Black Sea Rim Current Intensity Index

'''DEFINITION'''The Black Sea Rim Current index (BSRCI) reflects the intensity of the Rim current, which is a main feature of the Black Sea circulation, a basin scale cyclonic current. The index was computed using sea surface current speed averaged over two areas of intense currents based on reanalysis data. The areas are confined between the 200 and 1800 m isobaths in the northern section 33-39E (from the Caucasus coast to the Crimea Peninsula), and in the southern section 31.5-35E (from Sakarya region to near Sinop Peninsula). Thus, three indices were defined: one for the northern section (BSRCIn), for the southern section (BSRCIs) and an average for the entire basin (BSRCI).BSRCI=(V ̅_ann-V ̅_cl)/V ̅_cl where V ̅ denotes the representative area average, the “ann” denotes the annual mean for each individual year in the analysis, and “cl” indicates the long-term mean over the whole period 1993-2020. In general, BSRCI is defined as the relative annual anomaly from the long-term mean speed. An index close to zero means close to the average conditions a positive index indicates that the Rim current is more intense than average, or negative - if it is less intense than average. In other words, positive BSRCI would mean higher circumpolar speed, enhanced baroclinicity, enhanced dispersion of pollutants, less degree of exchange between open sea and coastal areas, intensification of the heat redistribution, etc.The BSRCI is introduced in the fifth issue of the Ocean State Report (von Schuckmann et al., 2021). The Black Sea Physics Reanalysis (BLKSEA_REANALYSIS_PHYS_007_004) has been used as a data base to build the index. Details on the products are delivered in the PUM and QUID of this OMI.'''CONTEXT'''The Black Sea circulation is driven by the regional winds and large freshwater river inflow in the north-western part (including the main European rivers Danube, Dnepr and Dnestr). The major cyclonic gyre encompasses the sea, referred to as Rim current. It is quasi-geostrophic and the Sverdrup balance approximately applies to it. The Rim current position and speed experiences significant interannual variability (Stanev and Peneva, 2002), intensifying in winter due to the dominating severe northeastern winds in the region (Stanev et al., 2000). Consequently, this impacts the vertical stratification, Cold Intermediate Water formation, the biological activity distribution and the coastal mesoscale eddies’ propagation along the current and their evolution. The higher circumpolar speed leads to enhanced dispersion of pollutants, less degree of exchange between open sea and coastal areas, enhanced baroclinicity, intensification of the heat redistribution which is important for the winter freezing in the northern zones (Simonov and Altman, 1991). Fach (2015) finds that the anchovy larval dispersal in the Black Sea is strongly controlled at the basin scale by the Rim Current and locally - by mesoscale eddies. Several recent studies of the Black Sea pollution claim that the understanding of the Rim Current behavior and how the mesoscale eddies evolve would help to predict the transport of various pollution such as oil spills (Korotenko, 2018) and floating marine litter (Stanev and Ricker, 2019) including microplastic debris (Miladinova et al., 2020) raising a serious environmental concern today. To summarize, the intensity of the Black Sea Rim Current could give valuable integral measure for a great deal of physical and biogeochemical processes manifestation. Thus our objective is to develop a comprehensive index reflecting the annual mean state of the Black Sea general circulation to be used by policy makers and various end users. '''CMEMS KEY FINDINGS'''The Black Sea Rim Current Index is defined as the relative annual anomaly of the long-term mean speed. The BSRCI value characterizes the annual circulation state: a value close to zero would mean close to average conditions, positive value indicates enhanced circulation, and negative value – weaker circulation than usual. The time-series of the BSRCI suggest that the Black Sea Rim current speed varies within ~30% in the period 1993-2020 with a positive trend of ~0.1 m/s/decade. In the years 2005 and 2014 there is evidently higher mean velocity, and on the opposite end are the years –2004, 2013 and 2016. The time series of the BSRCI gives possibility to check the relationship with the wind vorticity and validate the Sverdrup balance hypothesis. '''Figure caption'''Time series of the Black Sea Rim Current Index (BSRCI) at the north section (BSRCIn), south section (BSRCIs), the average (BSRCI) and its tendency for the period 1993-2020.'''DOI (product):''' https://doi.org/10.48670/mds-00326