Ventilation of the Black Sea pycnocline on seasonal and interannual time scales

Abstract

The paper is a description of temporal variability of winter cooling conditions and estimation of effective cross-isopycnal mixing rates in the Black Sea. Data averaging versus salinity / sigma-t scale was used to filter effects of local dynamics. It is shown that traces of winter mixing events appear well preserved in the temperature-salinity structure, due to the peculiarities of the Black Sea where temperature often acts as a passive tracer with a smaller contribution to density as compared to salinity. Vertical distribution of the magnitudes of temperature oscillations indicates that the convection events have limited effects in modifying the structure of the middle and lower pycnocline on a seasonal time scale. However, long-term fluctuations are well recognised. The magnitudes of the seasonal and long-term temperature fluctuations are comparable only in the upper pycnocline. Three major cooling events can be distinguished from the record of the pycnocline temperature for the past 75 years. The intensive cooling occurred in the late 1920s - early 1930s, early 1950s and late 1980s - early 1990s. Partial renewal of the water of the cold intermediate layer core took place approximately once in two years. The period when convection causes erosion of the pycnocline lasts for only a week. It is shown that a lateral source of heat and salt exists for the upper pycnocline, where it is the cold intermediate water, and for the lower pycnocline, the layer below S@ 20.5, where this lateral source of salt and heat is maintained by disintegrating Bosphorus plume.

Seasonal and inter-annual temperature variability in the bottom waters over the western Black Sea shelf

“…The Bottom Shelf Waters (BSW) are defined here as a near-bottom water mass which is located below the upper mixed layer down to the seabed. We use the density level σ θ = 14.2 kg m −3 as the lower boundary of the upper mixed layer following previous studies (e.g.Özsoy andÜnlüata, 1997; Ivanov et al, 1997Ivanov et al, , 2000Ivanov et al, , 2001. Our calculations (Fig.…”

“…Temperature of surface waters responds fairly quickly (on a daily to weekly time scale) to the changes in the meteorological forcing (Stanev et al., 1995; Enriquez et al, 2005;Schrum et al, 2001). However the waters below the pycnocline on the outer shelf experience only indirect influence from the atmosphere, and hence have greater inertia, so that time scales for local atmospheric forcing and lateral exchanges due to ocean dynamics could become comparable (Ivanov et al, 2000). In this section we contrast and compare vertical and "horizontal" (isopycnic) communications of the bottom waters on the western Black Sea shelf.…”

“…Blatov et al, 1984) or at its upper bounds by the isopycnic levels σ θ = 14.2 kg m −3 (e.g. Ivanov et al, 2000Ivanov et al, , 2001. For practical purposes these two definitions are effectively the same, as can be seen in Fig.…”

“…The studies of long-term (interannual or interdecadal) variations of physical parameters of the Black Sea are sparse, they are commonly restricted by analysis of the deep sea only, and usually deal with either sea surface temperature (Oguz, 2005;McQuatters-Gollop et al, 2008) or temperature variations in the Cold Intermediate Water (CIW) of the deep basin (Ivanov et al, 2000;Belokopytov, 1998).…”

“…We use temperature as an indicator for general physical conditions in the BSW. The physical reason for this is that inter-annual variations in the near-bottom temperature are directly related with the variations in the volume of cold waters (Ivanov et al, 2000) which are formed on the shelf and then exported into the deep sea. The other reason is that the variations in the strength of the thermocline above the BSW reflect variations in the vertical fluxes of heat and dissolved matter including oxygen (Vinogradov and Nalbandov, 1990;Yakushev et al, 2005).…”