engleski

Interannual to interdecadal variability of sea level in the North Sea

Long time series of monthly mean sea-level data from 34 tide gauges around the North Sea have been analysed in time and frequency domain, to determine principal spatial and temporal modes of variability. Principal component (PC) analysis shows that the North Sea is a coherent region where sea level varies almost uniformly, with more than 80% of variance being explained by the first PC mode. Power density spectra throughout the North Sea exhibit a strong signal at the period of 14 months, while peaks at periods of 3 yr and 6 yr are observed at a number of stations. These oscillations are more pronounced in the northern part of the region. Additionally, variability of an 18 yr period is detected at some sites. Running spectra reveal that none of the observed peaks is associated with a permanent oscillation. All the signals are strongly modulated. Band-pass filtered time series show that the 3 yr and the 6 yr oscillations possess a very similar interdecadal variability, with small amplitudes between 1880-1920 and large values after 1940. The 14 month oscillation follows a similar pattern. The impact of NAO on different scales of sea-level variability and how it changes over time, has also been examined. The strongest and most persistent relationship between NAO and sea level occurs around the period of 14 months. The influence of NAO is also reflected in the 6 yr and much weaker in the 3 yr sea-level variability. However, at these periods there is some other forcing, in addition to NAO, that drives sea-level variability.

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