Wavenumber spectra of a submesoscale front from ADCP ship-track data

Oceanic fronts are regions of intense activity which play a crucial role in connecting surface flows to the deeper ocean. However, the complex dynamics of these systems remain challenging to observe and quantify. The CALYPSO program aims to uncover the pathways by which surface water is transported into the ocean interior, addressing fundamental questions about the evolution of oceanic fronts. Previous studies have identified frontal regions as hotspots of variability, where sharp gradients and small-scale structures can enhance mixing and drive significant transport of physical and biogeochemical properties.

Here, we focus on the characterization of a frontal system in the northern Balearic Sea through a multi-instrumental approach involving CTD (UCTD) and Acoustic Doppler Current Profiler (ADCP). The survey was conducted from 18 February to 12 March 2022 aboard the R/V Pelagia and consisted of 19 repetitions of a high-resolution, small-scale rectangular sampling box. UCTD profiles were collected at a mean horizontal resolution of 1km enabling detailed resolution of temperature, salinity, and density gradients across the front.

As part of the analysis, vorticity was calculated from the ADCP velocity field to estimate the role of submesoscale processes within the frontal system. To further investigate the structure of the velocity field, we applied a spectral Helmholtz decomposition technique to the ADCP data, which separates the one-dimensional observed velocities into their rotational and divergent components, providing detailed insights into the flow kinematics. A crucial step in this process is assessing the anisotropy of the flow to ensure the correct implementation of the method. The analysis used models tailored for both isotropic and anisotropic flows, enabling us to examine how flow anisotropy influences submesoscale dynamics. Additionally, the analysis revealed signatures of inertial-gravity waves, highlighting their role in the observed velocity field and their interaction with submesoscale processes. This approach offers a spectral view of the energy distribution, flow instabilities, and wave dynamics, improving our understanding of the interplay between different scales within frontal systems and the evaluation of the balanced and unbalanced flows. Preliminary results reveal a dominant contribution of rotational motions to the kinetic energy spectra, while the influence of internal waves becomes increasingly significant at deeper levels.