An air-sea interaction surface mooring was deployed in the eastern tropical Pacific Ocean at 10°N, 125°W during the 1997/1998 ENSO to obtain accurate estimates of air-sea fluxes as part of the Pan American Climate Study (PACS). During the 18 month deployment, a full seasonal cycle was recorded, including the migration of the ITCZ over the mooring. The high resolution measurements of surface meteorological properties and incoming short and longwave radiation allow accurate computation of the air-sea fluxes of heat, freshwater, and momentum. The measurements are used to characterize the variability in the surface meteorology and air-sea fluxes at the site. The net heat flux into the ocean was higher while the ITCZ was over the site because reduced wind speeds contributed to a substantial decrease in evaporation. Furthermore, the airsea fluxes are used in conjunction with subsurface measurements of temperature, salinity, and velocity to examine the local oceanic response during two contrasting regimes of meteorological variability defined as ITCZ and non-ITCZ conditions. A one-dimensional oceanic boundary layer model forced with the observed fluxes suggests that the high winds of the non-ITCZ regime generated strong inertial oscillations in the oceanic boundary layer, and the superposition of these oscillations upon the large-scale geostrophic currents induced mixing and deepening of the thermocline by enhancing the vertical shear in the upper 100 m of the water column. Previous studies have shown that the rapid deepening of the thermocline that occurred near 10°N in early 1998 was part of a much larger pattern of variability which reliably precedes the transition from El Niño to La Niña states. Our analysis suggests that this rapid thermocline deepening observed at the site was due primarily to a combination of downward Ekman pumping and vertical mixing.