Seasenal And Interannual Variations Of The Thermocline Depth In The South China Sea

Seawater temperature is one of the most important and basic elements inresearch of physical oceanography. The vertical distribution of the seawatertemperature is especially important. Thermocline is defined as the water layer withthe most vertical seawater temperature gradient in ocean. Thermocline is locatedbelow the mixed layer. Research of thermocline in ocean is one of the mostimportant respects of the Physical Oceanography research. The thermocline is anindicator which reflects important physical properties of the ocean temperature field.The research of the basic characteristics of the thermocline is of great significance tomarine fisheries, the activities of the Navy’s submarine, the propagation of light andsound waves in the ocean and underwater communications. The research ofcharacteristics of the thermocline is also closely associated with ocean circulation,internal waves, water mass, air-sea exchange and other subdiscipline. Therefore, theinvestigation and research of the ocean thermocline, especially seasonal thermocline,has attracted many domestic and international scholars in the process of thedevelopment of the Physical Oceanography.The study of the characteristics and distribution pattern of the thermocline in theSouth China Sea has be made some achievements, but previous studies are mainlybased on analysis of the seasonal or intraseasonal thermocline variation in theSouth China Sea. There also are a small amount of research on the interannualvariability of the mixed layer in the South China Sea. However interannual variationsof the thermocline in the South China Sea is yet to be conducted.Based on SODA (Simple Ocean Data Assimilation) and NCEP data, waveletanalysis, EOF analysis, trend analysis and composit analysis will be used,interannual variation of the wind field and heat flux during the six times El Ni(?)oevents will be analyzed. Then the seasonal and interannual variation of the thermocline depth in the South China Sea will be discussed. The relationshipbetween the El Ni(?)o events and interannual variation of the thermocline depth in theSouth China Sea will be analyzed. Last but not the least, the main reason forthe thermocline depth anomalies during the El Ni(?)o event was discussed in this article.Conclusion as follows:(1) In the first mode of the EOF analysis, The distribution of thermoclinedepth in the South China Sea is substantially in the same phase. The results of thetime series and its wavelet analysis show that the main period is6~7years. As thefirst mode spatial distribution and time sequence show, the interannual variability ofthe South China Sea thermocline is due to the interannual variability of the monsoonand sea surface heat flux which is influenced by the interannual variability of thetropical Pacific. In the second mode spatial distribution of the EOF analysis, thethermocline depth Is in reverse distribution between southeast and northwest SouthChina Sea, which may be caused by Ekman transport and Ekman pumping caused bythe monsoon. The results of the time series and its wavelet analysis show that themain period is four years.(2) There is a negative correlation between the Nino3index and thermoclinedepth anomaly in the South China Sea in1958~2000. During the El Ni(?)o events,when the Nino3index show positive anomalies, the South China Sea thermoclinedepth always show negative anomalies especially in1965/1966,1972/1973,1982/1983,1986/1987,1997/1998.(3) During the El Ni(?)o events, the thermocline depth in most of the South ChinaSea shows negative anomalies from October [0] to March [+1], which is consistentwith the time range of the high value of Nino3index. The thermocline depth in mostof the South China Sea shows positive anomalies from April[+1] to June [+1]. Themain factors to affect the thermocline depth in the South China Sea are net heat flux,stir of wind stress,wind stress curl and Ekman transport. Stir of wind stress and thenet heat flux are dominant while the effect caused by wind stress curl and Ekman transport on the thermocline depth is weaker, which is basically canceled by otherfactors in the South China Sea during the El Ni(?)o events.(4) Northeast monsoon weaken in autumn and winter while southwest monsoonstrengthen in spring and summer, in most of the South China Sea during the El Ni(?)oevent. The decreasing wind speed shoals thermocline depth in autumn and winter [0].Meanwhile, the decreasing wind speed results latent heat flux to increase.Conversely, the increasing wind speed results in the depth of the thermocline inspring and summer [+1]. At the same time, the increaseing wind leads latent heat fluxto be negative anomalies.(5) Abnormal downdraft leads cloud cover to reduces, in the Western Pacificand the South China Sea during the El Ni(?)o event.Then shortwave radiationenhances net surface heat flux. In the autumn and winter [0], the decreasing windspeed increases the net surface heat flux too. At this point, the increasing net surfaceheat flux result in shallow of the thermocline in the South China Sea. Conversely, inspring and summer [+1], negative anomalies of net heat flux enhances thermoclinedepth.