Evolution of a submesoscale cyclone in the Balearic Sea

A submesoscale cyclonic eddy, less than 10 km in diameter, was observed in the Balearic Sea from inception to dissipation for more than a month in winter 2022 The feature was heavily sampled by freely-drifting (Lagrangian) instruments to study its kinematics, spatial structure and temporal evolution.  The Lagrangian sensors included drifters, wirewalkers and profiling floats. Additional shipboard data (underway CTD, ADCP) and remote sensing images revealed that the eddy was formed by doming isopycnals, extending more than 200 m in depth, with dense salty water at the center. The near-surface chlorophyll concentration was maximum in the eddy center, as clearly seen in ocean color satellite images.

The eddy formed in late February by pinching off from a larger cyclonic feature via the collapse of a sharp submesoscale ridge that connected it to a larger cyclone. Wavelet analysis of the drifter trajectories revealed that the submesoscale eddy had a maximum orbital speed of ~ 30 cm/s, at a radius of ~3 km. In its inner core, vorticity was as large as twice the local inertial frequency (Rossby number ~ 2). The surface drifters made looping tracks that were slightly elliptical. Tracked by the swarm of drifters until mid-March, the eddy remained surprisingly coherent as it moved to the South and then East with a drift speed between 2 and 5 cm/s. After a strong wind event, just a few drifters remained in the eddy until it weakened and dissipated in early April.

These observations were a part of the CALYPSO program, an ONR Departmental Research Initiative that addresses the challenge of observing, understanding and predicting the three-dimensional pathways by which water from the surface ocean makes its way into the deeper ocean. Vertical transports by submesoscale fronts and eddies can play an important role in ocean and climate dynamics.