Inversion of sea surface currents from satellite-derived SST-SSH synergies with 4DVarNets
This work addresses the limitation of satellite altimetry in observing ageostrophic sea surface currents for oceanography, offering an incremental improvement through a novel data assimilation method.
The paper tackled the problem of estimating sea surface currents, including ageostrophic dynamics, by developing a learning-based 4DVarNet scheme that synergizes satellite-derived sea surface height (SSH) and sea surface temperature (SST) data. The result showed that this approach could reveal velocities for time scales of 2.5-3.0 days and horizontal scales of 0.5°-0.7°, capturing about 47% of ageostrophic dynamics.
Satellite altimetry is a unique way for direct observations of sea surface dynamics. This is however limited to the surface-constrained geostrophic component of sea surface velocities. Ageostrophic dynamics are however expected to be significant for horizontal scales below 100~km and time scale below 10~days. The assimilation of ocean general circulation models likely reveals only a fraction of this ageostrophic component. Here, we explore a learning-based scheme to better exploit the synergies between the observed sea surface tracers, especially sea surface height (SSH) and sea surface temperature (SST), to better inform sea surface currents. More specifically, we develop a 4DVarNet scheme which exploits a variational data assimilation formulation with trainable observations and {\em a priori} terms. An Observing System Simulation Experiment (OSSE) in a region of the Gulf Stream suggests that SST-SSH synergies could reveal sea surface velocities for time scales of 2.5-3.0 days and horizontal scales of 0.5$^\circ$-0.7$^\circ$, including a significant fraction of the ageostrophic dynamics ($\approx$ 47\%). The analysis of the contribution of different observation data, namely nadir along-track altimetry, wide-swath SWOT altimetry and SST data, emphasizes the role of SST features for the reconstruction at horizontal spatial scales ranging from \nicefrac{1}{20}$^\circ$ to \nicefrac{1}{4}$^\circ$.