Vertical mixing at intermediate depths in the Arctic boundary current

Microstructure and hydrographic observations, during September 2007 in the boundary current on the East Siberian continental slope, document upper ocean stratification and along‐stream water mass changes. A thin warm surface layer overrides a shallow halocline characterized by a ∼40‐m thick temperature minimum layer beginning at ∼30 m depth. Below the halocline, well‐defined thermohaline diffusive staircases extended downwards to warm Atlantic Water intrusions found at 200–800 m depth. Observed turbulent eddy kinetic energy dissipations are extremely low (ε < 10−6 W m−3), such that double diffusive convection dominates the vertical mixing in the upper‐ocean. The diffusive convection heat fluxes FHdc ∼1 W m−2, are an order of magnitude too small to account for the observed along‐stream cooling of the boundary current. Our results implicate circulation patterns and the influence of shelf waters in the evolution of the boundary current waters.