The purpose of this study is to examine the variation of ocean surface layer hydrography on interannual timescales in the tropical Pacific Ocean using conductivity-temperature-depth measurements from 1976 to 1994. We demonstrate that associated with interannual variations in atmospheric forcing, there were distinct changes in mixed layer temperature, salinity, depth, and barrier layer thickness between normal, El Ni¿o, and La Ni¿a time periods. During El Ni¿o a warm, fresh mixed layer accompanied by an underlying barrier layer develops in the central and eastern Pacific in association with increased precipitation and reduced trade wind forcing. Conversely, during La Ni¿a, when unusually cold conditions prevail in the central and eastern equatorial Pacific and the warm pool is confined to the far western Pacific, a thick barrier layer is found west of 160 ¿E. Statistically, we demonstrate that barrier layers occur with increasing frequency as mixed layer temperatures increase from 20 ¿C to 30 ¿C, suggesting that barrier layer thickness may be one of the factors affecting surface temperatures. The physical mechanism underlying this relationship is likely to be related to the reduced efficiency of vertical turbulent mixing in cooling the surface via entrainment of thermocline water when the barrier layer is thick. Changes in mixed layer temperature, on the other hand, can affect precipitation and therefore mixed-layer salinity, leading to the possibility of feedbacks on interannual timescales involving the mixed-layer temperature balance and the hydrologic cycle over the ocean. The extent to which such feedbacks, if operative, may influence the detailed evolution of large-scale, lower-frequency variability in the tropical Pacific needs to be critically assessed in the context of coupled ocean-atmosphere models. ¿ 1997 American Geophysical Union