Frontal dynamics in a California Current System shallow front: 2. Mesoscale vertical velocity

Abstract

This is the second paper investigating the three‐dimensional dynamics from two consecutive, quasi‐synoptic surveys of a shallow front in the California Current System. The mesoscale vertical velocity (w) is obtained by solving a generalized ω equation using density and horizontal velocity observations. Highly nonlinear dynamics emerge from the ageostrophic forcing terms for w in an adiabatic generalized ω equation. The two main processes driving w are (1) wind‐induced cross‐frontal ageostrophic circulation (survey 1) and (2) ageostrophic kinematic deformation during frontogenesis (surveys 1 and 2). The horizontally averaged heat fluxes are positive in the whole water column with maxima at ∼50 m, which warms (cools) the upper (lower) water column and upwells (downwells) light (dense) water. Wind‐induced currents interact with the front, cooling the upper ocean and creating a divergent potential vorticity (PV) flux at the Ekman layer base which weakens the vertical heat and PV fluxes in survey 1. Vertical velocity extrema reach ∼10 m d−1 in both surveys. A diabatic ω equation is derived, which introduces an important new idea: the relation of the frictional w with the vertical diffusivity of the differential ageostrophic vorticity. This term is not found in the quasi‐geostrophic ω equation. By including vertical mixing, ∣w∣ is enhanced by a factor of 2 in the upper ∼100 m and reduced below. This effect is pronounced when the wind blows in the direction of the frontal jet, but it is sensitive to the vertical mixing parameterization.