Dynamical Analysis of Seasonal and Interannual Variability in the Equatorial Pacific*

Time series measurements from the Tropical Atmosphere Ocean array during the 1980s and 1990s are used to analyze the dynamics of the long-term mean, the seasonal cycle, and interannual variability in the upper 250 m of the equatorial Pacific. The analysis is based on a diagnosis of the zonal momentum equation using surface winds, ocean temperatures, and currents between 165°E and 110°W along the equator. In the mean, the balance is mainly between zonal wind stress and depth-integrated zonal pressure gradient, in good agreement with linear Sverdrup theory. For the seasonal cycle, the balance is mainly between wind stress, pressure gradient, and local acceleration, with differences between the stress and pressure gradient leading to accelerations and decelerations of the South Equatorial Current and Equatorial Undercurrent. On interannual timescales, local acceleration is much smaller than either zonal wind stress or zonal pressure gradient, indicating that to zeroth order, variations in the equatorial Pacific are in quasi-equilibrium with the wind forcing. Consistent with this quasi-equilibrium balance, Equatorial Undercurrent transports in the thermocline vary nearly in phase with the strength of the zonal pressure gradient and the easterly trade winds. Nonlinear effects are significant on all timescales, though of secondary importance in the depth-integrated zonal momentum balance.