Estimation of steric sea level variations from combined GRACE and Topex/Poseidon data.

We estimate the mean steric sea level variations over the 60S-60N oceanic domain for the recent period (from August 2002 to February 2006), by combining sea level data from Topex/Poseidon (T/P) altimetry with time- variable gravity data from GRACE. The observed global mean sea level change from satellite altimetry results in total from steric plus ocean mass change. As GRACE measurements averaged over the ocean represents the ocean mass change component only, their differences provides an estimate of mean steric sea level. Two different sets of GRACE geoid solutions (the GRGS EIGEN-GL04 and the GFZ EIGENGRACE04S products) have been used. Each GRACE data set ranges over approximately three years but shifted in time (August 2002 -September 2005 for the GRGS geoids and February 2003 - February 2006 for the GFZ geoids). We first focus on the seasonal variations. The two GRACE data sets agree well in terms of mean annual mass variation, both in amplitude and phase. The T/P sea level minus the GRACE ocean mass signal compares well with the steric annual sea level estimated from the World Ocean 2004 climatology and the Ishii et al (2006) ocean temperature data. We also examine the interannual fluctuations of the T/P-GRACE steric sea level. The inferred steric sea level curves exhibit a decreasing trend during the two years 2004-2005, as do the in situ hydrographic data, even if there is still some differences between both estimates.