Abstract
Current observations of ocean currents are mainly based on altimetric measurements of Sea Surface Heights (SSH), however the characteristics of the present-day constellation of altimeters are only capable to retrieve surface currents at scales larger than 50–70 km. By contrast, infrared and visible radiometers reach spatial resolutions thirty times higher than altimeters under cloud-free conditions. During the last years, it has been shown how the Surface Quasi-Geostrophic (SQG) approximation is able to reconstruct surface currents from measured Sea Surface Temperature (SST), but it has not been yet used to retrieve velocities at scales shorter than those provided by altimeters. In this study, the velocity field of ocean structures with characteristic lengths between 10 and 20 km has been derived from infrared SST using the SQG approach and compared to the velocities derived from the trajectories of Lagrangian drifters. Results show that the SQG approach is able to reconstruct the direction of the velocity field with observed RMS errors between 8 and 15 degrees and linear correlations between 0.85 and 0.99. The reconstruction of the modulus of the velocity is more problematic due to two limitations of the SQG approach: the need to calibrate the level of energy and the ageostrophic contributions. If drifter trajectories are used to calibrate velocities and the analysis is restricted to small Rossby numbers, the RMS error in the range of 10 to 16 cm/s and linear correlations can be as high as 0.97.
Highlights
The observation of currents is of key importance for understanding the ocean and managing human activities at sea
We focused on its evolution after leaving the vortex core because there were no clouds hiding any part of the vortex
The validation of surface current vectors at 1 km provided by future missions such as the SeaStar mission concept will require developing and exploring complementary approaches to provided currents at similar spatial resolutions, at least in enough places and/or periods of time to carry on the validation
Summary
The observation of currents is of key importance for understanding the ocean and managing human activities at sea. The surface circulation in the Mediterranean is characterized by the entrance of fresh waters through the Gibraltar strait that propagate anti-clockwise along the coast, in the Western Mediterranean sea, e.g., [15] The destabilization of these waters of Atlantic origin leads to the generation of vortices with sizes in the range of 50 to 100 km approximately, which have been studied with SSH and SST, e.g., [16,17,18,19]. We explore the capability of the SQG framework to reconstruct the velocity field in its lower limits exploiting both the optimal conditions for using SST measurements in the Mediterranean and the validity of the geostrophic approach at scales significantly smaller than those captured by current altimeters. We focus on the reconstruction of the velocity field associated to coherent structures smaller than 20 km that were sampled by drifting buoys
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