Surface ventilation of the Black Sea's cold intermediate layer in the middle of the western gyre

Abstract

Understanding the origin of the shallow temperature minimum, known as the Cold Intermediate Layer (CIL), in the Black Sea has long been hampered by the scarcity of winter observations. During March 2003, we observed a cold‐air outbreak over the center of the Black Sea's Western Gyre. Freezing winds drove convection that cooled the surface mixed layer to 6.1°C and deepened it to 40 m, directly ventilating the upper 80% of the CIL, whose lower boundary was at 49 m. Concentrations of dissolved oxygen were 350 μM in the mixed layer and decreased rapidly to 70 μM at the base of the CIL, 9 m below the mixed layer. A few meters deeper, at the top of the Sub‐Oxic Layer (SOL), both oxygen and hydrogen sulfide became undetectable (<5 μM and <1 μM, respectively). Microstructure profiles revealed intermittent turbulence in the oxycline below the mixed layer. Average rates of turbulent dissipation were 10−9–10−8 W kg−1. The accompanying mixing produced diapycnal diffusivities, Kρ, that were only (1–4) × 10−6 m2 s−1. Consequently, turbulent fluxes were too weak to renew significantly either the lower 20% of the CIL or the SOL, whose top was 4 m below the bottom of the CIL and hence well‐removed from direct surface replenishment.