Analysis Of The Structures And The Mechanisms Of The Cyclonic Eddies To The Northwest Of Luzon Islands

Both the local wind and the Kuroshio have great impacts on the circulation in the region to the northwest of Luzon Islands, as a result, the eddies in the region is active. Luzon Cold Eddy which is one of the two seasonal cyclonic eddies in the South China Sea also originates there. Understanding the structures and the mechanisms of the eddies to the northwest of Luzon Islands is meaningful. It is not only make a supplement to theories of the circulation in the South China Sea, but also contribute a lots to the study of the interaction between oceans and marginal seas.Based on the climatological temperature and salt data combined with the Satellite Altimeter data, The climatological characteristics of the main oceanic elements and the seasonal variance to the northwest of the Luzon Islands are discussed firstly. Taking advantage of the TOPEX/Poseidon data along with the mean sea level data supplied by the IPRC, the configuration and the structures of the cyclonic eddies are studied. Then we analyzed the characteristics of the flow field using the Okubo-Weiss function to confirm the structure of the cyclonic eddies. In addition, we discussed the relationship between the cyclonic eddies and the local wind stress as well as the Kuroshio based on correlation and power spectrum analysis. At last, we successfully simulated the cyclonic eddy to the northwest of Luzon Islands using HYCOM numerical experiments. Both the data and the model are all in high resolution, so we succeed in giving out a reasonable explanation to the existing controversy on the mechanism of Luzon cold eddy. The main conclusions are as follows:1) The Luzon Cold Eddy actually consists of two cyclonic eddies in winter. One (LCE1) is located to the west of the Luzon Islands and the other (LCE2) is to the northwest. The horizontal scale of LCE1 is about two times as large as LCE2, while the intensity of the LCE2 is a little greater.2) The cyclonic eddy to the west of Luzon Islands exists only during winter time, while the eddy northwest of Luzon Islands exists all year round. The two cyclonic eddies act as a big cyclonic eddy occasionally, However, they have their own centers most of the time (account for about 3/4 of the total samples).3) The characteristic of the flow field to the northwest of Luzon Islands are analyzed using Okubo-Weiss function. The results suggest that the flows in both the LCE1 region and LCE2 region are vorticity-dominated. However, the characteristic of vorticity-dominating is more obvious in region LCE1 than in region LCE2.4) The main period of variance for LCE2 is about 60 days, while the transport of the Kuroshio in the Luzon Strait not only has a intraseasonal oscillation of 60-70 days, but only has a oscillation of 100 days. The LCE2 are mainly composed of water from South China Sea, so we conclude that the LCE2 is a local cyclonic eddy in South China Sea induced by the Kuroshio.5) The studies also show that the mechanisms of the two cyclonic eddies to the northwest of Luzon Islands are different. The cyclonic eddy to the west of Luzon Islands (LCE1) is related to the positive wind stress curl to the northwest of the Luzon land in winter, while LCE2 is induced by the Kuroshio which is checked in HYCOM experiment. The cyclonic eddy occurs even though the wind on the South China Sea is set to zero in the numerical experiments. The intensity of the eddy become strong(weak) when we strenThe intensity of eddy will become doubled (half) as we strengthen (weaken) the Kuroshio by one fourth.To sum up, we propose that the Luzon Cold Eddy in winter is composed of two cyclonic eddies with different mechanisms for the first time. One of the cyclonic eddies are forced by the local winter wind stress curl to the west of the Luzon Islands, while the other is induced by the Kuroshio. The contribution of the former is 2/3, while the latter accounts for 1/3 to the Luzon Cold Eddy.