The Effect of Stokes Drift and Transient Rip Currents on the Inner Shelf. Part II: With Stratification

Abstract

his is Part II of a two-part study focused on Stokes drift and transient rip current (TRC) effects on the unstratified (Part I) and stratified (this paper) inner shelf. Part I focuses on funwaveC?Coupled Ocean?Atmosphere?Wave?Sediment Transport (COAWST) coupling and TRC effects on mixing and exchange on an unstratified inner shelf. Here, two simulations (R3 and R4) are performed on a stratified inner shelf and surfzone with typical bathymetry, stratification, and wave conditions. R3 is a COAWST-only simulation (no TRCs), while R4 has funwaveC?COAWST coupling (with TRCs). In R4, TRCs lead to patchy, near-surface cooling, vertical isotherm displacement, and increased water column mixing. For both R3 and R4, the mean Lagrangian circulation has two nearly isolated surfzones and inner-shelf overturning circulation cells, with a stronger, R4, inner-shelf circulation cell. The R4, inner-shelf, vertical velocity variability is 2?3 times stronger than a simulation with TRCs and no stratification. Relative to R3, R4 eddy diffusivity is strongly elevated out to three surfzone widths offshore due to TRCs and TRC-induced density overturns. The R4 inner-shelf stratification is reduced nearshore, and mean isotherms slope more strongly than R3 because of the TRC-enhanced irreversible mixing. At six surfzone widths offshore, both R3 and R4 are in geostrophic balance, explaining the stratified (summertime) observed deviation from Stokes?Coriolis balance. In this region, baroclinic pressure gradients induced by sloping isotherms induce an alongshore geostrophic jet offshore, strongest in R4. In R4, TRCs result in an enhanced (2?10 times) cross-shore exchange velocity across the entire inner shelf, relative to R3. Accurate, stratified, inner-shelf simulations of pollution, larval, or sediment transport must include transient rip currents.