Patterns of OBP variability are often related to climatic modes, especially the El Niño–southern oscillation (ENSO) in the Pacific. Climate dynamics also impact sea level and mass variation no matter the triggered mechanism. However, instances of regional-scale barotropic sea-level variability were reported. At lower latitudes, OBP variations potentially have a baroclinic contribution, typically at the subcentimetric level. Especially at mid-high latitudes such as in the southern ocean, regional-scale sea-level variations mostly correspond to barotropic, i.e., depth-independent, wind-driven mass variations. Besides land-ocean transfers, the ocean circulation, mass, and sea-level variations result from water density gradients or wind-driven Ekman transport. Oceanic barotropic signals are supposed to be prominently related to high frequencies but were also reported at intraseasonal and interannual scales. The effect of the atmosphere tends to cancel out at timescales longer than a few days. Regarding processes, the mass distribution in the ocean is governed by the hydrostatic equilibrium, which implies that OBP reflects the mass of the ocean-atmosphere fluid column. Still, there are reasons to expect relevant signals and processes at interannual and intraseasonal time scales. Conversely, empirical analyses of GRACE (-FO) products, or their assimilation in OGCMs, offer the opportunity of improving our understanding and model representation of the global ocean functioning. The physical representativeness of GRACE (-FO) product over oceans was acknowledged early on, even more today with recent mass concentration solutions that are better matching with in situ OBP sensors. OGCMs offer opportunities for correcting GRACE (-FO) solutions from aliasing errors. Yet, OGCMs and GRACE (-FO) products have co-evolved along with our understanding of the global ocean. Besides GRACE (-FO), oceanic mass distributions are estimated through a limited set of ocean bottom pressure (OBP) in situ sensors, or from sea-level variations, indirectly, by removing steric effects, or using ocean general circulation models (OGCMs). GRACE products show a sensitivity at the subcentimetric level when expressed in equivalent water height. Since 2002, the Gravity Recovery and Climate Experiment (GRACE) and its successor GRACE Follow-On (GRACE-FO) satellite missions allow monthly estimations of ocean mass distributions at the global scale, with a 3° × 3° spatial resolution.

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