CoAuthors

Lionel Zawadzki (CLS, France); Hélène Roinard (CLS, France); Anny Cazenave (CLS, France); Nicolas Picot (CLS, France)

The reference Mean Sea Level (MSL) record strongly relies on four missions: TOPEX/Poseidon and its successors Jason-1, Jason-2 and Jason-3 on the same historical orbit. The global trend uncertainty has been estimated close to 0.5 mm/yr (Ablain et al., 2017) over the whole altimetry period (1993-2015). However, this uncertainty rises (0.8 mm/yr) only considering the TOPEX period (1993-2002) (Ablain et al., 2013). Furthermore, considering only TOPEX-A period (Jan. 1993- Feb. 1999), a significant drift between 1.5 and 2 mm/yr has been highlighted comparing altimetry data with a global tide gauges network. (Watson, et al., 2015; Prandi et al., 2015). Furthermore, a recent MSL closure budget study (Dieng et al., 2017) has highlighted a strong discrepancy between the TOPEX-A global MSL and the steric+mass components. This strong drift might be explained by instrumental anomalies on TOPEX-A data as shown by (Beycley, et al, 2016) during the last OSTST in La Rochelle. It is also worth noting that the correction of this drift would allow to observe an acceleration of the global MSL over all the altimeter period for the first time (Nerem et al, 2016; Dieng et al., 2017).

In order to better understand TOPEX errors, JPL and CNES have been working together for several years in order to provide a reprocessed TOPEX altimeter dataset for users (update of level 2 M-GDR products). In 2017, a new version of this reprocessed dataset will be available with retracked TOPEX data (already provided by JPL in 2016) and including the best geophysical corrections available during this period (provided by CNES in 2017). In this study, supported by CNES, we propose to analyze the impact of this new TOPEX dataset on the global MSL thanks to different approaches which are all independent. After analyzing directly the impact on the global MSL, we will compare the TOPEX MSL time series: (1) with tide gauges with similar method developed by (Valladeau et al, 2012, Prandi et al., 2016); (2) with steric and mass components as in the recent study performed by (Dieng et al., 2017) ; (3) and by comparisons with Poseidon-1 data (Zawadzki et al., 2016). The main interests of all this independent approaches are on the one hand to cross-check the impact of reprocessed TOPEX data on the global MSL, and on the other hand to evaluate the uncertainties of such approaches to know the confidence in the results obtained.


References:

- Ablain et al., 2013: Why altimetry errors at climate scales are larger in the first decade [1993-2002]? OSTST 2013, Boulders, poster session.
- Ablain et al., 2017: Ablain, M., J. F. Legeais, P. Prandi, M. Marcos, L. Fenoglio-Marc, H. B. Dieng, J. Benveniste, and A. Cazenave (2017), Altimetry-based sea level at global and regional scales, Surv. Geophys., 38, 7–31, doi:10.1007/s10712-016- 9389-8.
- Beckley et al., 2016 : On the ‘cal mode’ correction to TOPEX altimetry and its effect on the global mean sea-level time series, OSTST 2016, La Rochelle
-Dieng, H. B., A. Cazenave, B. Meyssignac, and M. Ablain (2017), New estimate of the current rate of sea level rise from a sea level budget approach, Geophys. Res. Lett., 44, doi:10.1002/2017GL073308
-Nerem et al., 2016: Has the Rate of Sea Level Rise Accelerated During the Altimeter Era? , OSTST 2016, La Rochelle
-Prandi et al., 2015: Accuracy of Global Comparisons Between Altimetry and Tide Gauges. OSTST 2015, Reston
-Valladeau et al., 2012: G. Valladeau, J. F. Legeais, M. Ablain, S. Guinehut, N. Picot , 2012, Comparing Altimetry with Tide Gauges and Argo Profiling Floats for Data Quality Assessment and Mean Sea Level Studies, Marine Geodesy, Vol. 35, Iss. sup1, 2012
-Watson et al., 2015: Unabated global mean sea-level rise over the satellite altimeter era, Nature Climate Change 5, 565–568, doi:10.1038/nclimate2635