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

Abstract

[1] 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 mM in the mixed layer and decreased rapidly to 70 mM 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 mM and <1 mM, respectively). Microstructure profiles revealed intermittent turbulence in the oxycline below the mixed layer. Average rates of turbulent dissipation were 10 À9 -10 À8 W kg À1 . The accompanying mixing produced diapycnal diffusivities, K r , that were only (1 -4) Â 10 À6 m 2 s À1 . 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. Citation: Gregg, M. C., and E. Yakushev (2005), Surface ventilation of the Black Sea's cold intermediate layer in the middle of the western gyre, Geophys. Res. Lett., 32, L03604,

Mixing in the Black Sea detected from the temporal and spatial variability of oxygen and sulfide – Argo float observations and numerical modelling

“…These low values persist in the entire water column, in particular in summer, shielding the intermediate and deep layers from the heat fluxes at sea surface. The lowest values are 2.5 × 10 −6 m 2 s −1 , which agrees well with the results of measurements by Gregg and Yakushev (2005). The performance of GOTM is also consistent with the results of Lewis and Landing (1991), who estimated indirectly vertical diffusion from the tracer distribution in the Black Sea.…”

“…Anoxic conditions were formed about 8000 years ago because of the reconnection of Mediterranean and Black Sea and the intrusion of saltier Mediterranean water which followed the reconnection (Deuser, 1974). The micro-structure profiling measurements (Gregg and Yakushev, 2005) supported earlier estimates from analysis of tracers (Lewis and Landing, 1991) and numerical simulations (Stanev et al, 1997) revealing extremely low vertical turbulent exchange (coefficients of about 1-4 × 10 −6 m 2 s −1 ), approaching the values of molecular exchange. This explains the formation of a strong vertical stratification (Fig.…”

“…7) is a permanent layer at 50-100 m (see also Figs. 2a and 3) acting as a near-surface thermal reservoir, similar to the North Atlantic Subtropical Mode Water, which is renewed due to vertical mixing during cold winters (Stanev et al, 2003;Gregg and Yakushev, 2005). Winter cooling penetrates down to about 100 m in the western Black Sea (Fig.…”

Black Sea nitrogen cycling and the preservation of phytoplankton δ15N signals during the Holocene

“…The remaining PO 4 3À is assimilated by N 2 -fixing cyanobacteria. We assume that the PO 4 3À influx to the SML comes from the CIL, the maximum mixing depth of surface waters [Gregg and Yakushev, 2005;Murray et al, 1991]. This nutrient pool has an N:P ratio near 5:1 , so we expect upwelling CIL waters to stimulate significant cyanobacterial N 2 fixation in the SML.…”

“…Due to a weaker salinity gradient, mode-1 allows upward mixing of nutrients from the SOL and AOL where PO 4 3À is in excess of combined N. During mode-1 the chemocline at the top of the AOL would be deeper than in the modern water column. Upward mixing is limited to the CIL in mode-2, with the maximum depth derived from modern observations by Gregg and Yakushev [2005].…”