Long term couplings of winter index of North Atlantic oscillation and water level in the Baltic Sea and Kattegat

The paper analyzes long-term time series of average winter local water level variations at 21 stations around the Baltic Sea and Kattegat. It is compared with the corresponding time series of winter index of North Atlantic Oscillation. The analysis of sea level revealed the presence of two patterns: the multi-decadal variations and an oscillation with the period of 8 years. Northern locations contained the glacial isostatic adjustment effect, which produced the isostatic sink that was observed in the southern part of the Baltic Sea. At some locations multi-decadal variations of sea level were free from those factors, which allowed determination of those variations with reasonable accuracy. The 8-year oscillation is very pronounced in the winter NAO series. It was detected at all mareographic stations, so significant part of sea level variability could be attributed to the impact of winter NAO. Further examination determined an amplitude threshold in the 8-year cycle of winter NAO for which the associated variations of sea level are temporally synchronized. This can serve as a criterion of assessment whether the Baltic Sea experiences an epoch of coupling to pressure fields from over the Atlantic or not; the analysis identified such epochs in the past.