On the resolutions of ocean altimetry maps

Abstract

Abstract. The Data Unification and Altimeter Combination System (DUACS) produces sea level global and regional maps that serve oceanographic applications, climate forecasting centers, and geophysics and biology communities. These maps are generated using an optimal interpolation method applied to altimeter observations. They 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 does not have full 1∕4∘ spatial and 1 d temporal resolutions 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 (a.k.a. DUACS-DT2018). Our approach is based on the ratio between the spectral content of the mapping error and the spectral content of independent true signals (along-track and tide gauge observations), also known as the noise-to-signal ratio. 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 ∼34 d. The mean effective spatial resolution at midlatitude is estimated to be ∼200 km. The mean effective spatial resolution is ∼130 km for the regional Mediterranean Sea and for the regional Black Sea products. An intercomparison with previous DUACS reprocessing systems (a.k.a., 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 in resolving capabilities and accuracies in the DUACS maps with the number of missions.

Eddies in the Tropical Atlantic Ocean and Their Seasonal Variability

“…EKE shows almost the same evolution with very weak maximum values lower than 100 cm 2 /s 2 in the two off‐equatorial regions and ~500 cm 2 /s 2 in the equatorial region (Figure c). The relatively weak eddy amplitudes (<1 cm) and radii (<40–50 km) observed at the beginning and end of the eddy life cycles (Figures b and e) are supposedly below the altimeters' capability of resolving such small‐scale structure (e.g., Ballarotta et al, ; Chelton et al, ). However, our results are similar to Chen and Han () and likely robust, since eddy characteristics are associated with a relatively high signal‐to‐noise ratio during their mature phase, and the observed dome‐shape of their temporal evolution (Figure ) is effectively part of the eddy life cycles (e.g., Pegliasco et al, ; Samelson et al, ).…”

Can We Resolve the Basin‐Scale Sea Level Trend Budget From GRACE Ocean Mass?

“…The sum of GRACE, Argo, and deep steric SSH trend has similar power to altimetry observed SSHA trend between wavelengths of approximately 3,000 and 10,000 km, at the hemispheric scale (Figure ; see Figure S2 for all data sets). GRACE has a native resolution of 300 km (latitude dependant; Vishwakarma et al, ) that is subsequently processed and filtered resulting in a longer wavelength effective resolution, whereas satellite altimetry has a minimum native resolution of 7 km along track, which once filtered and processed to the multimission product has an effective resolution of between 100 and 800 km (latitude dependant; Ballarotta et al, ). The spatial coherency of the Argo SSHA products is also latitude dependant.…”

Long‐Term Monitoring of the Brazil Current Transport at 22°S From XBT and Altimetry Data: Seasonal, Interannual, and Extreme Variability

“…The SYNTH time series shows significant variability from mesoscale to decadal timescales (Figure b), since it is reconstructed from 25 years of daily altimetry data. Note, however, that the mean effective spatial and temporal resolutions of the DUACS product are estimated to be ~200 km and ~28 days period, respectively (Ballarotta et al, ). The most prominent timescales are the annual and semiannual cycles (e.g., Figure ), in addition to large amplitude eddy variability below 100 days.…”