Hydrographic data and acoustic Doppler current profiles collected from 150 ¿W to 85 ¿W in the equatorial Pacific during 1984 showed significant seasonal changes in the temperature and velocity fields. On the equator, the surface current was eastward in April up to 80 cm s-1, reversing to westward at 100 cm s-1 by November. Over the same period, the Equatorial Undercurrent (EUC) transport decreased, the equatorial zonal pressure gradient (ZPG) increased, and the depth of the mixed layer and EUC core deepened. Off the equator at 150 ¿W, the North Equatorial Counter-current (NECC) was absent in April/May but pronounced in October/November. Superimposed on this seasonal variability were smaller-scale (roughly 1000 km wavelength) correlated fluctuations in the upper ocean temperature and velocity fields. We identify these structures with the 20- to 30-day instability waves . The coincident high-resolution velocity and temperature data allowed the calculation of Reynolds' stresses due to the waves and resultant heat and momentum flux estimates as well as details of the vertical phase structure. Barotropic instability at the northern edge of the EUC is a likely source of energy for these waves. Estimated EUC transport decreased from 50 in April to 25¿106 m3 s-1 in November while the westward wind stress doubled and the 0-/400-dbar ZPG quadrupled. The data were used to estimate terms in the momentum balance in the upper 150 m, and it was found that nonlinear terms were often at least as important as the integrated ZPG in balancing the surface wind stress. East of 120 ¿W, the eastward advection of eastward momentum, UUx, was particularly important. These momentum equation terms were used to estimate a profile of the coefficient of vertical eddy viscosity; it was similar to profiles estimated by bulk methods and by parameterization by Richardson number.