One-dimensional modeling of vertical mixing in a seasonally ice-covered, stratified, high-latitude coastal ocean

Hyatt, J., Beardsley, R.B.

The PWP (Price et al., 1986) one-dimensional vertical mixed layer model was adapted for investigation of mixing beneath an ice-covered ocean. Simulations using an idealized storm forcing of a stratification representative of winter on the western Antarctic Peninsula (wAP) shelf revealed the relative roles of wind mixing and convection due to brine rejection in vertical mixing. When reasonable ice-ocean drag coefficients were used (0.015), the presence of ice did not greatly change the modeled response of the water column when compared with the open water case. Shear mixing driven by the initial wind event, not the subsequent shear associated with inertial oscillations, caused the mixing and subsequent deepening of the mixed layer.

The model results indicate that the combined effects of shear and static instability can be significant over short time scales. The model forced by observed atmospheric and ice conditions on the wAP shelf produces large vertical fluxes for short periods, a ~385 W/m² one-day averaged heat flux across the base of the mixed layer during a storm event. However, when averaged over the entire 6-day simulation, the flux reduces to a more modest 79 W/m² across the base of the mixed layer. Making the rough assumption of one storm event per month with the remainder of the month having a weak flux (17 W/m²) one estimates a monthly average flux of 29 W/m² across the base of the mixed layer.

The small (17 W/m2) fluxes estimated during the period of inertial oscillations following the storm event are consistent with the estimate made in Howard et al., 2004, of ~5 W m^-2 over the sharp pycnocline during a 4-day period of inertial oscillations in August, 2001. Furthermore, the results support the notion proposed in Howard et al, 2004, that intermittent but relatively energetic events may be the dominant contributors to mean turbulent diapycnal fluxes.