This data set includes two versions: 1.4.2 and 1.4.3. The general circulation ocean model on which the analysis is based uses the Geophysical Fluid Dynamics Laboratory Modular Ocean Model 2.b code, with conventional choices for mixing, etc. The constraint algorithm applied is an extension of optimal interpolation data assimilation, an algorithm that has been widely applied in meteorological numerical weather forecasting. The extension includes changes in the spatial dependence of the statistics and in the assumption of bias in the model forecast. The domain of this analysis extends from 628S to 628N. The main datasets used to constrain the model are the hydrographic data contained in the World Ocean Atlas 1994; additional hydrographic data, satellite, and in situ sea surface temperature; and altimetry from the Geosat, ERS-1, and TOPEX/Poseidon satellites. The historical dataset is sufficiently limited that no attempt has been made to resolve midlatitude mesoscale variability and have limited our attention to the upper 500 m of the water column. The control analysis and analysis experiments begin January 1950 and continue through December 1995. The basic analysis fields, thus, consist of 552 monthly averages of temperature, salinity, sea level, and the horizontal components of velocity.