Antarctic Circumpolar Transport Variability from a combination of precise altimetry and GRACE 'bottom pressure' data.
Jet Propulsion Laboratory,
Pasadena, CA 91109,
Efforts at monitoring the transport of the Antarctic Circumpolar Current (ACC) have intensified in recent years, as the ACC's important role has become more apparent. The ACC has the strongest bottom pressure signal of the world's oceans (together with N. Pacific), which makes it clearly detectable in GRACE data, interpeted as bottom pressure variability. When using precise altimetry, however, surface-intensified eddy variability acts as noise when trying to construct an index of ACC transport. Furthermore, while the barotropic horizontal transport across an arbitrary vertical cross section is proportional to the difference in bottom pressure between the two ends of the section, the southern boundary of the ACC is much better defined than the northern one, and there are physical reasons to believe that the fluctuations in the southern boundary's pressure are more important in determining the pressure gradient. However, the southern data alone are more sensitive to quantities that are still poorly defined by the GRACE data. Here we construct an index using both the GRACE data, which only exists after mid 2002 and whose quality increased at the beginning of 2003, and precise altimetry, which permits us to extend the time series to 1993-2005.