Using Argo to characterize altimetric products: a study of eddy-induced subsurface oxygen anomalies in the Black Sea

Mesoscale eddies are ubiquitous energetic features that alter the biogeochemical regimes of the oceans by blending large-scale gradients, isolating and transporting water masses over large distances, and by locally shallowing or deepening isopycnals [1]. While several studies highlighted mesoscale biogeochemical mechanisms in the open ocean, the difficulties affecting altimetry products in nearshore regions constitute a strong barrier to the observation-based characterization of nearshore biogeochemical eddy dynamics.

Because of their transitional nature, capturing observational snapshots of eddies with satisfactory coverage is challenging. To overcome this difficulty, composite analyses consist of gathering a large number of near-eddy data instances (from observation or model results) and analyzing their variations according to their relative position to nearby eddies. The method thus aims at characterizing average eddy-induced perturbations and provided the basis for many of the recent advances in the field [2].

The BGC-Argo program provides a powerful asset for eddy composite studies, resulting 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.

Here, we evaluate three Black Sea altimetry data sets (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 locations, contours, and properties of eddies are obtained by applying the py-eddy-tracker procedure [3] to three altimetric sets, that differ in terms of along-track and gridding processing, and spatial resolution. For comparison, we consider equivalent CMEMS BS-MFC model products [4]. Oxygen and salinity subsurface anomalies are then obtained from BGC-Argo profiles and relocated in eddy-centric coordinates specifically for each altimetric product.

The most recent altimetric data set, issued from the ESA EO4SIBS project, provides eddy properties that are closer to model simulations, in particular for coastal anticyclones. More importantly, subsurface signatures reconstructed from BGC-Argo are more consistent when EO4SIBS is used to express eddy-centric coordinates.

We propose that the estimated error on the reconstructed mean anomaly may serve to qualify the accuracy of gridded altimetry products and that BGC-Argo data provide a strong asset in that regard.

Besides, we reveal intense subsurface oxygen anomalies whose structure supports the hypothesis that the mesoscale contribution to Black Sea oxygen dynamics extends beyond transport and involves net biogeochemical processes.

 

[1] D. J. McGillicuddy, (2016), Ann. Rev. Mar. Sci., 8, 125–159.

[2] P. Gaube, D. J. McGillicuddy, Jr, D. B. Chelton, M. J. Behrenfeld, P. G. Strutton, (2014), J. Geophys. Res. C: Oceans, 119, 8195–8220.

[3] E. Mason, A. Pascual, J. C. McWilliams, (2014), J. Atmos. Ocean. Technol., 31, 1181–1188.

[4] Ciliberti, S. A., et al. (2021), Journal of Marine Science and Engineering, 9(10), 1146.