Detection of Long-Term Instabilities on Altimeter Backscatter Coefficient Thanks to Wind Speed Data Comparisons from Altimeters and Models

Abstract

As long-term stability of altimeter backscatter coefficient (σ ₀ ) can impact the accuracy of the Global Mean Sea Level (GMSL) trend through the Sea State Bias (SSB) correction, it is important to accurately monitor its evolution in order to detect potential instabilities. This study aims to characterize long-term uncertainties from several altimeter missions. The originality of our approach is to perform comparisons between wind speed estimations deduced from σ ₀ measurements and from ERA-interim reanalysis. Thanks to the good temporal correlation between the Global Wind Speed (GWS) evolution issued from altimeters and the atmospheric reanalysis, the detection of small drifts is possible in the altimeter time series. In particular, drifts have been detected on Jason-1 GWS between mid-2004 and 2005 and on Envisat prior to 2004. They are associated to σ ₀ jumps of approximately 0.03 dB. Larger anomalies have been detected on TOPEX σ ₀ data with variations reaching 0.2 dB. These low altimeter wind speed variations originate from small σ ₀ changes are within the instrumental stability requirements. However, via the SSB calculation, their impact on the GMSL evolution is estimated close to 0.1 mm/yr over the altimetry 20-year period. In addition, they strongly affect inter-annual GMSL variations with associated errors reaching 2 mm.

Keywords:

• Altimetry
• backscatter coefficient stability
• ocean wind speed
• global mean sea-level

Acknowledgements

This study has been performed at CLS in the frame of the SALP project supported by the CNES. It has been possible thanks to the availability of the ERA-Interim dataset from the ECMWF data server. Finally, Raymond Zaharia and José Gonnela from the “Club des Argonautes” have actively participated to this study through fruitful discussion.