji Północnoatlantyckiej na przepływy rzek europejskich [w:] Oscylacja Północnego Atlantyku i jej rola w kształtowaniu zmienności warunków klimatycznych i hydrologicznych Polski Akademia Morska w Gdyni, 163-172 KoŜ uchowski K., 1995 Głębokie cyklonu, antycyklony i cyrkulacja strefowa nad Europą (1900-1990) [Summary: Deep cyclones, anticyclones and zonal circulation over Europe (1900-1990)] Przegląd Geofizyczny, 40, 3, 231-246 Marsz A., 1999 Oscylacja Północnoatlantycka a reŜim termiczny zim na obszarze północnozachodniej Polski i polskim wybrzeŜu Bałtyku [Summary: The North Atlantic Oscillation and the thermal regime in the area of north-west Poland and the Polish coast of the Baltic Sea] Przegląd Geograficzny, 71, 3, 225-245 Rogers J.C., 1997 North Atlantic storm track variability and its association to the North Atlantic Oscillation and climate variability of Northern Europe Journal of Climate, 10, 7, 1635-1647 Rotnicka J., 1988 Taksonomiczne podstawy klasyfikacji reŜimu rzecznego (na przykładzie zlewni Odry i rzek Przymorza) [Summary: Taxonomic foundations of the classification of river regime; the example of the catchments of the Odra and Polish coastal rivers] Wyd UAM, Seria Geografia, 40 Rotnicka J., 1993 A typology of hydrological periods for use in river regime studies Quaestiones Geographicae, 15/16, 77-95 Serreze M.C., Carse F., Barry R.G., Rogers J.C., 1997 Icelandic Low cyclone activity: climatological features, linkages with the NAO, and relationship with recent changes in the Northern Hemisphere circulation Journal of Climate, 10, 453-464 Shorthouse C.A., Arnell N.W., 1997 Spatial and temporal variability in European river flows and the North Atlantic Oscillation; FRIEND’97: International Association of Hydrological Science Publications, 246, 77-85 Trigo R.M., Osborn T.J., Corte-Real J.M., 2002 The North Atlantic Oscillation influence on Europe: climate impacts and associated physical mechanism Climate Research, 20, 9-17 Wrzesiński D., 2004 Flow regimes of rivers of northern and central Europe in various circulation periods of the North Atlantic Oscilation (NAO) NHO Report No 48, 670-679 20 ... on the basis of an earlier analysis of the sequence of hydrological periods derived from material from the years of the highest and the lowest winter NAO values The pattern of the 1901-2000 hydrological. .. year on the basis of the similarity of one of their features The elementary time unit adopted is a five-day period of time, or a pentad, and the grouping criterion, the river discharge Thus, the. .. and on the stability of the characteristics of hydrological regimes The determination of the number and temporal structure of the hydrological periods distinguished in long time series (of about