Submesoscale Motions Driven by Down-Front Wind Around an Anticyclonic Eddy With a Cold Core

This study investigates the dynamics and diurnal variations of submesoscale motions around an anticyclonic eddy with a cold core using observations and simulations. Glider observations reveal vigorous submesoscale motions within the mixed layer (ML) at the eddy periphery due to strong frontal structure. The main driving force behind these motions is external atmospheric forcing, especially down-front wind, which injects negative Ertel potential vorticity (PV) into the ML, triggering frontal instability in the low-PV layer. Observations indicate a regular diurnal cycle of submesoscale motions at the eddy periphery in the absence of strong daytime winds. During the night, cooling and winds inject negative PV, leading to instability, while daytime heating and weak winds contribute to restratification. However, the presence of strong down-front winds during the day disrupts the regular diurnal cycle, causing frontal instability even during daylight hours. The observed eddy periphery is reconstructed in a turbulence-resolving large-eddy simulation, confirming and extending the observations. The combined results emphasize the crucial impact of atmospheric forcing, particularly down-front wind, on the diurnal variations of submesoscale motions around abnormal anticyclonic eddies. Diurnal heat fluxes play a key role in maintaining the diurnal cycle of these motions, while transient strong down-front winds are essential for breaking the regular cycle and rapidly triggering frontal instability.