Black Sea Overturning Circulation Index from Reanalysis

'''DEFINITION'''

This ocean monitoring indicator (OMI) provides a time series of Meridional Overturning Circulation (MOC) Strength in density coordinates, area-averaged and calculated for the period from 1993 to the most recent year with the availability of reanalysis data in the Black Sea (BS). It contains 1D (time dimension) maximum MOC data computed from the Black Sea Reanalysis (BLK-REA; BLKSEA_MULTIYEAR_PHY_007_004) (Ilicak et al., 2022). The MOC is calculated by summing the meridional transport provided by the Copernicus Marine BLK-REA within density bins. The Black Sea MOC indicator represents the maximum MOC value across the basin for a density range between 22.45 and 23.85 kg/m³, which corresponds approximately to a depth interval of 25 to 80 m. To understand the overturning circulation of the Black Sea, we compute the residual meridional overturning circulation in density space. Residual overturning as a function of latitude (y) and density (σ ̅) bins can be computed as follows: ψ^* (y,σ ̅ )=-1/T ∫(t_0)^(t_1)▒∫(x_B1)^(x_B2)▒〖∫_(-H)^0▒H[σ ̅-σ(x,y,z,t)] ×ν(x,y,z,t)dzdxdt,〗 where H is the Heaviside function and ν is the meridional velocity. We used 100 σ_2 (potential density anomaly with reference pressure of 2000 dbar) density bins to remap the mass flux fields.

'''CONTEXT'''

The BS meridional overturning circulation (BS-MOC) is a clockwise circulation in the northern part up to 150 m connected to cold intermediate layer (CIL) and an anticlockwise circulation in the southern part that could be connected to the influence of the Mediterranean Water inflow into the BS. In contrast to counterparts observed in the deep Atlantic and Mediterranean overturning circulations, the BS-MOC is characterized by shallowness and relatively low strength. However, its significance lies in its capacity to monitor the dynamics and evolution of the CIL which is crucial for the ventilation of the subsurface BS waters. The monitoring of the BS-MOC evolution from the BLK-REA can support the understanding how the CIL formation is affected due to climate change. The study of Black Sea MOC is relatively new. For more details, see Ilicak et al., (2022).

'''KEY FINDINGS'''

The MOC values show a significant decline from 1994 to 2009, corresponding to the reduction in the CIL during that period. However, after 2010, the MOC in the Black Sea increased from 0.07 Sv (1 Sv = 106 m3/s) to 0.10 Sv. The CIL has nearly disappeared in recent years, as discussed by Stanev et al. (2019) and Lima et al. (2021) based on observational data and reanalysis results. The opposite pattern observed since 2010 suggests that mechanisms other than the CIL may be influencing the Black Sea MOC. For the OMI we have used an updated version of the reanalysis (version E4R1) which has a different spinup compared to the OSR6 (version E3R1).

'''DOI (product):''' https://doi.org/10.48670/mds-00349