Ocean circulation from altimetry: progresses and challenges

The paper provides a review of recent progresses in physical oceanography, such as results on studying ocean circulation and mesoscale ocean dynamics derived from satellite altimetry. Since 1992, satellite altimetry has provided an unprecedented 16 years monitoring of sea level and ocean circulation variations. Continuous measurements from satellites like TOPEX/Poseidon and Jason-1 help us understand and foresee the effects of the changes in ocean circulation on climate and on extreme climate events such as El Niño and La Niña. Altimeter measurements have improved our understanding of both the dynamics and thermodynamics of western boundary currents by providing a synoptic view of the current systems and their interannual variations, and allowed scientists to quantify eddy-induced salt and heat transport, and seasonal and interannual variations in eddy kinetic energy. Altimeter measurements have also been used to map eddies, quantify their amplitudes and diameters, track their trajectories, and examine their eddy dynamics and roles in the ocean processes and climate variability. Recent advances in satellite altimetry, in synergy with other remote sensing techniques, constrain the uncertainty of mechanic energy driving meridional overturning circulation which regulates climate change. Moreover, altimetry has discovered a surprising sea level anomaly propagation speed which challenges the existing linear Rossby wave theory, and revealed the presence of elusive zonal fronts and jets in the ocean. However, challenges still exist in monitoring the ocean variability from satellite altimetry. Satellite altimeters are not able to measure the time-mean geostrophic currents due to the large uncertainty in geoid. This poses challenges for deriving the absolute geostrophyic flow in regions where bottom velocities are non-zero since hydrographic estimates of absolute dynamic topography are unable to capture the effects of the bottom. The uncertainties of satellite altimetry