Non-stationary analysis of water level extremes in Latvian waters, Baltic Sea, during 1961–2018

Abstract

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.

Bayesian hierarchical modeling of sea level extremes in the Finnish coastal region

“…This assumption is unlikely to be completely true. Studies have indicated that sea level extremes have exhibited variations in the Baltic Sea region over different time scales (Johansson et al, 2001;Ribeiro et al, 2014;Marcos and Woodworth, 2017;Kudryavtseva et al, 2021) and that these variations were associated with variations in large scale atmospheric conditions (Samuelsson and Stigebrandt, 1996;Johansson et al, 2001;Ribeiro et al, 2014;Marcos and Woodworth, 2017;Kudryavtseva et al, 2021;Passaro et al, 2021). As with changing mean sea level, future climate is also expected to bring changes in storm surges in the Baltic Sea region (e.g., Vousdoukas et al, 2016Vousdoukas et al, , 2017Vousdoukas et al, , 2018.…”

“…One remedy would be to model temporal dependence directly by allowing (e.g.) linear trends in the GEV parameters (e.g., Ribeiro et al, 2014;Marcos and Woodworth, 2017;Kudryavtseva et al, 2021). Physical covariates, which describe large scale atmospheric circulation conditions around the Baltic Sea region, could also be used to account for interannual-to-decadal scale variations in the annual maximum sea levels.…”

“…If block-maxima, such as the annual maximum sea levels are considered, extreme value theory tells us that the generalised extreme value distribution (GEV) is the only possible limiting distribution. GEV has been used to model return levels of annual maximum sea level heights in the Baltic Sea region in many previous studies (e.g., Ribeiro et al, 2014;Wolski et al, 2014;Marcos and Woodworth, 2017;Soomere et al, 2018;Kudryavtseva et al, 2021). Most of these studies, however, have concentrated on individual tide gauge locations.…”

“…Studies in the Baltic Sea region have suggested that the annual maximum sea levels should be calculated for a year-long block that covers all winter months instead of using the calendar year due to seasonality in storminess (e.g., Johansson et al, 2001;Suursaar et al, 2002;Soomere et al, 2018;Kudryavtseva et al, 2021). We took this approach and calculated the annual maximum values between 1 April-31 March.…”

Sea level dynamics and coastal erosion in the Baltic Sea region

“…Major inflow events are associated with typical volumes in the order of about 100 km 3 , corresponding to a Baltic sea level increase of about 24 cm (Matthäus and Franck, 1992). Typically, such variations have timescales of about 10 d or longer (Soomere and Pindsoo, 2016) while atmospheric variability on shorter timescales primarily leads to a redistribution of water masses within the Baltic Sea basin (Kulikov et al, 2015) or between the Baltic Proper and the Gulf of Riga (Männikus et al, 2019).…”