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In this paper it is the associations between mean annual discharges of the main rivers in Poland and the surface component of the Northern Atlantic Thermohaline Circulation (NA THC) that are analysed. Relevant data derived from hydrometric profiles on the country’s main rivers, i.e. the Vistula at Tczew (years 1901-2015), the River Bug at Wyszkow (1921-2015), the Oder at Gozdowice (1901-2015) and the Warta in Poznan (1822-2015). Use was also made of the time series for annual discharges of the Vistula at Torun (1822-2000). Results show that, despite marked inter-annual variability to discharges in two periods (1921-2015 and 1822-2000), there is a common rhythm to long-term discharge fluctuations (Table 1, Figure 1), with consecutive increases and decreases in annual discharges. The analysis shows that the changeability of annual discharges and variability in the NA THC are characterised by weak, but highly statistically signifi cant associations (Table 2). In periods in which the NA THC intensifies (as indicated by the DG3L index), river discharges are found to decrease; whereas a fall in the intensity of NA THC is associated with an increase in the intensity of river discharges (Figs. 2, 3 and 4). While small, these differences do assume a high level of statistical significance (Fig. 4). Figs. 2, 3 and 4 also show how the occurrence of positive values for the NA THC (+DG3L) sees the degree of variability of annual discharges reduced in relation to that present when values for the NA THC are negative (–DG3L). Given that the variability characterising the DG3Lindex (NA THC) displays multi-decadal oscillations (Fig. 6), it is not surprising that such oscillations also occur where discharges from Poland’s main rivers are concerned. The differences between discharges occurring in the positive and negative phases of the NA THC (+DG3L and –DG3L amount to around 10%. The main reason for such an association is that variability in the NA THC (with which changes in the thermal state of the Northern Atlantic are connected) is what is known to regulate some aspects of atmospheric circulation. As a result, in the circumstances of positive values for the DG3L index, mean annual air temperature in Poland is higher (and in April, July and August significantly higher) than at times when a negative DG3L index is present. When positive values for the DG3L index arise, there are greater losses due to evaporation (or evapotranspiration) in the river catchments in Poland, with the result that discharges are reduced. No statistically significant relationships were found between the NA THC and precipitation in Poland. However, while there are no statistically significant relationships between mean annual discharges from the rivers in Poland and the AMO index, associations between discharges and the DG3L index – characterising the intensity of the thermohaline circulation in the Northern Atlantic – do achieve statistical significance. The results obtained suggest that the relationship between the intensity of the NA THC and the occurrence of droughts in Poland should be investigated.

Długookresowe wahania przepływów rocznych głównych rzek w Polsce ich związek z cyrkulacją termohalinową Atlantyku Północnego