Mesoscale vortices, or eddies, are ubiquitous energetic features whose potential to alter the biogeochemical regimes of the oceans arise, among others, from their capacity to blend large-scale gradients (eddy stirring), to isolate and transport water masses over large distances (eddy trapping) and to locally shallow or deepen isopycnals (eddy pumping) . While many studies have been dedicated to highlighting and deciphering these mesoscale biogeochemical mechanisms in the open ocean, the difficulties affecting altimetry products in the nearshore area, constitute a strong barrier to the observation-based characterization of biogeochemical eddy dynamics near the shelf-slope.
Because of their transitional nature, capturing observational snapshots of eddies with a satisfactory degree of horizontal and vertical coverage is challenging. To overcome this difficulty, the method of composite analysis consists of gathering a large number of near-eddy data instances (from observation or model results) and exploring the variability of their local anomalies according to their relative position to eddies. The method thus aims at characterizing average eddy-induced perturbations and provided the basis for many of the recent advances in eddy biogeochemical studies .
The BGC-Argo program obviously provides a powerful asset for eddy composite studies, which derives from 1) the large availability of data provided under the hood of common technical protocols, 2) the richness of characterized biogeochemical variables, and 3) the continuity of data acquisition which facilitates the characterization of local anomalies.
The first necessary step to any eddy composite analysis lies in the identification and mapping of mesoscale eddies from remote sensing altimetry data products, in order to provide the required information to express in-situ data in eddy-relative coordinates. Typically, this constitutes a critical step in the near-shore domain, where altimetric products are challenged and may finally limit the outcome of downstream composite analysis attempts.
Here, we evaluate different altimetry data sets derived for the Black Sea (2011-2019) and compare their adequacy to characterize eddy-induced subsurface oxygen and salinity signatures by applying a common composite analysis framework exploiting in-situ data acquired by BGC-Argo profilers.
The identification of eddies locations, contours, and properties was obtained by applying the same py-eddy-tracker procedure  to three altimetric sets, that differ in terms of along-track preprocessing, optimal interpolation procedure (gridding), and spatial resolution. To complement the comparison, the same procedure was applied to equivalent model products issued from the CMEMS BS-MFC framework . Oxygen and salinity subsurface anomalies were obtained from BGC-Argo profiles, by applying a temporal high-pass filter to the original time-series, and relocated in eddy-centric coordinates specifically for each altimetric product.
The most recent altimetric data set, prepared with a coastal concern in the frame of the ESA EO4SIBS project, provides statistics of eddy properties that, in comparison with earlier products, are closer to those obtained from model simulations, in particular for coastal anticyclones.
More importantly, the eddies subsurface signatures reconstructed from BGC-Argo are more consistent when the EO4SIBS data set is used to relocate the profiles into the eddy-centric framework, in sense of the spatial structure and statistical significance of the obtained subsurface mean anomaly.
We propose that the estimated error on the reconstructed mean anomaly may serve as an argument to qualify the accuracy of gridded altimetry products and that Argo and BGC-Argo data provide a strong asset in that regard.
Besides, the method allowed us to reveal intense subsurface oxygen anomalies associated with the Black Sea near-shore anticyclones, whose structure supports the hypothesis that the contribution of mesoscale circulation to the Black Sea oxygen cycles extends beyond oxygen transport processes and involves net catalytic effects on biogeochemical processes.
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