How Well Do We Know the Seasonal Cycle in Ocean Bottom Pressure?

We revisit the nature of the ocean bottom pressure (pb) seasonal cycle by leveraging the mounting GRACE-based pb 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 pb 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 pb. 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 pb estimates. We revisit the nature of the ocean bottom pressure (pb) seasonal cycle by leveraging the mounting data from space gravity missions and their use in constraining model-based 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. The pb annual cycle tends to dominate but the semi-annual cycle can also be important in some regions. Amplitudes are enhanced near shallow coastal regions and can also vary more strongly across the coastal zone, pointing to the importance of changes in ocean bottom topography in shaping the seasonal cycle in pb. Comparisons of data and ECCO estimates indicate good qualitative agreement, but considerable quantitative differences in the magnitude and timing of maxima remain. Largest differences, which tend to occur near coastal regions, can be related to enhanced data uncertainties and also to the absence in ECCO of effects from gravitational attraction of land ice and water storage and related deformation of the ocean bottom. Substantial differences remain in satellite and model-based estimates of the mean seasonal cycle in ocean bottom pressure (pb) Differences between two satellite gravimetry products suggest largest uncertainties around many continental boundaries Absence of gravitational attraction and loading effects and intrinsic ocean variability can lead to errors in model-based pb estimates