Seasonal variability in the isothermal and isopycnal surface mixed layers of the North Pacific Ocean is examined using the Naval Research Laboratory Ocean Mixed Layer Depth (NMLD) Climatology. A comparison with observations from 11 ocean weather stations in the northeast Pacific Ocean is performed that validates the NMLD climatology in this region. The general features of the isothermal layer depth (ILD) and mixed layer depth (MLD) obtained from these mixed layers are explained with wind stress, surface net heat flux, and freshwater flux climatologies, given guidance from a mixed layer model. Departures from a surface-forced interpretation of turbulent mixing are found near the Kuroshio, where horizontal heat transport is important. The much deeper ILD in the northeast Pacific in winter and spring relative to the MLD reveals a 50 m barrier layer between the bottom of the MLD and the top of the thermocline. A detailed analysis shows this barrier layer extends over most of the North Pacific subpolar gyre. It forms when the seasonal thermocline is deepened in winter by surface cooling, such that salinity stratification due to evaporation minus precipitation less than zero (E-P<0) becomes important in the formation of the MLD. A shallower halocline forms over the subpolar gyre than in other regions of the North Pacific because of precipitation dominating over evaporation in the annual mean. A mechanism for maintaining the shallow halocline is provided by upward vertical motion driven by positive wind stress curl in the presence of diapycnal mixing. Numerical models show this as part of a shallow meridional overturning cell. ¿ 2000 American Geophysical Union