What sea-level drifts can be detected at global and regional scales by comparing recent altimetry missions together: S3A, Jason-3 and Saral-Altika?

Rémi Jugier (Magellium, France)

CoAuthors

Robin Fraudeau (Magellium, France); Michaël Ablain (Magellium, France); Matthias Raynal (CLS, France); Adrien Guerou (CLS, France); Pierre Femenias (ESA/ESRIN, Italy)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Quantifying Errors and Uncertainties in Altimetry data

Presentation type: Type Forum

The correct assessment of the GMSL trend is of paramount importance to climate studies. Therefore, climate missions must be carefully monitored, using comparison to independent sources, to ensure that the altimeter instrument ageing has the lowest possible impact on the GMSL trend.
Comparison to tide gauges are often used to evaluate the stability of the GMSL indicator (see Valladeau et al. 2012, Watson et al. 2015). However, it suffers from poor accuracy for short periods of time : Ablain et al. 2018 estimate, through an error budget approach, that drifts under 0.6mm/yr (68% C.L.) can only be detected for a period of over 7 years for the recent altimetry era, and over 10 years for the Topex era. While Ablain et al. 2017 showed that this is the most accurate method to detect the Topex-A drift, the more recent altimetry period is seeing a higher number of missions operating over the same periods and this allows for direct GMSL comparison between missions.
The uncertainties of the direct comparison method have been already estimated (Ablain et al. 2019, b) and applied to the detection of the Sentinel-3A drift versus Jason-3, SARAL/Altika and Jason-2. This approach is more accurate than comparison to tide gauges : 0.5 mm/yr (68% C.L.) over a 3 years period. Its main drawbacks, however, are that the drifts can only be evaluated on 2 missions’ common periods, and on the other hand that the drifts are estimated in a relative way. It is therefore necessary to have at least 3 missions over concomitant periods in order to know which mission is drifting.
The current study first presents an update of the study presented by (Ablain et al.( 2019, b) at last OSTST (Chicago, 2020) with 1 year of additional data for S3A, Jason-3 and SARAL-Altika. The now well-known GMSL S3-A drift, detected last year, is estimated as well as its associated uncertainty over a longer period of 1 year, and still compared to JAson-3 and SARAL-Altika. In this study we also discuss some limitations of the proposed method in order to evaluate the uncertainties over very short periods (< 2-3 years), due to the precise elaboration of the error budget of the method for high frequencies.
In addition, we analysed the uncertainties of the direct method at regional scales. From experience, we know that regional drifts in altimetry are very difficult to detect by such a direct approach because of the ocean variability that is not observed in the same way by the different altimetry missions. Such an approach is especially effective during the tandem phase when altimetry missions observe the same sea surface height (for example during the tandem phase between the Jason and TOPEX missions, also known as the calibration phase). However, our intention in this study is to quantify precisely the level of uncertainty reached as a function of the period duration and also of the size of the regional area analysed by the direct method. The results obtained confirm what we thought and show that it is impossible to detect drifts of less than 1.8 mm/year (68% C.L.) over a period of 5 years or 0.9/year (68% C.L.) over a period of 10 years, for regions of box size 18°x18°.
As there is no other precise approach for estimating regional sea level drifts, we want to highlight that it will be impossible to verify the stability requirements expressed by C3S at regional scales (<0.5 mm/yr over 10 years) if new methods for calibrating altimeter missions are not implemented in the future.

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Rémi Jugier
Magellium
France
remi.jugier@magellium.fr