Dynamics of seasonal and interannual variability in the equatorial Pacific

The goal of this thesis is to investigate dynamics of seasonal-to-interannual variability in the equatorial Pacific upper ocean mainly using the observations from the TAO array over 1980-1997. The investigation is conducted in three relevant perspectives. First, the variability is described. At seasonal time scales, the normally westward flowing South Equatorial Current (SEC) reverses its direction with significant magnitude in boreal spring in the eastern Pacific. Seasonal variations in both the SEC and Equatorial Undercurrent (EUC), and thermocline depth at 5{dollar}spcirc{dollar}N and 5{dollar}spcirc{dollar}S propagate westward, but variations in thermal structure propagate eastward along the equator in the eastern and central Pacific. On interannual time scales, zonal wind anomaly is largest and in phase over the western and central Pacific. SST anomalies are significant in the region 165{dollar}spcirc{dollar}E-110{dollar}spcirc{dollar}W, and interannual variations in thermal structure propagate eastward, with most of the phase shift occurring in the central Pacific.; The zonal momentum balance is then diagnosed for seasonal-to-interannual variability. At the seasonal cycle, the momentum balance is mainly between wind stress, pressure gradient and local acceleration. On interannual time scales, to zeroth order, the wind stress is balanced by the pressure gradient, indicating that the equatorial Pacific varies as a quasi-steady equilibrium. Nonlinear effects are significant at some depths on all time scales, though of secondary importance in the depth integrated momentum balance.; Finally, seasonal variations are studied in terms of equatorial waves through a simple wave dynamical model. Model results indicate that seasonal variability between 5{dollar}spcirc{dollar}N and 5{dollar}spcirc{dollar}S is dominated by wind-forced equatorial Kelvin waves and the first two meridional mode Rossby waves. The first two baroclinic modes dominate the solutions. The sum of the Rossby waves and Kelvin waves results in westward propagation in the equatorial zonal currents and off-equatorial thermal structure, but eastward propagation in thermal structure along the equator in the eastern and central Pacific. The simultaneous response to the zonal surface stress from the frictional dynamics also contributes to the westward propagation in the surface currents.