A synoptic analysis of the dynamic topography of the sea surface in a (531 km)2 domain including the sites of the Polymode Synoptic Dynamics Experiment is presented. This study is based on all available GEOS 3 and Seasat altimeter passes between May and August 1978 referenced to an altimetric mean sea surface. The altimeter synoptic time series are compared to in situ estimates calculated from Polymode expendable bathythermographies, conductivity-temperature-depth probes, deep floats, and current meters. Wherever the altimeter coverage is good, both analyses show remarkable agreement. The following results are obtained: (1) the altimeter/in situ correlation in 0.56 on average and never falls below 0.38, (2) the beginning of operations of Seasat in the first days of July 1978 has a dramatic effect on all indicators, (3) the variance for the altimeter estimate is no more than 9% higher on average than the variance of the in situ estimate, and (4) the daily mean sea level is measured to within 2.4 cm on average. A first guess of the four-dimensional stream function was calculated using the altimeter estimate at the surface and an empirical extension scheme. The resulting field was specified as initial and boundary values of the Harvard quasi-geostrophic model. It is shown that the assimilation makes the altimeter estimate converge toward the in situ estimate especially at the subthermocline levels where spatial correlations of 0.60 are achieved after 8 days. A simultaneous assimilation of the altimeter stream function and 1400-m Polymode current meter stream function was also performed, with even better results. ¿ American Geophysical Union 1989