Hybrid Decade-mean Global Sea Level

Improved global mean sea level is obtained by combining two datasets: large-scale mean sea level based on measurements of twin-satellite mission GRACE and mesoscale sea level tilt derived from the momentum balance as seen in drifter, satellite altimeter and wind data. Hybrid product reveals complex structures of main currents even after averaging over 10 years and shows their places in the large-scale near-surface circulation. INTRODUCTION Shape of the sea level determines geostrophic currents in the upper ocean, which play important role in the global circulation. Before the beginning of satellite altimetry, direct observations of the sea level on large scales were not available. Indirect techniques (such as the dynamical method) were based on assumptions (such as the assumption of no-motion level), which are not acceptable any more by many modern studies. Even after accuracy of satellite altimeter reached a few centimeters, extraction of the signal related to ocean dynamic is still a challenge. Indeed, absolute sea level deviates from the Earth’s ellipsoid by up to 100 meters due to gravitational anomalies and only by up to 2 meters due to the ocean currents. Decades of satellite and in situ gravity measurements produced multiple models of the Earth's equipotential surface (geoid), but even most advanced of them, such as EGM96 (Lemoine et al., 1998) contained large-scale errors reaching 1 meter in some areas. Recently released GRACE (Gravity Recovery and Climate Experiment Mission) Gravity Model 01 (GGM01, http://www.csr.utexas.edu/grace) corrected that error (Showstack, 2002) and allowed more accurate estimate of the large-scale geostrophic circulation (Tapley et al., 2003). Based on this model, a number of improved models of the mean sea level were released by different institutions, varying in the used ensembles of satellites and in the employed techniques. General for all these models is their coarse spatial resolution not exceeding a few hundred kilometers. This limitation is clearly set by the design of the GRACE, whose satellites are orbiting the Earth at altitude of 485 km. Independently, Niiler et al. (2003) developed the technique for computation of the mean sea level based on joint analysis of drifter, satellite altimeter and wind data. Their dynamically balanced