Formation and changes of the Black Sea cold intermediate layer

Abstract

The local properties of the cold intermediate layer (CIL) in the Black Sea are examined over the period 1960–2015 using a 3D numerical model and realistic forcing. The simulations demonstrate a substantially large multi-annual CIL variability and confirm two major mechanisms: (i) cooling of the surface waters in the basin interior in winter and (ii) transportation of cold water masses, formed in winter on the North Western Shelf (NWS), transported by the main cyclonic current and by mesoscale eddies on the shelf break. Both processes are crucially important for the CIL formation, as was suggested by previous studies. The novel element of the present study is the quantification of the relative importance of these mechanisms during a prolonged time-period. In particular, (i) plays a key role for the CIL formation in the basin interior, while (ii) controls CIL renewal along the main cyclonic current and basin exterior, as well as in the south-eastern region. In order to isolate the effects of basin circulation and the NWS cold water masses contribution, the distribution of passive tracer originating from the NWS is studied. Tracer distribution in the basin interior indicates that a large fraction of the cold NWS water is transported via the main cyclonic current to the eastern convergence and anticyclonic areas. A smaller fraction of the cold NWS water is transported to the central part of the basin. The cooling capacity of the CIL is highly variable and decreases drastically in the last decade of simulation, approaching zero due to changes in regional weather conditions.