Mean dynamic topography in the Southern Ocean: Evaluating Antarctic Circumpolar Current transport

Mean Dynamic Ocean Topography (MDT) is the difference between the time-averaged sea surface height and the geoid. Combining sea level and geoid measurements, which are both attained primarily by satellite, is complicated by ocean variability and differences in resolved spatial scales. Accurate knowledge of the MDT is particularly difficult in the Southern Ocean as this region is characterized by high temporal variability, relatively short spatial scales, and a lack of in situ gravity observations. In this study, four recent Southern Ocean MDT products are evaluated along with an MDT diagnosed from a Southern Ocean state estimate. MDT products differ in some locations by more than the nominal error bars. Attempts to decrease this discrepancy by accounting for temporal differences in the time period each product represents were unsuccessful, likely due to issues regarding resolved spatial scales. The mean mass transport of the Antarctic Circumpolar Current (ACC) system can be determined by combining the MDT products with climatological ocean density fields. On average, MDT products predict higher ACC transports than inferred from observations. More importantly, the MDT products imply an unrealistic lack of mass conservation that cannot be explained by the a priori uncertainties. MDT estimates can possibly be improved by accounting for an ocean mass balance constraint.