Inversion of sea surface currents from satellite-derived SST-SSH synergies with 4DVarNets

TL;DR

This paper introduces a learning-based 4DVarNet scheme that combines satellite-derived sea surface height and temperature data to improve the inversion of sea surface currents, capturing significant ageostrophic components at small scales.

Contribution

It develops a novel variational data assimilation method leveraging SST and SSH synergies, enhancing the retrieval of ageostrophic surface currents at sub-weekly scales.

Findings

SST-SSH synergies can reveal currents at 2.5-3 days and 0.5°-0.7° scales.

The method captures approximately 47% of ageostrophic dynamics.

SST features are crucial for reconstructing small-scale surface currents.

Abstract

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}$^\circ$.