3D dynamics from uCTD observations of an anticyclonic eddy in the Alboran Sea

Eugenio Cutolo1, Ananda Pascual2, Simon Ruiz2, Shaun Johnston3, Mara Freilich4 and Amala Mahadevan5, (1)IMT ATLANTIQUE, Brest, France, (2)IMEDEA(CSIC-UIB), Esporles, Spain, (3)UCSD/SIO, La Jolla, United States, (4)Massachusetts Institute of Technology, Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (5)Woods Hole Oceanographic Institution, Woods Hole, MA, United States
Intensive underway CTD (uCTD) sampling took place in the Alboran Sea during 2019. A 40-km diameter anticyclonic eddy was observed from satellites (altimetry and chlorophyll data) inside the campaign area. The same feature appeared in the high frequency uCTD salinity and temperature profiles at 1 km resolution and down to 200 m depth. These data were mapped with a variational method and used to reconstruct the 3D dynamics of the area.

The data were mapped using both a standard isotropic interpolation on a regular grid with axes parallel to the meridional and zonal directions and in a rotated coordinate system with axes parallel to the along/across-front directions. This strategy better preserved the gradients and allowed for a small correlation length of 2 km in the across-front direction. Along front we used a correlation length scale of 80 km due to the larger distances between profiles in the along front direction. Since the sampled area was covered in 4 days also the time was considered as a dimension of the interpolation. The mapped hydrography data have been used to compute dynamic height and geostrophic velocities. ADCP data confirmed the validity of 200 m as a no-motion depth.

Vertical velocities have been estimated from the Quasi-Geostrophic and Semi-Geostrophic frameworks. The SG omega equation gives total vertical velocities with a magnitude up to 50 m/day while the QG reach 35 m/day. We tested the sensitivity to the correlation scales and found a reduction in vertical velocity estimates of more than 50% when using standard isotropic interpolation.

Furthermore the QG and SG equations provided estimates of the ageostrophic horizontal velocities component that has been compared with ADCP data and with the lagrangian trajectories obtained by the drifters deployed during the campaign. Finally, the validity of the QG and SG approximations in the Alboran Sea is investigated using synthetic data coming from a Process-Study Model Simulation.