A strictly geostrophic product of sea-surface velocities from the SWOT fast-sampling phase

TL;DR

This paper develops a method to accurately extract the geostrophic component of sea-surface velocities from SWOT satellite data, addressing spatial scale ambiguities and providing a robust, global dataset for oceanographic analysis.

Contribution

It introduces a DMD-based approach combined with IT corrections to isolate geostrophic velocities from SWOT data, enhancing the reliability of altimetric observations.

Findings

Robust extraction of geostrophic velocities from SWOT data.

Global dataset of SSHa-derived velocities and related statistics.

Spectral analysis of SSHa in key regions.

Abstract

While geostrophy remains the simplest and most practical balance to extract velocity information from sea-surface height anomaly (SSHa), confusions remain within the oceanographic community to what extent this balance can be applied to altimetric observations with the launch of the Surface Water and Ocean Topography (SWOT) satellite. Given the limited temporal resolution of SWOT, many studies have resorted to claiming that the spatially filtered SSHa fields correspond to the geostrophic component. This introduces the ambiguity of which spatial scale to choose. Here, we build upon the recent developments in internal tide (IT) corrections (Yadidya et al., 2025) and apply a dynamic mode decomposition (DMD)-based method introduced by Lapo et al. (2025) to robustly extract the geostrophic component associated with sub-inertial frequencies from the SWOT one-day-repeat orbit; we distribute the global dataset as a public good. We provide the joint probability density function (PDF) of vorticity and strain, and spectra of SSHa at a few cross-over regions.