Glider Survey reveals the evolution of the mesoscale and submesoscale structures in the Balearic Sea: Formation, Intensification, and Decay.

Abstract

Mesoscale features and their corresponding submesoscale structures may generate a substantial vertical transport of carbon and biogeochemical tracers from the surface to the interior. The baroclinic instabilities formed from the Northern and Balearic Currents are associated with vigorous mesoscale eddy activity in the Balearic Sea. In the CALYPSO 2022 experiment, eight gliders were programmed to dive as deep as 700 m from 25 March until 21 June 2022. The glider fleet measured temperature, salinity, velocity, chlorophyll fluorescence, oxygen, and acoustic backscatter. The glider data was mapped utilizing objective mapping of the across-front, along-front, and time on 10 m vertical levels. The geostrophic velocity was inferred using a variational approach. We estimate the vertical and ageostrophic horizontal velocities using the omega equation. The glider observations provide a complete description of the evolution of the eddy field. The analysis of the uplifted isopycnal surface as the eddy formed showed consistency between the movement of the dynamic and biogeochemical tracers. Glider data show the evolution of a cyclonic eddy (20-30 km) in the area, where vertical velocities tend to be downward. As the eddy developed, its axis shifted westward. The isopycnal uplift inside the pycnocline can provide nutrients to the euphotic zone. While the initial cyclonic feature has dissipated, a second shoaling of the 28.9 isopycnal occurred in the east toward the end of April 2022, where the biggest relative vorticity (~0.5 ζ/f) was observed in the region. The eddy extended during its growing phase, showing a westward shift of the eddy axis. The observed peak downward vertical velocities were near 30 m day-1 during the eddy intensification, with the size of the cyclonic eddy varying between 20-30 km. The new cyclonic feature has been spreading in the area for a few days before dividing into two separate cyclonic eddies (15km) around the beginning of May. The two smaller cyclonic eddies moved north and west until they vanished from our study area. However, an anticyclonic structure (20km) was developed within their separation. The vertical velocity tended to be downward on the dense side of the front and upward on the light side, flattening the eddy characteristic. The glider observations reveal horizontal density gradients up to 0.5 kg m-3 over ~10 km. The obtained maximum velocities were up to 30 cm/s in the region. Upwelling and downwelling were also detected by chlorophyll fluorescence, oxygen, and acoustic backscatter near the frontal interface. Glider observations were integrated with remote sensing and modeling simulations to evaluate mesoscale and submesoscale variability in developing vertical velocities in the Balearic Sea and their impact on biological carbon storage.