COMBINING T P ALTIMETRIC DATA WITH HYDROGRAPHIC DATA TO ESTIMATE THE MEAN DYNAMIC TOPOGRAPHY OF THE NORTH-ATLANTIC AND IMPROVE THE GEOID/

The mean dynamic topography of the surface of the North Atlantic is es timated using an inverse model of the ocean circulation constrained by hydrographic and altimetric observations. In the North Atlantic, alti metric observations have no significant impact on the topography estim ate because of the limited precision of available geoid height models. They have a significant impact, however, when uncertainties in the de nsity field are increased to simulate interpolation errors in regions where hydrographic data are scarce. This result, which moderates the c onclusion drawn by Ganachaud and co-workers of no significant contribu tion of altimetric observations to the determination of the large-scal e steady circulation, reflects the simple idea that altimetric data ar e most useful near the surface of the ocean and in areas where the hyd rography is poorly determined. One application of the present inverse estimate of the mean dynamic topography is to compute a geoid height c orrection over the North Atlantic which reduces the uncertainty in the geoid height expanded to spherical harmonic 40 down to a level of abo ut 5 cm.