Preliminary Analysis Of Origin And Variation Characteristics Of Ryukyu Current

Ocean circulation is the permanent flow of the sea water. As an important component of the golbal circulation, the western boundary currents play an significant role of the mass and heat transport, and affect the climate change deeply. The Ryukyu Current is a northeastward and western boundary current east of Ryukyu Islands. Compared with the Kuroshio, there are less observation and study on the Ryukyu Current.In this paper, the output between January 1977 to December 2006 of Ocean General Circulation Model for the Earth Simulator (OFES) were used to investigate the structure and transportation of the Ryukyu Current. Totally 18 sections were studied in the specific area to research on the details of the velocity fields. Besides, the transport of section were calculated with the potential density restriction.To be exact, the dots with a potential density greater than 27.15,which were obtained from the potential temperature and salinity, were not invloved to compute the transportation. The result shows that:(1)There are four origins of the Ryukyu Current, namely a branch of the Kuroshio flowing east to the Taiwan Island, the westward flow south to the Miyako Strait, the subsurface outflow of the Kuroshio through the Kerama Channel, and the westward flow east to the Okinawa Islands and the Amami Islands, which occupy 15.0%,48.3%,2.7% and 32.0% of the total transport, respectively.(2)The velocity as well as the transport of the Ryukyu Current increases from the southwest to the northeast. It is about 3.33 Sv of the transport of the section south to the Yaeyama Islands while it is 15.07Sv along the section north to the Miyako Strait.The transport of the secton southeast to the Amami Islands, which has beyond 16.7% of the Kuroshio through the Osumi Strait and Tokara Strait, is approximate 22.17 Sv(3) It basiclly locates from the surface to the depth of 400 m of the core of the Kuroshio in the East China Sea, while the core of Ryukyu exists between 200 m to 600m. The Ryukyu is relatively thinker and deeper than Kuroshio. For examply, the transport of the Kuroshio hardly increases when deeper than 500m ,however, it is necessary to calculate the Ryukyu Current's transportaion to 850m.(4) The transport of the Ryukyu Current is bigger in the late winter, early spring, late autumn and early autumn compared with other seasons; The Kuroshio is strongest in late summer and gets the weakest in winter.(5) It is significantly affected by the eddies of the Ryukyu Current and the transport of the current is less stable than Kuroshio. The lower the latitude it exists, the more influence it gets from the eddies and the bigger of the relative standard error of the transport.Sometimes the positive transport through some sections is less than the negtive ones. Specifically, there are no positive transport of the section south of the Yaeyama Islands, which means some eddies could cut off the first origin that feeds the Ryukyu Current.(6) The similar significant cycle appears both in the Ryukyu Current and Kuroshio, which is about 120 months according to the filter analysis.By the tracing analyzing of the eddies south to the Shikoku, the conclusion has gotten that the eddies break away from the Kuroshio once every 10 years, then moves southwestward. The movements of the eddies influence the sea off the Ryukyu Islands.(7) Bigger eddies mainly distribute east off the Taiwan Island, in the upper space of the Ryukyu Trench and in the Shikoku Basin. Majority of the eddies are anticyclonic and the anticyclonic eddies are propitious to the development of the Ryukyu Current. The transport of the water east off the Ryukyu Islands are mainly northeastward.In this study, a hybrid coordinate ocean model(HYCOM) is used to simulate the climatological golbal ocean circulation. By comparing and summarizing the result of HYCOM and OFES, it is confirmed that the existance of the Ryukyu Current and its four sources.The sensitive experiments indicate that the Ryukyu is more affected by the North Equatorial Current of the Pacific than the Kuroshio.