摘要：Analysis and prediction of water level extremes in theeastern Baltic Sea are difficult tasks because of the contribution ofvarious drivers to the water level, the presence of outliers in time series,and possibly non-stationarity of the extremes. Non-stationary modeling of extremes was performed tothe block maxima of water level derived from the time series at sixlocations in the Gulf of Riga and one location in the Baltic proper, BalticSea, during 1961–2018. Several parameters of the generalized-extreme-value(GEV) distribution of the measured water level maxima both in the Baltic properand in the interior of the Gulf of Riga exhibit statistically significantchanges over these years. The most considerable changes occur to the shapeparameter ξ. All stations in the interior of the Gulf of Rigaexperienced a regime shift: a drastic abrupt drop in the shape parameterfrom ξ≈0.03±0.02 to ξ≈-0.36±0.04 around 1986followed by an increase of a similar magnitude around 1990. This means asudden switch from a Fréchet distribution to a three-parameter Weibulldistribution and back. The period of an abrupt shift (1986–1990) in theshape parameters of GEV distribution in the interior of the Gulf of Rigacoincides with the significant weakening of correlation between the waterlevel extremes and the North Atlantic Oscillation (NAO). The water levelextremes at Kolka at the entrance to the Gulf of Riga reveal a significantlinear trend in shape parameter following the ξ≈-0.44+0.01(t-1961)relation. There is evidence of a different course of the water levelextremes in the Baltic proper and the interior of the Gulf of Riga. Thedescribed changes may lead to greatly different projections for long-termbehavior of water level extremes and their return periods based on datafrom different intervals. Highlights. Water level extremes in the eastern Baltic Sea and the Gulf of Riga are analyzedfor 1961–2018. Significant changes in parameters of generalized-extreme-value distributionare identified. Significant linear trend in shape parameter is established at Kolka. The shape parameter changes in a step-like manner. The shape parameter of GEV has regime shifts around 1986 and 1990 in thegulf.
