Restratification at a California Current Upwelling Front. Part II: Dynamics

A coordinated multiplatform campaign collected detailed measurements of a restratifying surface intensified upwelling front within the California Current System. A companion paper outlined the evolution of the front, revealing the importance of lateral advection at tilting isopycnals and increasing stratification in the surface boundary layer with a buoyancy flux equivalent to 2000 W m−2. Here, observations were compared with idealized models to explore the dynamics contributing to the stratification. A 2D model combined with a reduced form of the horizontal momentum equations highlight the importance of transient Ekman dynamics, turbulence, and thermal wind imbalance at modulating shear in the boundary layer. Specifically, unsteady frictional adjustment to the rapid decrease in wind stress created vertically sheared currents that advected horizontal gradients to increase vertical stratification on superinertial time scales. The magnitude of stratification depended on the strength of the horizontal buoyancy gradient. This enhanced stratification due to horizontal advection inhibited nighttime mixing that would have otherwise eroded stratification from the diurnal warm layer. This underscores the importance of near-surface lateral restratification for the upper ocean buoyancy budget on diel time scales.