04 Apr 2023
 | 04 Apr 2023
Status: this preprint is open for discussion.

Regional mapping of energetic short mesoscale ocean dynamics from altimetry: performances from real observations.

Florian Le Guillou, Lucile Gaultier, Maxime Ballarotta, Sammy Metref, Clément Ubelmann, Emmanuel Cosme, and Marie-Helène Rio

Abstract. For over 25 years, satellite altimetry has provided invaluable information about the ocean dynamics at many scales. In particular, gridded Sea Surface Height (SSH) maps allow to estimate the mesoscale geostrophic circulation in the ocean. However, conventional interpolation techniques rely on static optimal interpolation schemes, hence limiting the estimation of non linear dynamics at scales not well sampled by altimetry (i.e. below 150–200 km at mid latitudes). To overcome this limitation in the resolution of small-scale SSH structures (and thus small-scale geostrophic currents), a Back and Forth Nudging algorithm combined with a Quasi-Geostrophic model, a technique called BFN-QG, has been successfully applied on simulated SSH data in Observing System Simulation Experiments (OSSEs), showing a significant reduction in interpolation error and an improvement of space-time resolutions of the experimental gridded product compared to operational products. In this study, we propose to apply the BFN-QG to real altimetric SSH data in a highly turbulent region spanning a part of the Agulhas current. The performances are evaluated within Observing System Experiments (OSEs) that use independent data (such as independent SSH, Sea Surface Temperature and drifter data) as ground-truth. By comparing the mapping performances to the ones obtained by operational products, we show that the BFN-QG improves the mapping of short, energetic mesoscale structures and associated geostrophic currents both in space and time. In particular, the BFN-QG improves (i) the spatial effective resolution of the SSH maps by a factor of 20 %, (ii) the zonal and (especially) the meridional geostrophic currents and (iii) the prediction of Lagrangian transport for lead times up to 10 days. Unlike the results obtained in the OSSEs, the OSEs reveal more contrasting performances in low variability regions that are discussed in the paper.

Florian Le Guillou et al.

Status: open (extended)

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  • RC1: 'Comment on egusphere-2023-509', Anonymous Referee #1, 26 Apr 2023 reply

Florian Le Guillou et al.

Florian Le Guillou et al.


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Short summary
Altimetry provides Sea Surface Height (SSH) data along one dimensional tracks. For many applications, the tracks are interpolated in space and time to provide gridded SSH maps. The operational SSH gridded products filter out the small scale signals measured on the tracks. This paper evaluates the performances of a recently implemented dynamical method to retrieve the small scale signals from real SSH data. We show a net improvement in the quality of SSH maps when comparing to independent data.