Analysis Of Thermocline Variability In The South China Sea And Its Response To The ENSO Events

A thermocline is defined as a transition layer between the warmer mixed water of the upper ocean and the cooler subsurface water below.Thermocline is one of the major physical phenomena in the upper ocean and is the major interface of marine life and circulation.Its spatio-temporal variations can greatly influence climate change,activities of naval vessels,marine fishery and underwater communication,attracting many scholars at home and abroad to investigate the thermocline.As a semi-closed basin,the South China Sea(SCS)is the largest and deepest marginal sea of China.According to the previous studies,some achievements have been obtained in the study of thermocline in the SCS,especially in the thermocline depth.However,many researchers focus mainly on the seasonal variation of thermocline depth in the SCS,few investigates the monthly and interannual variations of thermocline in the SCS.What's more,the variation of the lower boundary depth(Zlow),the thickness(?Z)and the intensity(Tz)of the thermocline in the SCS,which are also the important parameters of thermocline,have rarely been studied.The study is based on 51-year(1960-2010)monthly seawater temperature and surface wind stress data from Simple Ocean Data Assimilation(SODA),together with heat flux,precipitation and evaporation data from the National Centers for Environmental Prediction(NCEP),the National Oceanic and Atmospheric Administration(NOAA)and the Woods Hole Oceanographic Institution,respectively.This study analyzes monthly variability of Zup,Zlow,?Z and Tz,in the South China Sea(SCS).What's more,we qualitatively explain the main mechanism for the Zup variation and quantitatively analyze that variation using the multiple linear regression method.We also investigate the relationship between Zup and the ENSO events.The conclusions are as follows:(1)The Zup,Zlow,?Z and Tz of thermocline in the SCS show remarkable monthly variability.Being averaged for the deep basin(>200 m)of SCS,Zup deepens gradually from May to the following January and then shoals from February to May,while Zlow varies little throughout the whole year.?Z gradually become thin from May to the following January,but become thick from February to May.Tz gradually strengthens from March to September,but weakens from September to the following March.(2)Further diagnostics indicates that the monthly variability of Zup is mainly caused by the buoyancy flux and wind stress curl.Using the multiple linear regression method,the impacts of the buoyancy flux and wind stress curl on Zup can be quantitatively distinguished.The results suggest that Zup tends to deepen about 5.4 m when the buoyancy flux increases by 1 × 10-5kg/ms3,while it shoals about 2.2 m when the wind stress curl strengthens by 1 × 10-7N/m3.(3)It is indicated that the response of the SCS TD to the El Nino and La Nina events is in opposite phase.On one hand,the spatial-averaged TDs in the SCS(deeper than 200 m)appear as negative and positive anomalies during the mature phase of the El Nino and La Nina events,respectively.On the other hand,from June of the El Nino year to the subsequent April,the spatial patterns of TD in the north and south of 12°N appear as negative and positive anomalies,respectively,but present positive and negative anomalies for the La Nina case.However,positive and negative TD anomalies occur almost in the entire SCS in May of the subsequent year of the El Nino and La Nina events,respectively.It is suggested that the response of the TD in the SCS to the ENSO events is mainly caused by the buoyancy flux and the wind stress curl.