Meso- and Sub-mesoscale Wind–Front Interactions and Their Impacts on Ocean Vertical Velocities

Abstract

Coherent surface ocean temperature and velocity structures, with scales spanning 10–100 km, induce surface wind stress anomalies that generate near-surface vertical velocities through Ekman dynamics. These wind–front interactions are known as Thermal (TFB) and Current (CFB) feedbacks. Their relative contributions to ocean vertical velocities and their variability across scales remain underexplored. This study decomposes and attributes Ekman vertical velocity variability to TFB and CFB at O (100) km and O (10) km scales using output from a global 4–6 km ocean–atmosphere coupled simulation, focusing on the California Current System. Total Ekman velocities are decomposed into a linear component associated with wind stress curl and a nonlinear component involving ocean vorticity derivatives. At 30–200 km scales, TFB and CFB contribute similarly to the linear component, which is comparable in magnitude to the nonlinear term ( O (1–5) m day −1 in instantaneous value). At <30 km scales, nonlinear Ekman velocities exceed linear terms by an order of magnitude, reaching instantaneous values of O (10–50) m day −1 and matching the total near-surface vertical velocities. The decomposition reveals that the nonlinear enhancement is driven by ocean vorticity gradients at <30 km scales. These results emphasize the importance of resolving small-scale wind–front interactions and ocean surface features that modulate upper-ocean vertical velocities, demonstrating, as a first step, the potential to infer subsurface vertical motions from surface measurements and Ekman dynamics. However, such inferences should be interpreted with caution where the surface Rossby number exceeds unity.