Ocean Currents Reconstruction from a Combination of Altimeter and Ocean Colour Data: A Feasibility Study

Daniele Ciani,Elodie Charles,Marie-Hélène Rio,Rosalia Santoleri,Bruno Buongiorno Nardelli

doi:10.3390/rs13122389

Abstract

Measuring the ocean surface currents at high spatio-temporal resolutions is crucial for scientific and socio-economic applications. Since the early 1990s, the synoptic and global-scale monitoring of the ocean surface currents has been provided by constellations of radar altimeters. By construction, altimeter constellations provide only the geostrophic component of the marine surface currents. In addition, given the effective spatial-temporal resolution of the altimeter-derived products (O (100 km) and O (10 days), respectively), only the largest ocean mesoscale features can be resolved. In order to enhance the altimeter system capabilities, we propose a synergistic use of high resolution sea surface Chlorophyll observations (Chl) and altimeter-derived currents’ estimates. The study is focused on the Mediterranean Sea, where the most energetic signals are found at spatio-temporal scales up to 10 km and a few days. The proposed method allows for inferring the marine surface currents from the evolution of the Chl field, relying on altimeter-derived currents as a first-guess estimate. The feasibility of this approach is tested through an Observing System Simulation Experiment, starting from biogeochemical model outputs distributed by the European Copernicus Marine Service. Statistical analyses based on the 2017 daily data showed that our approach can improve the altimeter-derived currents accuracy up to 50%, also enhancing their effective spatial resolution up to 30 km. Moreover, the retrieved currents exhibit larger temporal variability than the altimeter estimates over annual to weekly timescales. Our method is mainly limited to areas/time periods where/when Chl gradients are larger and are modulated by the marine currents’ advection. Its application is thus more efficient when the surface Chl evolution is not dominated by the biological activity, mostly occurring in the mid-February to mid-March time window in the Mediterranean Sea. Preliminary tests on the method applicability to satellite-derived data are also presented and discussed.

Highlights

This study relied on numerical hydrodynamical and biogeochemical models distributed within the Copernicus Marine Service CMEMS, providing gap-free data that can be manipulated to simulate satellite-derived observations and, at the same time, to rely on gap-free fields that can be used as a validation benchmark
The effective temporal resolution of the optimal currents is enhanced compared to the altimeter estimates
The difference between the OPtimal Currents (OPC) and Synthetic Altimeter-derived Currents (SAC) standard deviations (STDs), computed on both annual and weekly timescales, is positive in 80% of the Mediterranean, demonstrating an enhanced temporal variability; the spectral analyses of the SAC, OPC and CMEMS Physics Analysis and Forecast (PHY) Kinetic Energy fields suggest that the OPC fully recovers the surface dynamics until scales of 30 km that we defined as the OPC effective spatial resolution

Summary

Introduction

Global-scale, synoptic, high-resolution and accurate estimates of sea surface variables are necessary for a better understanding and prediction of a variety of processes occurring in the Earth system. Observations from individual sensor/platform types, generally provide incomplete information, either due to their non-homogeneous space–time sampling, to limited spatial-temporal resolution and coverage, and/or to measurement representativeness issues. Achieving global coverage at high spatial-temporal resolution still remains a challenge and requires advanced techniques to produce gap-free, highresolution analyses based on multiple observations. To this scope, several approaches have been developed.

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Conclusion