Complex 3-D oceanic velocities at SWOT scales exhibited during the spring 2023 BioSWOTMed cruise

Abstract

During the SWOT fast sampling phase in spring 2023, the BioSWOTMed cruise (https://doi.org/10.17600/18002392) sampled for four weeks a front located on both the western and eastern swaths of the 003 SWOT pass, about 100 km northeast of Menorca, in the Western Mediterranean Sea. The front coincided to a portion of the north Balearic front. The front was modulated by the presence of eddies of small Rossby radius (of the order of 20-30km), not visible in conventional altimetry maps. The availability of cloud-free images of ocean color (OLCI) for five consecutive days also revealed the presence of various submesoscale structures and part of their life cycle. The front consisted of a strong roughly eastward meandering jet, separating cold, salty and more productive waters (modified Atlantic Water) in the north from warm, fresher and more oligotrophic waters (younger Atlantic Water) in the south. In addition to classical hydrological, glider, drifters/floats and moving vessel profiler (MVP) measurements, 3D oceanic velocities were measured by 3 ship-mounted acoustic Doppler current profilers (ADCPs), 2 lowered-ADCPs, 1 free-falling newer generation 5-beam ADCP and 2 autonomous vertical velocity profilers. The jet had horizontal velocities up to ~0.35 m.s-1 (0-300 m average), with a cross-jet distance of ~40 km and a vertical extension of 200 m. 1m-surface drifters deployed in the core of the jet traveled at a speed of ~0.5 m.s-1. Two northerly storms generated intense near-inertial waves interacting in the mesoscale field of several eddies sampled by the ship. Cyclogeostrophic velocities derived from SWOT are in good agreement with the measured ADCP (horizontal) velocities. The normalized relative vorticity provides a regional view of the complex oceanic circulation of the jet meandering between various mesoscale eddies and interacting with submesoscale structures. The high variability of vertical velocities (+/- 1.5 cm.s-1) of various origins masks the expected cross-frontal ageostrophic circulation. The finescale 3D circulations observed and well captured at the surface by SWOT are also associated with complex biological content distribution.