Spectroscopic analysis of global tide gauge sea level data

Yearly and monthly tide gauge sea level data from around the globe are fitted to numerically generated tidal data to search for the 18.6-yr lunar nodal tide and 14-month pole tide. Both tides are clearly evident in the results, with amplitudes and phases that are consistent with a global equilibrium response. Global atmospheric pressure data are fitted to global, monthly sea level data to study the response of the ocean to pressure fluctuations. The global response of sea level to pressure is found to be inverted barometer at periods greater than 2 months. A large coherence at 437 days between pressure and sea level data is found for the north Sea, Baltic Sea and the Gulf of Bothnia. The results are not entirely conclusive, but they tend to support O'Connor's (1986) suggestion that the apparent enhanced pole tide in these basins may be due to meteorological forcing rather than to a basin-scale resonance. Finally, global averages of tide gauge data, after correcting for the effects of post glacial rebound on individual station records, reveal an increase in sea level over the last 80 yr of between 1.1 and 1.9 mm yr-1. As part of the process of removing the effects of post-glacial rebound, we fit those effects to the global tide gauge data and obtain very good agreement with results predicted from post-glacial rebound models. This tends to support the post-glacial model results and it suggests that the global tide gauge data are, indeed, capable of resolving changes in sea level at the mm yr-1 level.