Surface evolution and wind effects during a cyclonic eddy splitting event in the Balearic Sea

Abstract. During the period of 23–28 February 2022, a cyclonic eddy in the Balearic Sea was observed to split into two smaller eddies. Serendipitously, a wealth of data was collected of the event, including satellite chlorophyll maps, Lagrangian drifters at several depths, hydrographic sections intersecting the splitting eddy, and wind speed and direction. Sufficiently many drifters were in the area to estimate kinematic properties (divergence, vorticity, and strain rate) from clusters along the edge of the eddy before and during the elongation period that led to the splitting. The vertical velocity w can be computed from colocated divergence values from surface drifters (CARTHE and CODE, within the top meter) and near-surface drifters (SVP, at 15 m depth). Together, the observations delineate the process of eddy elongation, leading to vorticity and strain-rate intensification on February 25, followed by the collapse of the ridge in the center of the eddy and the emergence of smaller eddies on February 26, terminating with the splitting into submesoscale cyclones on February 28. The consecutive daily hydrography and drifter observations supplement the remotely sensed descriptive view of the eddy splitting process. In particular, they confirm dominant internal dynamics, consistent with isopycnal doming, but also point to a role played by the winds, which shifted from predominantly southwesterly to predominantly northeasterly and strengthened significantly before weakening again in the area of interest during the eddy splitting period. Nonlinear Ekman pumping W_Eknl is estimated from the wind data and drifter-derived vorticities to capture the contribution of the wind effects to the patterns of up- and downwelling accompanying the eddy splitting. The W_Eknl patterns are consistent with the drifter-based w and divergence estimates. Moreover, the nonlinear Ekman pumping is found to be of the same order of magnitude as (though generally less than) w, suggesting that the wind likely plays a role in the observed surface processes.