Southern Ocean mass variation studies using GRACE and satellite altimetry

The Southern Ocean is a major link between the world oceans via complicated processes associated with the melting and accumulation of the vast Antarctic ice sheets and the surrounding sea ice. The Southern Ocean sea level is poorly observed except from recent near-polar orbiting space geodetic satellites. In this study, the Southern Ocean mass variations at the seasonal scale are compared using three independent data sets: (1) the Gravity Recovery And Climate Recovery Experiment (GRACE) observed ocean bottom pressure (OBP), (2) steric-corrected satellite altimetry (ENVISAT) and, (3) the Estimating the Circulation and Climate of the Ocean (ECCO) model OBP data. The height difference between sea level derived from altimetry and steric sea level contains the vertical displacement of the Earth surface due to elastic loading. Here we provide a formulation of this loading term which has not been considered previously in other studies and demonstrate that it is not negligible, especially for regional studies. In this study, we first conduct a global comparison using steric-corrected JASON-1 altimetry with GRACE to validate our technique and to compare with recent studies. The global ocean mass variation comparison shows excellent agreement with high correlation (similar to 0.81) and with discrepancies at 3-5 mm RMS. However, the discrepancies in the Southern Ocean are much larger at 12-17 mm RMS. The mis-modeling of geocenter variations and the second degree zonal harmonic (J(2)) degrade the accuracy of GRACE-derived mass variations, and the choice of ocean temperature data sets and neglecting the loading correction on altimetry affect the OBP comparisons between GRACE and altimetry. This study indicates that the satellite observations (GRACE and ENVISAT) are capable of providing an improved constraint of oceanic mass variations in the Southern Ocean.