Analysis of the relationship between the North Atlantic oscillation and sea-level changes in northwest Europe



The impact of the North Atlantic oscillation (NAO) on sea level around northern European coasts is explored on the basis of mean monthly sea level time series back to the 19th century at 10 sites. Two types of relationship with the NAO are found. Most of northern Europe shows a positive relationship with higher sea levels under stronger NAO conditions, which is clearer for the winter season. In contrast, a negative relationship exists to the south, covering southwest England. Both the relationships are variable in time and were enhanced during recent decades, possibly due to movements of the NAO-related pressure centres. The analysis of the NAO and sea level variability in different frequency bands suggests that the relationship is strongest at the annual and interannual time scales at all sites. The NAO is also strongly correlated with decadal and longer term sea level changes in the Baltic Sea. The wavelet spectral power peaks at about 12 months for the NAO and 13–15 months for sea level. Unusually strong and weak annual cycles in sea level are closely related to two types of seasonal variation in the NAO. The anomalous annual cycles in sea level tend to lag those in the NAO by a month for most of the sites analysed, suggesting that sea level does not simply respond to the pressure forcing, but is additionally responsive to changes in thermal conditions and fluxes associated with the NAO. Copyright © 2004 Royal Meteorological Society.