The Role of Internal Gravity Waves in the Equatorial Current System

Eric D. Skyllingstad Battelle/Marine Sciences Laboratory, Sequim, Washington

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Donald W. Denbo Battelle/Marine Sciences Laboratory, Sequim, Washington

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Abstract

Using a two-dimensional nonhydrostatic model, experiments were performed to investigate the formation and maintenance of internal waves in the 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 number dependence, the simulated internal waves exhibit a diurnal cycle, following the daily stability change in the mixed layer. The diurnal cycle is not evident when the wind stress is eastward because of a decreased mixed layer shear and corresponding Reynolds number. The amplitude, wavelength, frequency, and diurnal variability of the simulated waves are in agreement with high-resolution thermistor chain measurements. Linear theory shows that the horizontal wavelength of the internal waves depends on both the thermocline stratification and the strength of the Equatorial Undercurrent.

The simulations show that internal waves can provide an efficient mechanism for the vertical transport of horizontal 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 level resulting in a deceleration below the undercurrent core. Otherwise, the waves tend to decrease the current velocity above the undercurrent core.

Abstract

Using a two-dimensional nonhydrostatic model, experiments were performed to investigate the formation and maintenance of internal waves in the 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 number dependence, the simulated internal waves exhibit a diurnal cycle, following the daily stability change in the mixed layer. The diurnal cycle is not evident when the wind stress is eastward because of a decreased mixed layer shear and corresponding Reynolds number. The amplitude, wavelength, frequency, and diurnal variability of the simulated waves are in agreement with high-resolution thermistor chain measurements. Linear theory shows that the horizontal wavelength of the internal waves depends on both the thermocline stratification and the strength of the Equatorial Undercurrent.

The simulations show that internal waves can provide an efficient mechanism for the vertical transport of horizontal 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 level resulting in a deceleration below the undercurrent core. Otherwise, the waves tend to decrease the current velocity above the undercurrent core.

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