Are DUACS products corrected for the TOPEX-A instrumental drift?

Between 1993 and 1998, the global mean sea level (MSL) has been known to be affected by an instrumental drift in the TOPEX-A measurements which has been quantified by several studies (Watson et al., 2015, Dieng et al. 2017; Beckley et al., 2017). This is the most significant error that affects the first 6 years (January 1993 to February 1999) of the Topex/Poseidon GMSL measurements.

The correction of this drift leads to a reduced rate of global mean sea level rise from 3.4 to 3.1 ± 0.4 mm/yr (ata 90% confidence level) over the total altimetry era (1993–present). Correcting for the drift also modifies the global mean sea level time-series over the past 25 years from linear to quadratic, with an acceleration of the associated rise estimated as 0.12 ± 0.073 mm/yr2 (90% confidence interval) (WCRP Sea Level Budget Group, 2018 and Legeais et al., 2020).

A correction based on the mean of the different corrections available (tide gauges, sea level budget closure, see WCRP,2018; and Legeais et al., 2020) has been added to the DT2021 sea level products as a separate correction. The WCRP sea level budget group (2018) indicates that whatever the approach used to determine the drift’s estimate,the global mean sea level trend and acceleration are similar (within the uncertainty interval) and in agreement with climate models. Users are warned that the proposed value is a global correction to be applied a posteriori on the globally averaged MSL (and not before computing the average) derived from the gridded sea level maps (following the method presented in It is acknowledged that the correction can be applied to regional or local MSL derived from the maps since it may be better than without any correction. However, the obtained MSL will not be correct since the TOPEX-A instrumental drift is expected to vary regionally (due to dependence to the temperature), although these regional variations are unknown. A convention has been initially applied so that the global mean sea level is set to zero during the year 1993. However, the proposed correction of the TOPEX-A instrumental drift has been chosen so that the correction is null after the end of the lifetime of the TOPEX mission in 1999. With this approach, the corrected GMSL does not equal zero in 1993, but this approach is preferred to ensure the continuity of the initial and corrected GMSL after 1999 (e.g.for ocean modelers).

The level2 measurements from the TOPEX mission are currently being reprocessed by CNES/JPL. Once available these input data will be used in the future version of the DUACS altimeter sea level product distributed by Copernicus services. The assessment of the impact of these reprocessed measurements on the TOPEX-A instrumental drift is planned.