The Black Sea Mixed Layer Depth Variability and Its Relation to the Basin Dynamics and Atmospheric Forcing

Abstract

Spatio-temporal variability of the mixed layer depth (MLD) in different areas of the Black Sea in 1985-2017, its relationship with basin dynamics and atmospheric forcing are studied. Methods and Results. The study is based on the hydrological data archive for 1985-2017 including the measurements of the ship expeditions, the Argo buoys and the moored buoy "Aqalog". Seasonal and interannual variability of the mixed layer depth was determined using the density criterion (dr = = 0.07kg/m 3) between the surface layers and the base of the upper mixed layer. Conclusions. In January-March, the large-scale and mesoscale dynamics significantly affects the mixed layer depth variability. Minimum monthly average values of the mixed layer depth in winter are observed in the mesoscale cyclonic eddies and in the center of the sea (20-30 m), the moderate valueson the periphery of the basin (40-45 m) and the maximum onesin the mesoscale anticyclones (60-70 m). Several times the mixed layer depth values exceeding 150 m were detected in the downwelling areas of the basin. Analysis of the whole period (1985-2017) shows that the mixed layer density was never more than 1015 kg/m 3. This isopycnal limits the maximum possible depth of the upper mixed layer. The impact of wind velocity on the spatial and temporal variability of the mixed layer thickness is the largest in spring and autumn when the seasonal thermocline is weak. It is less important in summer when solar heating stabilizes the upper layer, and in winter when the mixed layer depth is large. Rise of the mixed layer depth in summer is observed in recent years that is associated with rise of the wind speed in a warm period of a year.

Untitled

“…The most significant waves and high wind speeds were recorded in 2001, characterized by anomalous phenomena in the marine ecosystem. In particular, the intensification of storms in 2001 caused a significant increase in the depth of the upper quasi-uniform layer of the sea [31] and the appearance of anomalous phytoplankton blossom in the summer [32]. Significant interannual variability can cause differences in the reported trend estimates based on the literature data for different periods.…”

Warmer winters causes an increase of chlorophyll-a concentration in deeper layers: the opposite role of convection and self-shading on the example of the Black Sea

“…MLD was estimated using a potential density threshold of 0.007 kg/m 3 . This criterion was justified by Kubryakov, Belokopytov, et al (2019) on the base of the composite analysis of the long-term data for the Black Sea in 1985-…”

“…S1). At the same time, the winter mixed layer in the Black Sea is only about 40-45 m (Titov, 2004;Kubryakov, Belokopytov, et al, 2019) (Fig. 3a).…”

“…Usually, the intensity of the large-scale circulation in the basin is observed in the cold years (Blatov et al, 1984;, as both of them are driven by the same atmospheric patternsintensification of northeast winds from Eurasia . The rise of cyclonic circulation and uplift of the pycno-halocline on the opposite decreases MLD (Kubryakov, Belokopytov, et al, 2019). That is why, in the Black Sea the MLD is not correlated with sea surface temperature (Titov, 2004), but strongly depends on dynamic forcing, such as eddies, large-scale circulation.…”

“…A strong vertical gradient of salinity suppresses the vertical mixing and the mixed layer depth (MLD) in the Black Sea in winter on average does not exceed 40-50 m. The spatial and temporal variability in winter MLD is strongly related to the vertical displacement of the pycno-halocline (Kubryakov, Belokopytov, et al, 2019). Due to the strong haline stratification, the position of chemical layers in the Black Sea is tightly coupled to certain isopycnals and the variations of their concentration in density coordinates are significantly less than in z-coordinates (Konovalov et al, 2005;Tuğrul et al, 2014).…”