||Antarctic Circumpolar Current Signals in GRACE: sensitivity to lowest degree coefficients.
The Antarctic Circumpolar Current (ACC) has the strongest bottom pressure signal, and thus is clearly detectable in GRACE data. 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. In previous work, this author did so by differencing GRACE data inside rectangular boxes. Here it is done by differencing the verage pressure along paths to the South and North of the ACC. However 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: the degree 1 and 2 terms, while the Northern minus southern difference is less sensitive to those uncertainties. Here this sensitivity is used to advantage in order to ascertain the quality of various GRACE solutions, and various non-GRACE estimates of the lowest degree and order (for example, degree 2 data from SLR) when used together with GRACE data for higher degrees. . 'Truth' here is a combination of output from a numerical ocean model (ECCO) and zonal wind data, the main driver of such variability.