We revisit the nature of the ocean bottom pressure (OBP) seasonal cycle by leveraging the mounting GRACE-based OBP record and its assimilation in the ocean state estimates produced by the project for Estimating the Circulation and Climate of the Ocean (ECCO). We focus on the mean seasonal cycle from both data and ECCO estimates, examining their similarities and differences and exploring the underlying causes. Despite substantial year-to-year variability, the 21-year period studied (2002–2022) provides a relatively robust estimate of the mean seasonal cycle. Results indicate that the OBP annual harmonic tends to dominate but the semi-annual harmonic can also be important (e.g., subpolar North Pacific, Bellingshausen Basin). Amplitudes and short-scale phase variability are enhanced near coasts and continental shelves, emphasizing the importance of bottom topography in shaping the seasonal cycle in OBP. Comparisons of GRACE and ECCO estimates indicate good qualitative agreement, but considerable quantitative differences remain in many areas. The GRACE amplitudes tend to be higher than those of ECCO typically by 10%–50%, and by more than 50% in extensive regions, particularly around continental boundaries. Phase differences of more than 1 (0.5) months for the annual (semiannual) harmonics are also apparent. Larger differences near coastal regions can be related to enhanced GRACE data uncertainties and also to the absence of gravitational attraction and loading effects in ECCO. Improvements in both data and model-based estimates are still needed to narrow present uncertainties in OBP estimates.

A dynamic response of the ocean to surface pressure loading by the well-known 5-day Rossby-Haurwitz mode in the atmosphere has been inferred from limited in situ tide gauge and bottom pressure data, but a global characterization of such response, including details at mid and high latitudes, has been lacking. Here we explore two daily data products from the Gravity Recovery and Climate Experiment (GRACE) mission to obtain a first quasi-global look at the associated ocean bottom pressure (OBP) signals at 5-day period. The previously reported in-phase behavior over the Atlantic basin, seesaw between the Atlantic and Pacific, and westward propagation in the Pacific are all seen in the GRACE solutions. Other previously unknown features include relatively strong responses in the Southern Ocean and also some shallow coastal regions (e.g., North Sea, East Siberian shelf, Patagonian shelf). Correlation analysis points to the Rossby-Haurwitz surface pressure wave as the main forcing for the observed large-scale OBP anomalies, while wind-driven signals are more spatially confined. The GRACE observations are found to be consistent with in situ OBP data and also with model simulations of the 5-day ocean variability where no in situ data is available. Inferences on energetics based on data and model results point to decay time scales shorter than the oscillation period, with substantial kinetic energy and dissipation located over a few topographic features in the Southern Ocean. Results illustrate the potential of space gravity measurements for examining large-scale oceanic variability at sub-weekly periods.

In many places around the world, tide gauges have been measuring substantial non-astronomical changes. Here we document an exceptional large spatial scale case of changes in tidal range in the North Sea, featuring pronounced trends between -2.3 mm/yr in the UK and up to 7 mm/yr in the German Bight between 1958 and 2014. These changes are spatially heterogeneous, suggesting a superposition of local and large-scale processes at work within the basin. We use principal component analysis to separate large-scale signals appearing coherently over multiple stations from rather localized changes. We identify two leading principal components (PCs) that explain about 69% of tidal range changes in the entire North Sea including the divergent trend pattern along UK and German coastlines, which suggest movement of the region’s semidiurnal amphidromic areas. By applying numerical and statistical analyses, we can assign a baroclinic (PC1) and a barotropic large-scale signal (PC2), explaining a large part of the overall variance. A comparison between PC2 and tide gauge records along the European Atlantic coast, Iceland and Canada shows significant correlations on time scales of less than 2 years, which suggests an external and basin-wide forcing mechanism. By contrast, PC1 dominates in the southern North Sea and originates, at least in part, from stratification changes in nearby shallow waters. In particular, from an analysis of observed density profiles, we suggest that an increased strength and duration of the summer pycnocline has stabilized the water column against turbulent dissipation and allowed for higher tidal elevations at the coast.