Variations in the mean, seasonal and extreme water level on the Latvian coast, the eastern Baltic Sea, during 1961-2018

摘要

High-resolution in situ water level data is one of the core sources for the identification and understanding the reaction of the sea to climate change. We analyse digitised recordings of water level measurements from all 10 currently functioning coastal tide gauges on the Latvian shores of the Baltic proper and in the Gulf of Riga for the period of 1961-2018. The frequency and temporal coverage of measurements vary greatly for these stations. The most complete hourly data are available from Liepaja on the Baltic proper coast and from Daugavgriva in the south-eastern bayhead of the Gulf of Riga. The water level regime is analysed from the viewpoint of (i) the entire range of water level variations, (ii) empirical probability distributions of different water levels, (iii) the seasonal course of water level, (iv) trends in the annual, seasonal, and monthly means and extremes of water level (in terms of the relative and uplift corrected absolute values), and (v) correlations of the main properties of water level with the North Atlantic Oscillation (NAO Gibraltar) index.The empirical probability distributions of different water levels have become narrower in 1991-2018 compared to 1961-1990 whereas very low water levels are now less frequent. The amplitude of the seasonal course has greatly decreased over these time intervals. The annual mean and maxima of water level have increased in 1961-2018. The rate of increase is smaller than the rate of increase in the sea level in the North Atlantic suggesting that changes in the local drivers of water level mitigate the sea level rise in Latvian waters. Variations in the NAO index can explain 1/3 of the annual variability of the main properties of water level and up to 2/3 of this variability in wintertime (December-March). The changes in the statistical properties of water level are consistent with alterations to the directional structure of strong winds.