Diagnosis of Ocean Near-Surface Horizontal Momentum Balance From Pre-SWOT Altimetric Data, Drifter Trajectories, and Wind Reanalysis

Along-track and gridded altimetric observations of sea level are colocated and combined with data from drifter observations and wind reanalysis to reconstruct global instantaneous near-surface horizontal momentum balance. This reconstruction includes not only geostrophic terms but also Lagrangian accelerative terms and turbulent stress terms. The methodology developed quantifies the degree of closure, distinguishes statistically balanced components from errors and estimates compensation between pairs of terms. The links between statistically derived closure diagnoses and dynamical ones are established. Overall, the variance of the residual acceleration is about 20% of the sum of individual acceleration variances. We carry out a detailed exploration of the misclosure, which is dominated by unbalanced variance in drifter observations (resolution mismatch accounts for 41% of the total error) followed by instrumental and spatial colocation errors. Except for the turbulent stress term, errors are sufficiently small to ensure safe interpretation of statistically derived balanced contributions as dynamical ones. Although geostrophy is the leading order equilibrium, ageostrophic contributions associated with nonlinear balanced motions, internal tides, and near-inertial waves account for one third of the global balanced acceleration variance. Momentum balance reconstructions and the methodology developed here for that purpose hold promise for validating Surface Water Ocean Topography sea level observations, for quantifying our ability to estimate the ocean circulation from these observations, and for improving our understanding of ocean near-surface dynamics.