Tests of Geoid Height Skill Through Estimates of the Ocean Circulation
Detlef Stammer(1) and Armin Koehl(1)
University of Hamburg,
New geoid height estimates are now available from the Gravity Recovery and Climate Experiment (GRACE) spacecraft. Such estimates are central to ongoing efforts to calculate the ocean circulation and its time variability, and it thus becomes important to understand the degree of skill in geoid knowledge. Here one of the GRACE estimates (GGM01s) is compared to that from a previous geoid estimate (EGM96) by examining the resulting estimates of the ocean circulation available from the ECCO and GECCO efforts now since several years and the residual misfit of the estimated sea surface height (SSH). Circulation estimates were obtained over the period 1992 - 2002 by combining most of the available ocean data sets with a global general circulation model on a 1 degree horizontal grid. When combined with altimetric data, as compared to EGM96, the new GRACE geoid produces fields that are more consistent with temperature and salinity climatologies, and thereby requires smaller adjustments to the initial model conditions. Because in both cases, the misfit of the the mean dynamic topography is small compared to other model-data misfits, differences in the time mean seasurface heights (SSH) from the two geoids are relatively modest. Therefore, an experiment was done in which the relative geoid error was artificially reduced, thus forcing the model circulation to be much closer to the absolute altimetric surface determined by GRACE. Adjustments occur in all aspects of the ocean circulation, including changes in the meridional overturning circulation and the corresponding meridional heat transport in the Atlantic of about 10% of their mean values. Overall this model state shows an improved skill in simulating in situ temperature and salinity profiles. Improved estimates of the dynamic sea surface height (altimetric surfaces - geoid) would be useful for quantitative improvements in estimates of the circulation and its climate impacts. As a by-product of this work, it appears that at the present time, the errors in the geoid height are comparable to uncorrected errors in the altimetric data sets.