Study Of Abyssal Mixing In The South China Sea Based On Thorpe Scale

As the largest marginal sea of northwest Pacific Ocean, the South China Sea(SCS) has been one of the research hotspots because of its special geographicallocation and economic and military values. Deep mixing (means diapycnal turbulentmixing in this paper) in SCS has a significantly impact on transport of material andenergy and stratification maintaining. In this paper, several temperature and salinityprofiles obtained from field observation carried out in SCS from2005to2012areprocessed and analyzed using Thorpe scale and inverse method. Potential densityprofiles are the mainly used when judging overturns while potential temperatureprofiles as the assistant profiles. Based on dissipation rate and diapycnal mixing rateof each profile, the distributions of the two parameters in deep SCS (mainly deeperthan1000m) and seasonal variation of the two parameters in Luzon Strait duringspring, summer and autumn are shown and analyzed.Statistical properties of overturns’ scale and distribution suggest that overturnswith large Thorpe scale become more when deepening in depth and densitycoordination, which agrees well with previous studies. Overturns with Thorpe scalebetween10m and20m occupied the largest proportion. The vertical scale of overturnsis about3times to Thorpe scale while the maximum of Thorpe displacement is about2times to Thorpe scale. Furthermore, the result suggests that the maximum of Thorpedisplacement can characterizing the property of Thorpe scale and overturn because thelinear relationship between it and Thorpe scale is fitted better than the relationshipbetween the vertical scale and Thorpe scale. Therefore, the relationship between themaximum of Thorpe displacement and Thorpe scale can be a reference to examine theresult of overturn discrimination.From the distributions of dissipation rate and diapycnal mixing rate in deep SCS,the following conclusions can be obtained. The dissipation and mixing near LuzonStrait are intense in almost whole depth with values reaching10-8Wkg-1and10-3m2s-1.In the layer of1000m-2500m, dissipation on the northwest continental slope is alsoreaching10-8Wkg-1while that in north deep basin is weak relatively. The dissipation and mixing in the layer below2500m are enhanced relative to the upper layer in mostregions. In the north SCS, the mixing rate in this layer is more intense than that in theupper layer, while higher by about2orders of magnitude than background value inthe open ocean. For the south SCS, the mixing rate of east part is enhanced by about2orders of magnitude. The matching of the two layers which below2500m and500mabove bottom illustrates the vital effect of bottom topography. The main charactersagree well with the result using GK96method. The variations in meridional and zonaldirection show that the mixing rate becomes higher gradually from west to east in alllayers while from south to north in upper two layers of1000-1500m and1500-2000m.The distribution appears low values around17°~18°N. After separating the study areato four regions of Luzon Strait (region1), the north part (region2), the middle part(region3) and the south part (region4), it can be found that the amplification ofmixing rate is the largest in region4of about two orders of magnitude, while withinone order of magnitude in other three regions. The distribution of Richardson numberbelow0.25is agree well with that of mixing rate, which suggests the big occurrenceprobability of instability.The unique double-ridge feature in Luzon Strait has a greatinfluence in generation, propagation and dissipation of local internal waves, whichmay account for the enhanced mixing here. The west-propagating internal wavesprovide energy for mixing of the north SCS, while the process of evolving tononlinear waves may be account for the dynamic mechanism. Moreover, the internaltides produced around Dongsha Island may be another reason for the enhanced mixnghere. Internal tide, which can provide energy for mixing, is weak in the middle SCS,resulting in weak dissipation and mixing here than the north. In the south SCS,Richardson numbers are almost smaller than0.25below2000m, suggests a largeprobability of instability. It can be inferred that small perturbation may result inintense mixing. The weak stratification and complex topography may be theaccounting aspects.From the vertical distribution of dissipation and mixing on the section of120.5°Ein Luzon Strait in spring, summer and autumn, it can be concluded that mixing ismore intense in autumn than that in spring and summer, especially to the north of20°N. The dissipation and mixing near the slope in the most southern of section areenhanced by an order of magnitude. The weak stratification and the intensity and east-west direction variation of energy flux of internal tide in autumn may account forthe seasonal variation of dissipation and mixing on this section.