Quantitative Evaluation of Mesoscale Eddies in the North Atlantic Using Satellite Altimetry and Ocean Reanalyses

Abstract. 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 spatial 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 SSALTO/DUACS (1/4°) and the higher-resolution AVISO 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 for example amplitude, radius, centroid, shape 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 DUACS and SWOT MIOST Science, the POD increases by more than 30 % when moving from GLORYS2V4 to GLORYS12V1, while the FAR also rises by about 20 %, mainly due to the detection of more small and weak eddies. The hit-cost metrics also improve with SWOT, the total hit cost decreases by more than 9 %, and the amplitude error is reduced by about 6 %. Small increases in radius and distance between centroids are still observed, but these are smaller in the SWOT comparison, showing better compatibility with the high-resolution reanalysis. Overall, POD values remain above 50 % and reach around 60 % with SWOT, while FAR stays below 30 %. These results underline the added value of high model resolution for the representation mesoscale eddies in ocean reanalyses and highlight the value of wide-swath altimetry for model verification.