On the resolutions of ocean altimetry maps

Abstract. The DUACS system produces sea level global and regional maps that serve oceanographic applications, climate forecasting centers, geophysics and biology communities. These maps are constructed from optimal interpolation of altimeter observations and are provided on a global 1/4° × 1/4° (longitude × latitude) and daily grid resolution framework (1/8° × 1/8° longitude × latitude grid for the regional products) through the Copernicus Marine Environment Monitoring Service (CMEMS). Yet, the dynamical content of these maps is not ensured to have a full 1/4° spatial and 1-day resolution, due to the filtering properties of the optimal interpolation. In the present study, we estimate the effective spatial and temporal resolutions of the newly reprocessed delayed-time DUACS maps (aka, DUACS-DT2018). Our approach is based on the spectral coherence between maps and independent datasets (along-track and tide gauge observations), which represents the correlation between two sea level signals as a function of wavelength. We found that the spatial resolution of the DUACS-DT2018 global maps based on sampling by three altimeters simultaneously ranges from ~ 100 km-wavelength at high latitude to ~ 800 km-wavelength in the Equatorial band and the mean temporal resolution is ~ 28 days period. The mean effective spatial resolution at mid-latitude is estimated to ~ 200 km. The mean effective spatial resolution is ~ 120 km for the regional Mediterranean Sea product and ~ 140 km for the regional Black Sea product. An inter-comparison with former DUACS reprocessing systems (aka, DUACS-DT2010 and DUACS-DT2014) highlights the progress of the system over the past 8 years, in particular a gain of resolution in highly turbulent regions. The same diagnostic applied to maps constructed with two altimeters and maps with three altimeters confirms a modest increase of resolving capabilities and accuracies in the DUACS maps with the number of missions.

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