Dangendorf, S., Wahl, T., Mudersbach, C. and Jensen, J., 2013. The Seasonal Mean Sea Level Cycle in the Southeastern North Sea

The seasonal cycle is a prominent feature in Mean Sea Level (MSL) time series with considerable influences on flood risk in coastal areas. When analyzing MSL it is often assumed that the seasonal cycle is a stationary process, independent from inter-annual variations, but there is no obvious reason for such an assumption. In this paper the seasonal cycle of MSL at 13 tide gauges in the German Bight is investigated for its average character as well as its time dependence over the past 166 years. A seasonal trend decomposition method is used to analyze the inter-annual fluctuations in amplitudes and phases of the seasonal cycle. In the German Bight the seasonal cycle accounts for up to 44 % of the observed monthly MSL variability. The mean seasonal cycle peaks during November at all stations. The mean amplitude varies between 14 and 20 cm and increases from the south-western to the north-eastern stations. Throughout the last 166 years it is found that the amplitudes as well as the phases of the seasonal cycle are marked by a considerable inter-annual variability. While most records, all starting in the 1930's or later, do not exhibit a significant trend the longest record at Cuxhaven displays a significant long-term trend of 0.2 ± 0.1 mm/yr. This trend is mainly caused by large values at the end of the 1970's and the beginning 1980's. Simultaneously, the annual peaks shift from the late autumn to winter months (December to February). These changes are caused by extraordinary large trends during the months from January to March, exceeding those in the remaining months by up to 4 mm/yr. These changes are in phase with an intensification of large-scale atmospheric circulation patterns over the North Atlantic bringing more frequent westerly winds over the North Sea.