A comparison of predicted and observed ocean tidal loading displacements around the Puget Sound

Around coastlines and in shallow oceans, models of ocean tidal loading (OTL) are not highly accurate and can create sources of error in OTL analysis. OTL is tides moving ocean water that cause the surface of Earth to deform. In this study, forward-modelled predictions of OTL are compared to observations from Global Positioning System (GPS) data to explore the elastic deformation response of Earth to OTL around the Puget Sound. Data from 75 Global Navigation Satellite Systems (GNSS) stations were processed to yield position estimates at intervals of 5 minutes for a year. The OTL model used for comparison was the FES2014b ocean-tide model loading a spherically symmetric, non-rotating, elastic, and isotropic (SNREI) Earth model. The three tidal frequency bands used were the semidiurnal (M2), diurnal (O1), and fortnightly (Mf). The M2 tide is the largest and Mf the smallest. The model and observations have the largest residual displacements of 5 mm for the M2 tide, and the smallest 2 mm for the O1 tide. The particle motion residuals have strong spatial coherence in the M2 and O1 tide but are less coherent with the Mf tide. This data indicates that either the models of OTL and/or the observations of OTL have deficiencies. Model deficiencies are likely due, in part, to the FES2014b model not extending fully into and accounting for the complex morphology of the Puget Sound. Another element explored was quantifying the difference in observed OTL when a sidereal filter is applied to the GNSS time series. A sidereal filter is used to remove multipath errors that occur every sidereal day when GNSS satellite orbits repeat. The largest residual displacements were 1 mm in the Mf tidal band and the smallest 0.5 mm in the M2 band. These differences can alter the observations used in OTL analysis, which impacts model comparisons. Multipath errors can overlap in frequency space with tidal frequencies, so the use of a sidereal filter must be carefully considered before its application