Study On Wind-induced Turbulent Mixing On The South China Sea Shelf And Long-term Mixing Observation

The South China Sea (SCS) is one of the broadest marginal seas adjacent to the Northwest Pacific, which contains a variety of oceanic dynamic processes. Breaking of internal waves due to local wind field and tidal currents usually leads to strong turbulent mixing which is an important physics processes to study the water mass properties, continental shelf circulation, nutrition flux and pollution distribution. The continental shelf in the SCS provides a good platform to explore the enhanced mixing by strong wind and the corresponding influences on the dynamic processes in the ocean side when a storm passes by. Despite the importance of internal waves and mixing induced by wind to the energy flux and mass transportation is acknowledged, it is difficult to obtain continous measurements of mixing restricted by means of the present observation, especially for the integrated observation under extreme weather conditions. Thus, the energy propagation and enhanced mixing induced by strong wind is still to open.In this paper, we addressed two major questions as follows: (1) Based on the observation obtained on the shelf in the SCS, including small-scale measurements, moored ADCP data and temperature data, we analyzed the processes of ocean mixing indueced by the passing strom, as well as how the engery of internal wave spread to generate turbulent mixing and the evolution of the ehanhacd mixing. (2) In order for long term and continous observation of small-scale processes under a variety of weather conditions, a newly built instrument and computation method are introduced in this paper.An integrated observation in August 2005 was unfolded on the SCS shelf at 19o37' N, 112o04' E , including current obtained by mooring ADCP (Acoustic Doppler Current Profiler), turbulent kinetic energy (TKE) dissipation rate by TurboMap II and temperature by thermistor chains. It is the first time to get the direct continuous measurements with high time resolution in the SCS, especially under a storm weather condition. The enhanced mixing depth increased about 10 m after the storm, and the average dissipation rates in the mixed layer of pre-storm and post-storm were 1. 5×10?6Wkg ?1 and 2. 0×10?6Wkg ?1 , respectively. The enhanced mixing can penetrate more than 60 m deep after the storm. Near-inertial currents were excited by the storm and lasted more than 1.5 inertial periods. The energy of near-inertial internal waves generated more shear instabilities in higher modes which are considered as the source of the enhanced mixing.In order for better understanding the response in the ocean side to a variety of extreme weather conditions, in particular the evolution of small-scale mixing processes, how to obtain continuous long-term measurements is the biggest bottleneck. A newly built instrument,χpod, is introduced and obtained a series of data of 4-month at equatorial Pacific Ocean. Estimates of dissipation rates of thermal variance and turbulent kinetic energy are made by scaling temperature gradient spectra in the inertial-convective subrange from well-resolved measurements on the equatorial ocean mooring. These are compared to estimates derived from scaling spectra at high wavenumbers. It is found that these values agree to within a factor of 5 on a 90% confidence level. We suggest that reasonable estimates of thermal variance can be made from temperature measurements resampled at 2 Hz, significantly slower than the 120 Hz data sampled for this study, thereby reducing data storage requirements for long deployments.