Effect of Breaking Waves on Near-Surface Mixing in an Ocean-Wave Coupling System under Calm Wind Conditions

Ji-Seok Hong,Jae-Hong Moon,Taekyun Kim

doi:10.3390/jmse8070540

Ji-Seok Hong, Jae-Hong Moon + Show 1 more

Open Access

https://doi.org/10.3390/jmse8070540

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Abstract

Estimating wave effects on vertical mixing is a necessary step toward improving the accuracy and reliability of upper-ocean forecasts. In this study, we evaluate the wave effects on upper-ocean mixing in the northern East China Sea in summer by analyzing the results of comparative experiments: a stand-alone ocean model as a control run and two ocean–wave coupled models that include the effect of the breaking waves (BW) and of the wave–current interaction (WCI) with a vortex-force formalism. The comparison exhibits that under weak wind conditions, the BW effect prescribed by wave dissipation energy significantly enhances near-surface mixing because of increased downward turbulent kinetic energy (TKE), whereas the WCI has little effect on vertical mixing. Increased TKE results in a mixed-layer depth deepened by ~46% relative to the control run, which provides better agreement with the observed surface thermal structure. An additional experiment with local wind–based BW parameterization confirms the importance of nonlocally generated waves that propagated into the study area upon near-surface mixing. This suggests that under calm wind conditions, waves propagated over distances can largely affect surface vertical mixing; thus, ocean–wave coupling is capable of improving the surface thermal structure.

Highlights

Vertical mixing in the upper-ocean in mid-latitude is one of the main processes in heat transfer between the warm surface layer and the cold subsurface layer, and it greatly influences the ocean–atmospheric interaction through changes in sea surface temperature
Carniel et al (2009) [22] incorporated the injection of turbulent kinetic energy (TKE) by breaking waves (BW) into an ocean model by parameterizing vertical mixing using a generic length scale (GLS) scheme and highlighted the need for wave-induced turbulence to improve the accuracy of temperature structures in the ocean surface
The BW run allowed the BW effect on thermal mixing through changes of TKE because of wave dissipation energy transferred from the wave model

Summary

Introduction

Vertical mixing in the upper-ocean in mid-latitude is one of the main processes in heat transfer between the warm surface layer and the cold subsurface layer, and it greatly influences the ocean–atmospheric interaction through changes in sea surface temperature. Carniel et al (2009) [22] incorporated the injection of TKE by BW into an ocean model by parameterizing vertical mixing using a generic length scale (GLS) scheme and highlighted the need for wave-induced turbulence to improve the accuracy of temperature structures in the ocean surface. Using a wave-circulation coupled model, Yang et al (2004) [25] reported that enhanced vertical mixing by non-BW can improve the thermal structure in the upper 50 m in the northern ECS during summer. This study highlights the contribution of nonlocally generated waves to surface thermal mixing by comparing a coupled model simulation with an uncoupled ocean model that includes local wind-based BW parameterization

Model Configurations

Experimental Design

Model Validation

Effect of Waves on the Ocean-Surface’s Thermal Structure