Observations of upper ocean boundary layer dynamics in the marginal ice zone

Abstract

Vertical profiles of turbulent kinetic energy dissipation and small‐scale hydrography were collected in the upper ocean boundary of both ice‐covered and ice‐free stations in the marginal ice zone of the Barents Sea in spring 2005. Together with shipboard wind measurements and current profiles, the mixed and mixing layer dynamics are studied. During the survey, shear production by the stress at the surface or under the ice dominated. Large upward turbulent heat fluxes ∼300–500 W m−2 were calculated for the mixing layers overlying the warm Atlantic Water which compared well with those obtained from an independent parameterization. At the top of the pycnocline, corresponding heat fluxes were 10–20 W m−2. Significant stabilizing buoyancy fluxes were estimated for ice‐drift stations owing to the melting of ice in response to the large heat fluxes. The mean dissipation profile in the ice‐free reference station agreed with the constant‐stress wall layer scaling within 30%. On the contrary, the observed dissipation profiles were enhanced in the upper half of under‐ice mixing layer or within 2.5 times the assessed pressure‐ridge keel depth. Deeper in the mixing layer the profiles relaxed toward the wall scaling. The variability of dissipation in the mixed layer was better captured by the shear production profile when local friction speed was used together with a mixing length profile modified by buoyancy fluxes. Significant correlations were found between dissipation integrated over the mixing layer and work done by stress under the ice and the wind work at 10 m height. The low correlation between the mixed layer depth and length scale for neutral conditions significantly increased when the reduction in the mixing length due to buoyancy fluxes was accounted for. The mixing depth is observed to be strongly correlated with the outer neutral planetary length scale.