Turbulent Kinetic Energy and Its Dissipation Rate in the Equatorial Mixed Layer

Abstract

The authors apply a one-dimensional mixed layer model, based on second moment closure of turbulence, to study the characteristics of turbulence in the equatorial mixed layer. In particular, they focus on characteristics such as the TKE and the dissipation rate in the upper layers of the equatorial ocean, especially the phenomenon of deep episodic penetration well below the mixed layer of elevated dissipation rates commonly observed in microstructure measurements in the central tropical Pacific. Model simulations for the Tropic Heat experiments in 1984 and 1987 and Tropical Instability Wave Experiment in the central Pacific as well as the Tropical Oceans Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (TOGA COARE) Intensive Operating Period (IOP) observations in the western Pacific warm pool are performed. Modeled dissipation rates are found to be in reasonable agreement with those measured by microstructure profilers during these observations. Sensitivity studies are carried out to investigate the mechanisms underlying the deep penetration process. The authors find that the deep penetration of turbulence arises simply due to the instability and mixing resulting from the background vertical shear of the Equatorial Undercurrent enhanced by the easterly winds. The process is present whether or not there is nocturnal cooling. Deep penetration of turbulence is absent during westerlies, and then the behavior of the equatorial mixed layer is more like that of a high-latitude mixed layer. The modeled dissipation rates in the tropical western Pacific warm pool are in agreement with values observed during the TOGA COARE IOP. Both TKE and dissipation rates are significantly elevated in the mixed layer during westerly wind bursts. However, because of the weak background shear due to an undercurrent that is too weak and too deep in this region, turbulent mixing does not penetrate much below the mixed layer in the warm pool.