Michaël Ablain (CLS, France); Pierre Thibaut (CLS, France); Pierre Prandi (CLS, France)

The reference Mean Sea Level (MSL) record strongly relies on four missions: TOPEX/Poseidon and its successors Jason-1, Jason-2 and soon Jason-3 on the same historical orbit. The global trend uncertainty has been estimated close to 0.5 mm/yr (Ablain et al., 2015) over the whole altimetry period (1993-2013). 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). This strong drift might be explained by instrumental anomalies on TOPEX-A. However, the uncertainty of such a method (tide gauge versus altimetry) is significant due to -for instance- the scarce spatial coverage, gaps, or drift in tide gauges records. Over the 22 years of the altimetry time period, this uncertainty has been estimated close to 0.7 mm/yr (Prandi et al., 2015). However, over the shorter 6-year period of TOPEX, with a scarcer tide gauges network, , we think this uncertainty is larger (likely higher than 1 mm/yr), and maybe close to the estimated drift.

Therefore, in our study, we suggest another a priori more accurate approach to estimate the TOPEX-A GMSL drift: we propose to estimate this drift using the measurements from Poseidon-1. This altimeter has indeed a very similar design to Poséidon-2, Poséidon-3, respectively onboard Jason-1 and Jason-2, which have proven their reliability. Moreover, no altimeter anomalies have been highlighted on this altimeter. The study first demonstrates that even though Poseidon-1 recorded 1 out of 10 cycles, it is theoretically significant to estimate TOPEX-A drift. Then, TOPEX-A drift w.r.t Poseidon-1 is estimated after applying the same geophysical corrections on both datasets, and compared to the drift obtained with tide gauges.

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., 2015: Improved sea level record over the satellite altimetry era (1993–2010) from the Climate Change Initiative project, Ocean Science, 11 (1). 67-82. doi: 10.5194/os-11-67-2015.
Prandi et al., 2015: Accuracy of Global Comparisons Between Altimetry and Tide Gauges. OSTST 2015, Reston.
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.