Abstract. Analysis and prediction of water level extremes in the
eastern Baltic Sea are difficult tasks because of the contribution of
various 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 to
the block maxima of water level derived from the time series at six
locations in the Gulf of Riga and one location in the Baltic proper, Baltic
Sea, during 1961–2018. Several parameters of the generalized-extreme-value
(GEV) distribution of the measured water level maxima both in the Baltic proper
and in the interior of the Gulf of Riga exhibit statistically significant
changes over these years. The most considerable changes occur to the shape
parameter ξ. All stations in the interior of the Gulf of Riga
experienced a regime shift: a drastic abrupt drop in the shape parameter
from ξ≈0.03±0.02 to ξ≈-0.36±0.04 around 1986
followed by an increase of a similar magnitude around 1990. This means a
sudden switch from a Fréchet distribution to a three-parameter Weibull
distribution and back. The period of an abrupt shift (1986–1990) in the
shape parameters of GEV distribution in the interior of the Gulf of Riga
coincides with the significant weakening of correlation between the water
level extremes and the North Atlantic Oscillation (NAO). The water level
extremes at Kolka at the entrance to the Gulf of Riga reveal a significant
linear trend in shape parameter following the ξ≈-0.44+0.01(t-1961)
relation. There is evidence of a different course of the water level
extremes in the Baltic proper and the interior of the Gulf of Riga. The
described changes may lead to greatly different projections for long-term
behavior of water level extremes and their return periods based on data
from different intervals. Highlights.
Water level extremes in the eastern Baltic Sea and the Gulf of Riga are analyzed
for 1961–2018. Significant changes in parameters of generalized-extreme-value distribution
are 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 the
gulf.
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