Ocean Science | 2019
On the resolutions of ocean altimetry maps
Abstract
Abstract. The Data Unification and Altimeter Combination System (DUACS) produces sea level global and regional\nmaps that serve oceanographic applications, climate forecasting centers,\nand geophysics and biology communities. These maps are generated using an\noptimal interpolation method applied to altimeter observations. They are\nprovided on a global 1∕4 ∘ × 1∕4 ∘ (longitude\u2009 × \u2009latitude) and daily grid resolution framework\n( 1∕8 ∘ × 1∕8 ∘ longitude\u2009 × \u2009latitude grid for the regional products) through the Copernicus Marine Environment Monitoring Service\xa0(CMEMS). Yet, the dynamical content of these maps does not have full 1∕4 ∘ spatial and 1\u2009d temporal resolutions due to the\nfiltering properties of the optimal interpolation. In the present study, we\nestimate the effective spatial and temporal resolutions of the newly\nreprocessed delayed-time DUACS maps (a.k.a. DUACS-DT2018). Our approach is\nbased on the ratio between the spectral content of the mapping error and the\nspectral content of independent true signals (along-track and tide gauge\nobservations), also known as the noise-to-signal ratio. We found that the\nspatial resolution of the DUACS-DT2018 global maps based on sampling by\nthree altimeters simultaneously ranges from ∼100 \u2009km wavelength\nat high latitude to ∼800 \u2009km wavelength in the equatorial band\nand the mean temporal resolution is ∼34 \u2009d. The mean\neffective spatial resolution at midlatitude is estimated to be ∼200 \u2009km. The mean effective spatial resolution is ∼130 \u2009km for\nthe regional Mediterranean Sea and for the regional Black Sea products. An\nintercomparison with previous DUACS reprocessing systems (a.k.a., DUACS-DT2010\nand DUACS-DT2014) highlights the progress of the system over the past 8\xa0years, 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 in resolving capabilities and accuracies in the DUACS maps with the number of missions.