Non-stationarity of sea level extremes caused by abrupt changes in atmospheric circulation in the Gulf of Riga (Baltic Sea)

Abstract:

Understanding and predicting the characteristics of sea level extreme events and their potential changes in frequency and magnitude is of fundamental importance in estimating the coastal flooding risks and the management of the coastal areas. Due to climate change, the properties of the sea level extremes are expected to change and exhibit non-stationary behaviour. However, in many cases, the flooding risks and coastal planning decisions are based merely on the stationary approach to the sea level extremes. Therefore, it is crucial to understand how common is the non-stationary behaviour in the case of sea level extremes and identify the main drivers behind it. Non-stationary modelling of extremes was performed to the block maxima of water level derived from the time series at six locations in the Gulf of Riga, Baltic Sea, during 1961–2018. Several parameters of the Generalized Extreme Value distribution of the measured water maxima 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 strong regime shift: a drastic abrupt drop of 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 extraordinary change in the shape of the distribution was linked to the significant weakening of the connection with the North Atlantic Oscillation (NAO) pattern. During the weak relationship with the NAO in the region, a strong correlation with the Scandinavian and Pacific/North American patterns appeared, changing the shape of the distribution of the sea level extremes. The observed phenomenon raises a question of whether this is a common phenomenon that the atmospheric regime shifts cause the non-stationary behaviour of the sea level extremes. We also discuss the implications of not taking into account the non-stationary behaviour of the sea level extremes.