Equatorial Pacific Ocean Variability -- Seasonal and El Nino Times Scales

摘要

A nonlinear, single-layer numerical model is used to examine the baroclinic response of the equatorial ocean to time-variable winds. The rectangular model basin extends 1,500 km south of the equator and 15,000 km zonally; open boundary conditions are employed at the southern boundary. Attention is focused on the equatorially trapped wave response to symmetric, zonal time-dependent winds. The effects of mean currents, topography, coastline variation, thermodynamics and thermohaline mixing are neglected. In order to interpret the numerical solutions, analytic expressions for the vertical motion of the model pycnocline along the equator are derived from the linear theory of equatorial waves. Two types of wind distribution are considered: (1) a uniform wind across the entire basin and (2) a longitudinally bounded patch of wind stress within which the wind is uniform. The results given by the numerical model, linear theory, and the derived analytical expressions are also used to examine the general linear response of the equatorial ocean to time-variable winds as a function of the period of the forcing, the basin length and the stratification. The numerical model reproduces the observations of the average seasonal vertical displacement of the pycnocline at the eastern boundary. The analytical expressions reveal that equatorially trapped Kelvin waves excited between 180 and 120 degrees W are responsible for the large semi-annual response at the eastern boundary of the model.