Residual M2 and S2 ocean tide signals in complex coastal zones identified by X-Track reprocessed altimetry data

Abstract

While the reliability of reprocessed altimetry data from X-Track has been examined for sea level studies, it has not previously been used to study ocean tides in tidally complex coastal zones. Using a new dataset of short-term tide gauges, the aim of this study is to demonstrate the improvements in X-Track reprocessed data for studying ocean tides for the first time in tidally complex regions in northwest Australia. We show that X-Track can be used to evaluate and potentially correct ocean tide models in challenging coastal areas. Our study comprises three components: (1) evaluate the tidal constants from X-Track and RADS datasets using nearby (<12 km) short-term tide gauge data, (2) assess three ocean tide models (FES2014, DTU10, TPXO9) using the X-Track altimetry dataset and (3) extract residual M2 and S2 tidal constants from sea level anomalies (SLAs) computed using the optimal model (FES2014) for ocean tide correction and then use these to predict the residual tidal height to use as an additional correction for the recomputation of the SLAs.Our results show that X-Track data provides users with more valid observations close to coastlines (as close as 5 km) so that the derived M2 and S2 tidal constants show better agreement with nearby tide gauges compared to RADS for both Jason-3 (J3) and GFO missions. In Van Diemen Gulf, where the footprints were 7 km or less from the coast, RADS did not have any valid observations, whereas tidal constants of M2 computed from X-Track J3 and GFO data agreed with the tide gauges within ∼6% of the M2 tidal height. In contrast, X-Track J3 and GFO extracted S2 tidal constants agreed with nearby tide gauges within ∼11–24% (GFO extreme of 47%) of the S2 tidal height. The FES2014-detided SLA time series from X-Track were then analysed to detect the remaining signals of M2 and S2 tidal constituents, with co-range maps suggesting the unmodelled amplitudes for M2 reaches 40 cm and S2 up to 30 cm in Collier Bay. In Van Diemen Gulf, M2 contributes more to the residual tidal signal than S2 with an amplitude of 40 cm compared to 10 cm respectively, in contrast to Joseph Bonaparte Gulf with ∼10 cm for both. When the SLAs, are corrected for the residual tide heights of M2 and S2, the standard deviation of the SLA residuals is decreased by as much as ∼70% in Collier Bay indicating that X-Track altimetry can identify M2 and S2 tidal residuals in complex coastal regions.