Black Sea Rim Current Intensity Index

'''DEFINITION'''

The Black Sea Rim Current Index (BSRCI) is an ocean monitoring indicator (OMI) that reflects the intensity of the Rim Current – one of the main features of Black Sea circulation and a basin-scale cyclonic current. The index is computed using sea surface current speed averaged over two areas of intense currents based on the Black Sea reanalysis data. These 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 both sections (BSRCI). The formulation used to estimate both indices is provided in the Quality Information Document (QUID) of the corresponding OMI.
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 Product (BLKSEA_MULTIYEAR_PHY_007_004) has been used as a database 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. The Rim Current is quasi-geostrophic in nature, and its dynamics are approximately governed by the Sverdrup balance. Its 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 Layer, water masses 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, and 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 the 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. 

'''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 suggests that the Black Sea Rim current speed varies within ~30% in the period 1993-2024. During the first two decades, a peak is observed in 2005, while higher values become more frequent in recent years, particularly in 2014, 2022, and 2024, when the BSRCI reaches values around 0.15. In contrast, minimum values are recorded in 2004, 2013, 2016, and 2019. The general tendency of the BSRCI time series is positive, e.g. to intensification of the circumpolar current. 

'''DOI (product):''' 
https://doi.org/10.48670/mds-00326