THE ROLE OF INTERNAL GRAVITY-WAVES IN THE EQUATORIAL CURRENT SYSTEM

Using a two-dimensional nonhydrostatic model, experiments were perform ed to investigate the formation and maintenance of internal waves in t he equatorial Pacific Ocean. The simulations show that internal waves are generated in the surface mixed layer by a type of Kelvin-Helmholtz instability that is dependent on both the flow Reynolds number (i.e., shear strength) and Richardson number. Because of the Richardson numb er dependence, the simulated internal waves exhibit a diurnal cycle, f ollowing the daily stability change in the mixed layer. The diurnal cy cle is not evident when the wind stress is eastward because of a decre ased mixed layer shear and corresponding Reynolds number. The amplitud e, wavelength, frequency, and diurnal variability of the simulated wav es are in agreement with high-resolution thermistor chain measurements . Linear theory shows that the horizontal wavelength of the internal w aves depends on both the thermocline stratification and the strength o f the Equatorial Undercurrent. The simulations show that internal wave s can provide an efficient mechanism for the vertical transport of hor izontal momentum. In the surface mixed layer, the internal waves gain westerly momentum at the expense of the background flow. In some cases , this momentum is transferred back to the mean flow at a critical lev el resulting in a deceleration below the undercurrent core. Otherwise, the waves tend to decrease the current velocity above the undercurren t core.