Quantitative evaluation of mesoscale eddies in the North Atlantic using satellite altimetry and ocean reanalyses

Ocean mesoscale eddies (10–250 km) play a key role in transporting heat, momentum, and nutrients. Accurately evaluating their representation in ocean reanalysis becomes important given that ocean reanalysis is widely used in ocean and climate research due to their temporal and spatially consistent coverage. This study aims to assess two global ocean reanalysis products GLORYS12V1 (1/12° resolution) and GLORYS2V4 (1/4° resolution) available from the Copernicus Marine Service against satellite altimetry observations. We have used both AVISO DUACS (1/8°) and SWOT MIOST Science product (1/8°), as reference datasets that allow us to understand better the advantage of high-resolution altimetry data and have a fair evaluation for eddy-resolving ocean reanalysis (1/12°). The evaluation approach is based on a feature-based eddy-verification method to compare eddy properties, such as amplitude, radius, and centroid position, using a cost-function metric. Probability of Detection (POD) and False Alarm Ratio (FAR) are then used to quantify the reanalysis skill. GLORYS12V1 demonstrates better agreement with observations than GLORYS2V4, especially for eddies with amplitudes > 10 cm. For both AVISO DUACS 1/8° and SWOT MIOST Science, the POD increases by more than 30 % when moving from GLORYS2V4 to GLORYS12V1, while the FAR increases by approximately 22 %–26 %, mainly due to the detection of more small eddies. For matched eddies, GLORYS12V1 shows a lower matching cost than GLORYS2V4 for both satellite products, with reductions of approximately 26 %. The mean errors in amplitude, radius, and centroid distance are reduced by approximately 35 %, 32 %, and 13 %, respectively. GLORYS12V1 achieves POD values of about 62 % with both references, compared with about 46 % for GLORYS2V4. FAR ranges from approximately 18 % for GLORYS2V4 to 24 % for GLORYS12V1. The results underline the added value of high model resolution for the representation of mesoscale eddies in ocean reanalyses and highlight the value of wide-swath altimetry for model verification in regions such as the North Atlantic.