Simulation of oceanic bottom pressure for gravity space missions

Oceanic bottom pressure is affected by mass redistribution in the ocean and atmosphere, and it influences gravity field determinations by the new sate llite missions CHAMP, GRACE and GOCE from seasonal up to short-period times cales. Thus, mass redistribution in the ocean needs to be accounted for to obtain the mean gravity field. With the Hamburg ocean model for circulation and tides (OMCT), time-depende nt oceanic bottom pressure fields are calculated using atmospheric fluxes o f momentum, heat and freshwater from ECHAM3 real-time simulations. The resu lting bottom pressure fields are expanded in gravity field spherical harmon ic coefficients as a function of time. The temporal resolution is 5 days fo r extracting annual and semi-annual amplitudes and 6 hr for studying high-f requency variations. In order to estimate the influence of oceanic mass variations on gravity fi eld determination, degree variance spectra of simulated bottom pressure are calculated and compared with the expected error spectra of space missions. Furthermore, variations in geoid height AN from the modelled Stokes coeffi cients are illustrated. The numerical results suggest that ocean-induced lo ng-wavelength gravity variations become detectable with the CHAMP and GRACE gravity missions.