Investigation of the non-stationary ocean loading with ARTOFS and STORMTIDE ocean models

Based on ocean models, ARTOFS ( Atlantic Real-Time Ocean Forecast System) and STORMTIDE, we calculate the effect of ocean loading and attraction at several European superconducting gravimeter stations. A moving window tidal analysis is applied to the sum of the signal and synthetic body tides. The results are compared to results from the analysis of observed data. Thereby we test, whether observed temporal variations of the tidal parameters of M2 for gravity observations can be explained by the time variable response of the oceans to tidal forcing.

An annual variation of the tidal parameters of wave group M2 is observed globally and is in the order of 10^-4 for the gravimetric factor and 10^-2 ° for  the phase at almost all European stations. Annual changes of the M2-amplitude in the oceans, which are larger than expected from the astronomical forcing, might cause this variation. They are observed in tide gauge and satellite altimetry data and are probably due to meteorological influences and non-linear ocean response, especially in shelf areas. The ocean models used here contain both effects. ARTOFS additionally assimilates observations and provides hourly values of sea surface height for the Atlantic ocean. STORMTIDE provides the global distribution of amplitudes and phases for eight constituents and for an additional annual variation of the M2 tide.

By using the Green's function formalism, we calculate the time dependent gravity change due to the loading and attraction of the water masses at the stations  Black Forest Observatory, Strasbourg, Moxa and Membach. Moving window analysis with Eterna 3.4 results in time dependent tidal parameters. Beside annual variations, changes on shorter time scales occur for the ARTOFS model, which are not present in the observed data. The amplitude of the annual variation of the tidal parameters is in the same order as the results obtained from observations.