| Introduction of the Author: Jia Ying—lai,was born on Jan 4,1975. Under the guidance of professor Qinyu—Liu, she was awarded the Dr.degree of Science at Ocean University of Qingdao on June 28, 2002. Now, she continues her research works in the field of the circulation in the South China Sea and its interaction with the Kuroshio. Abstract: The Luzon Strait, which is about 2500m deep and 3 degree wide, is an important western boundary gap in the path of the Kuroshio. Due to the lack of boundary support, the kinematics and dynamic features of the Kuroshio will change, which is called the deformation of Kuroshio in the Luzon Strait. The study of the deformation of Kuroshio is an instructive complementarity to the theory of the western boundary current. Furthermore, as the Luzon Strait is the main strait through which the South China Sea(SCS) connects and interacts with the Pacific Ocean, the features of the Kuroshio deformation in Luzon Strait are very important to the variation of the circulation in the north SCS. So the study of the spatial-temporal features and dynamics of the deformation of the Kuroshio in Luzon Strait is not only a complementarity to the theory of the western boundary current, but also a foundation to understanding the circulation in the SCS and its role in the variation of the East-Asia monsoon. Nowadays, three are three kinds of opinions on this topic: 1) There is a branch of the Kuroshio into the middle part of the SCS; 2) there is a current loop of the Kuroshio into the SCS. There may be anticyclonic eddies separating from the loop and then entering the SCS; 3) There is only the Kuroshio front, bending slightly to the SCS, and the Kuroshio does not directly enter the SCS. However, there is still no reasonable explanation about the mechanism of the three opinions. Based on the former studies, this paper promotes an innovation about this topic using new satellite altimetry data and hydrographic data along with numerical experiments: Firstly, the temporal variation of the deformation of Kuroshio accompanied by the periodically shed anticyclonic eddies from the Kuroshio bend is discovered; Secondly, the major mechanism of the anticyclonic eddy separating from the Kuroshio bend, which is the frontal instability, is brought forward and then proved; Thirdly, the factors with important impact on the periodical variation of the Kuroshio bend and the eddy shedding, which are the strength of the Kuroshio transport and the circulation in the winter SCS, are determined. Based on the POCM(Parallel Ocean Climate Model) result and the SODA(Simple Ocean Data Assimilation) data, there is Kuroshio bend toward the SCS in Luzon Strait which effect can reach 118°E. By analyzing the 7-year long TOPEX/POSEDIENT-ERS satellite altimeter data, it is found that the degree of the Kuroshio bend varies with time and is accompanied by the anticyclonic eddies periodically shed from the Kuroshio bend; There are three stages during the deformation procedure of the Kuroshio: In the first stage, the Kuroshio front bends slightly toward the SCS at the east of 118°E; In the second stage, the bend of the Kuroshio is increased and extends to the west of 118°E; In the third stage, an anticyclonic eddy is separated from the Kuroshio bend and the main branch of the Kuroshio flows directly northward in the Luzon Strait. In this stage, anticyclonic eddies with similar hydrographic features as the Kuroshio can be observed in the SCS. The analysis of the ARGOS buoy data, Conductivity-Temperature-Depth (CTD) data during two oceanographic surveys in 1998, and the objective analysis data from the Navy's Master Oceanographic Observational Data Set (MOODS) come to the same conclusion. The analysis of the T/P-ERS satellite altimetry data also shows that there will be a cyclone at the south of the Kuroshio front when the Kuroshio front forms a bend. The cyclone will increase during the westward extending of the Kuroshio bend. The Kuroshio bend will be separated by the cyclone and thus the eddy shedding occurred. The breaking off of the anticyclonic eddies from the Kuroshio bend is very closely related with the cyclone formed at the south of the Kuroshio bend. During the 7 years from October 1992 to December 1999, there are totally 38 eddy shedding occurred, in which 18 eddy shedding occurred during the summer monsoon period, and 20 occurred during the winter monsoon period. There is no difference between the occurrences of the eddy shedding during the winter and summer monsoon. The main period of the eddy shedding varies from 60 to 90 days and the main eddy shedding locations are in two areas: one is to the west of 118°E (116.5°E~117.5°E), the other is to the east of 118°E (119°E~121°E). In winter, anticyclonic eddies are mainly shed to the east of 118°E and the eddy shedding period mainly varies within 90 days. In summer, the eddies are more likely shed to the west of 118°E while the eddy shedding period mainly varies from 40 to 120 days. The formation mechanism of the deformation of Kuroshio in the Luzon Strait is determinedafter the study of the forming procedure of the Kuroshio and the Kuroshio bend, the importance of the nonlinear mechanism to the periodical variation of the Kuroshio bend and the eddy shedding, as well as the spatial-temporal features and the energy source of the deformation of the Kuroshio using a 21/2 layer nonlinear numerical model (McCreary & Yu,1992) forced by steady zonal wind with no local circulation in the SCS. The results show that: 1) In the 21/2 layer model, the Kuroshio bend forms with the developing of the Kuroshio, which is the result of the accumulation of the layer thickness at the western boundary due to the trap of the Rossby wave energy. 2) There will be no variation of the Kuroshio bend after the model reaches equilibrium in the linear experiment under the forcing of steady wind. However, in the nonlinear experiment under the forcing of the same steady wind, the Kuroshio bend will vary periodically and eddy shedding will occur after the fully formation of the Kuroshio when the model reaches quasi-equilibrium. So the nonlinear mechanism is the necessary condition for the periodical variation of the Kuroshio bend and the eddy shedding. 3) There will be a cyclone at the south of the Kuroshio front when the Kuroshio front forms a bend. The cyclone will increase during the westward extending of the Kuroshio bend. The Kuroshio bend will be separated by the cyclone and the eddy shedding occurred. The period of the variation of the cyclone and the anticyclone are both 60-70days. So, there must be increasing of the cyclone when the anticyclonic eddies is shed from the Kuroshio bend. 4) The instability analysis in the area of the Kuroshio bend shows that the energy source for the increasing of the cyclone south of the Kuroshio bend is the kinetic energy of the perturbation field converted from the potential energy of the mean field due to the frontal instability at the front in the south of the Kuroshio bend. In a word, the periodical variation of the deformation of the Kuroshio in the Luzon Strait is the result of the frontal instability. The factors which have impact on the periodical variation of the Kuroshio bend and the eddy shedding are determined by analyzing the variation of the Kuroshio transport, the local circulation in the SCS and the seasonally varying wind in the SCS and Pacific area through numerical experiments and perturbation energy analysis. It is found that the eddy shedding from the Kuroshio bend is mainly caused by the frontal instability in the area of the Kuroshio bend, while the intra-seasonal variation in the area of the north equator current and the Pacific Ocean east of the Luzon Strait, which has little impact on the eddy shedding, is not the main reason. The variation of the deformation of the Kuroshio can be affected by the strength of the Kuroshio...
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