Improved Estimates of Southern Ocean Transport by Combining Satellite Altimetry and Temperature/Salinity Profile Data
- (University of South Florida)
Climate models predict significant changes in ocean transport in a warming world, but finding observational evidence of these has been difficult. Ocean transports are most commonly estimated by applying the geostrophic relation to temperature and salinity data collected on hydrographic sections. However, such calculations are usually limited by the need to know a reference velocity at some level. To get around this problem, it is often assumed that the velocity at the deepest measured level is zero, or by adjusting the velocity field to be consistent with water-property distributions. However, in areas with deep-reaching currents, such as the Antarctic Circumpolar Current (ACC), such assumptions are fraught with uncertainty.
We propose to use a method first described in 1980 by Wunsch and Gaposchkin (WG80) to resolve the problem and calculate improved estimates of ocean transport for important regions. WG80 demonstrated that if the surface topography from satellite altimetry and a geoid were both accurately known, then the surface geostrophic currents could be used as a reference to compute currents at any depth by combining the data with density from hydrographic sections. This idea has never been fully tested, however, because marine geoids have been highly inaccurate until recently. Geoids have improved by several orders of magnitude in the last 5 years, due to the recent GRACE and GOCE gravity missions, and the Argo array has reported many more upper ocean temperature-salinity profiles in that time. Thus, we now have sufficient data to evaluate if the method will work.
Our investigation will first test the method in the Southern Ocean over the last decade when Argo temperature/salinity profiles and velocities at the Argo parking depth (~1000 dbar) are available. We will first process mapped altimetry and Argo data across the ACC, but will also examine along-track altimetry at crossover points and individual Argo profiles with a goal to quantify the uncertainty in the technique at different spatial resolutions. Based on our analysis with the altimetry and Argo data in the last decade, we will extend our analysis to the full ocean depth using repeat hydrographic sections occupied since 1993 at select locations crossing the ACC in order to quantify whether there are significant trends in the full-depth ocean transport over the last two decades.