Suppressed Thermocline Mixing in the Center of Anticyclonic Eddy in the North South China Sea

Yongfeng Qi,Xia Wang,Linhui Yu,Xiaodong Shang,Shumin Lian,Xianpeng Li,Huabin Mao

doi:10.3390/jmse9101149

Abstract

Direct microstructure observations and fine-scale measurements of an anticyclonic mesoscale eddy were conducted in the northern South China Sea in July 2020. An important finding was that suppressed turbulent mixing in the thermocline existed at the center of the eddy, with an averaged diapycnal diffusivity at least threefold smaller than the peripheral diffusivity. Despite the strong background shear and significant wave–mean flow interactions, the results indicated that the lack of internal wave energy in the corresponding neap tide period during measurement of the eddy’s center was the main reason for the suppressed turbulent mixing in the thermocline. The applicability of the fine-scale parameterization method in the presence of significant wave–mean flow interactions in a mesoscale eddy was evaluated. Overprediction via fine-scale parameterization occurred in the center of the eddy, where the internal waves were inactive; however, the parameterization results were consistent with microstructure observations along the eddy’s periphery, where active internal waves existed. This indicates that the strong background shear and wave–mean flow interactions affected by the mesoscale eddy were not the main contributing factors that affected the applicability of fine-scale parameterization in the northern South China Sea. Instead, our results showed that the activity of internal waves is the most important consideration.

Highlights

Oceanic mesoscale eddies, which play an important role in dynamical oceanography across a range of scales [1,2] and are key transporters of oceanic materials [3,4,5,6,7], are ubiquitous on a global scale [8]
Our findings indicated that the elevated turbulent mixing in the thermocline in the periphery of the eddy was not furnished by the sub-mesoscale eddy
The results of the present study suggest that weak background mixing in the center of the anticyclonic eddy is preserved by the reduced internal waves breaking with weak available potential energy (APE)

Summary

Introduction

Oceanic mesoscale eddies, which play an important role in dynamical oceanography across a range of scales [1,2] and are key transporters of oceanic materials [3,4,5,6,7], are ubiquitous on a global scale [8]. Mesoscale eddies contain more than 90% of the ocean’s kinetic energy; they play important roles in ocean energy distribution and the regulation of ocean mixing processes [7,9,10,11,12]. Turbulence microstructure observation is the most direct and effective method for studying the mixing processes of ocean mesoscale eddies; few microstructure observation datasets are available, which is not conducive to in-depth analyses of the interaction mechanism between mesoscale eddies and ocean-mixing. Fine-scale parameterization, built on the wave–wave interaction theory [13], facilitates exploration of the distribution of turbulent mixing in the open ocean [14,15,16,17]; hydrographic data derived by conductivity-temperature-depth (CTD) profilers are much easier to work with, compared with previous datasets. Fine-scale parameterization is one of the main methods used to study ocean mixing variation regulated by mesoscale eddies. Zhang et al [27]

Methods

Results

Discussion

Conclusion