Regional differences in the influence of the North Atlantic Oscillation on seasonal river runoff in Poland
In this paper, an analysis of monthly and seasonal runoff volumes in two stages of the North Atlantic Oscillation are presented. The analysis embraced runoff at 146 profiles located on 96 Polish rivers during the years 1951-2000. The changes in the runoff conditions of Polish rivers in the two NAO stages and their spatial diversity were determined based on the differences between runoff observed in the years of exceptionally high (NAODJFM > 2,0) and low (NAODJFM < -2,0) values of the winter NAO index. The results of the research indicate that the influence of the North Atlantic Oscillation on the runoff of Polish rivers is diverse in terms of time and space. A classification of rivers was made in terms of the similarity of deviations of their seasonal runoff between the different NAODJFM stages. In the classification procedure use was made of Ward's method of hierarchical grouping. In this way, five classes of the river profiles under analysis were obtained. Environmental conditions in the catchments and hydrological regime features clearly influence the regional differences in the impact of the North Atlantic Oscillation on the flow of rivers.
The aim of the paper is to describe spatial differences in the uncertainty of features of the flow regimes of rivers in Europe on the basis of measures whose methodological assumptions derive from Shannon’s information entropy theory (1948). They included: the entropy of monthly flow volumes, the entropy of the flow distribution over time, and the entropy of maximum and minimum monthly flows. An analysis was made of monthly flow series for the years 1951-1990 from 510 gauging stations located on 369 rivers in Europe. It allowed a quantitative determination of the degree of uncertainty of the four regime characteristics, indirectly establishing the predictability, regularity and stability of their appearance and their spatial variability. In the procedure of identification of spatial differences among rivers concerning the uncertainty of their flow regime features, use was made of local indices of spatial dependence. On application of LISA (Local Indicators of Spatial Association) based on Moran’s local Ii statistic, a typology of rivers was obtained in terms of the kind and statistical significance of spatial associations involving the uncertainty of the flow regime variables in question.
The aim of this study is to detect changes in flow regime of rivers in Poland. On the basis of daily discharges recorded in 1951-2010 at 159 gauging stations located on 94 rivers regularities in the variability of the river flow characteristics in the multi-year period and in the annual cycle were identified and also their spatial uniformity was examined. In order to identify changes in the characteristics of river regime, similarities of empirical distribution functions of the 5-day sets (pentads) of discharges were analyzed and the percent shares of similar and dissimilar distributions of the 5-day discharge frequencies in the successive 20-year sub-periods were compared with the average values of discharges recorded in 1951-2010. Three alternative methods of river classification were employed and in the classification procedure use was made of the Ward’s hierarchical clustering method. This resulted in identification of groups of rivers different in terms of the degree of transformation of their hydrological regimes in the multi-year and annual patterns.
The paper presents the effect of North Atlantic Oscillation of macroscale atmospheric circulation (NAO) on the hydrological conditions of Lake Morskie Oko located at an altitude of 1392.8 m a.s.l. in the highest range of the Carpathians. The paper applied detailed hydrometric information from the years 1971-2010 concerning water level fluctuations, water temperature, terms of the commencement and end of ice phenomena and ice cover, as well as meteorological data concerning air temperature and atmospheric precipitation, and monthly and seasonal NAO indices. The performed analysis suggests that the majority of analysed hydrological characteristics of Lake Morskie Oko was not prone to variability of NAO intensity in its various phases. The situation results from the local conditions, particularly responsible for the course of processes and phenomena in Lake Morskie Oko, simultaneously obscuring the effect of macroscale factors.
An analysis is presented of changes in the parameters of ice phenomena on selected Polish lakes over the years 1961-2010 against the background of variations in the intensity of the North Atlantic Oscillation. The following features were analysed: the start and end of ice phenomena and ice cover, their duration, and the stability of ice cover at observation sites of the Institute of Meteorology and Water Management on lakes located in various physical-geographic regions. Multi-year tendencies of change in the ice parameters under study were identified. The most statistically significant negative trends were observed for the ending dates of ice phenomena and their duration. Changes in the ice characteristics in the different NAODJFM stages were determined on the basis of their deviations: between years with high (> 1.7) and low (< -1.09) values of the winter NAO index, and from average values from the years 1961-2010. Changes in the intensity of the North Atlantic Oscillation affect all the parameters of ice phenomena except their starting dates. In a negative NAODJFM stage, ice phenomena and ice cover end later and last longer; also, the maximum thickness of ice cover is greater, its forms later and is more stable than in a positive stage and in average conditions
The paper presents the characteristics of hydrological periods in an average annual cycle in Polish lakes. The types of period and their sequence determine the regime of water stages in lakes. The article applies the unsupervised approach to analysis of water level fluctuation patterns, where the regime is identified by grouping analytical parameters. Hydrological periods were designated by grouping elementary time units of the hydrological year (pentads) based on the similarity of their parameters, namely water level frequency distributions. The analysis covered daily water stages in 33 lakes in Poland from the period from 1984 to 2012. Five types of hydrological period were designated. The studied lakes differ in the number, type and sequence of hydrological periods in an average annual cycle. Most of the lakes (19 lakes) have a 4-period temporal structure of water stages with the course of water stages in a year characteristic of this geographical zone. No spatial patterns occurred in the location of lakes from particular groups. This suggests the dominant role of local factors in determining the seasonality of water stages.
The paper discusses changes in the hydrological regime of high mountain Lake Morskie Oko located in the Tatra Mountains, in the Tatra Mountains National Park, a UNESCO biosphere reserve (MaB). According to the research conducted in the years 1971–2015, its water stages decreased by 3.5 cm·dec−1, mean annual water temperature increased by 0.3°C·dec−1 and the duration of ice phenomena and ice cover was reduced by 10 day·dec−1. No considerable changes in maximum values of ice cover thickness were recorded. Such tendencies are primarily caused by long-term changes in climatic conditions – air temperature and atmospheric precipitation. The hydrological regime of the lake was also determined by changes in land use in the lake’s catchment and its location in high mountains.