Secular trends in ocean tides: Observations and model results

Abstract

During the last century the response of the oceans to tidal forces has changed significantly. This study focuses on an analysis of long-term sea level records located in the Atlantic and Pacific, primarily in the Northern Hemisphere. It shows that changes of tidal amplitude and/or phase have taken place over large scales. The principal solar semidiurnal (S-2) tide shows the largest trends. At some locations, the change in the mean tidal range due to tidal trends is significant compared with the trend in mean sea level. Thus, it might be advisable to consider these changes in studies of the impact of rising sea level. Numerical simulations of the principal lunar semidiurnal tide (M-2) demonstrate a model sensitivity in the North Atlantic to changes in glacial isostatic adjustment and sea level rise, which captures 30-40% of the magnitude of the trends in observations. However, the spatial patterns do not agree well with those inferred from observations, suggesting that forward global models are currently useful for qualitative but not quantitative understanding of the observed trends. A global free oscillation synthesis indicates that sea level rise due to glacial isostatic adjustment leads to decreasing global resonant periods and increasing damping in the system and a coupled oscillator model shows that changes in sea level on the shelf are much more effective at perturbing shelf and ocean tides than sea level changes in the deep ocean.