Numerical Study Of The Kuroshio Front To The East Of Taiwan

In this paper, based on the stronger judgment criterion for oceanic thermal front, the thermal front information is extracted from the temperature profile observations in the study area. By using the hydrographical atlas and referring to related literatures, the temperature distribution trend of different levels is also analyzed. The results show that the surface thermal front to the east of Taiwan is not obvious, while there is an all-year strong Kuroshio thermal front under the surface. The sea surface height (SSH),3-dimensional (3D) temperature, salinity and current in that area are numerically simulated. And the final result is consistent in the tendency with that from observations and atlas. There is more obvious Kuroshio thermal front from subsurface to 350m to the east of Taiwan; with depth increasing, both intensity and width of this thermal front has the tendency of decrease following increase; and the position of such thermal front tends to be eastward with the depth's increasing; such front is characterized by seasonal changes that the strongest strength and width are in summer and the weakest in winter.The oceanic thermal front information is extracted from the satellite remote sensing sea surface temperature (SST) data of China Seas. It is found that the surface thermal front is not obvious if the strong criterion is adopted. In order to study the 3D multi-scale temperature variation rule of the Kuroshio front under the surface to the east of Taiwan and discuss the mechanism of frontogenesis, the SSH,3D temperature, salinity and current are numerically reanalyzed in this frontal zone. The paralleled hybrid coordinates POMgcs is applied in this reanalysis. A new full-space multi-scale data assimilation method applicable to the analysis of the ocean front is developed. Through adopting the penalized smooth term, the "bull's eye" is settled, which ensures the objectivity of this new method. This new method can extract different wave-length information in turn and can make full-space 3D variational (3D-Var) data assimilation, so it overcomes the shortcomings of the traditional 3D-Var data assimilation method which only extracts the specific wavelength information and results in false analysis gradient due to the stratified assimilation.As for the reanalysis results above, by using the same stronger judgment criterion for oceanic front, the oceanic thermal front information is extracted. The climatology statistic results of the Kuroshio thermal front to the east of Taiwan are basically consistent with the above simulated results. From the maximum entropy spectral analysis, this thermal front not only has the significant annual cycle, but also has the significant multi-scale variation cycle, like 2.1a,195d,124d,90d and 59d.By using the reanalysis results and the extracted thermal front information, the frontogenesis and changing mechanisms of the Kuroshio thermal front to the east of Taiwan are studied. The Kuroshio to the east of Taiwan carries vast water with high temperature and high salinity, and the Kuroshio warm advection raise the water temperature at the upper level ocean to the east of Taiwan, which can enlarge the gradient of the water temperature at the both side of the current axis of the Kuroshio. At the same time, the upwelling is strong due to water depth varying sharply in the coast to the east of Taiwan, and the elevating cold water convenes with the high-temperature Kuroshio water, forming stronger horizontal temperature gradient, thereafter, the Kuroshio thermal front is created in the offshore area to the east of Taiwan. The cross spectral analysis of the thermal front intensity and the Kuroshio flux and that of thermal front intensity and the upwelling velocity of flow show that the stronger the upwelling and the Kuroshio flux are, the stronger the intensity of the Kuroshio thermal front is. Because the upwelling and the Kuroshio flux are strongest in summer, the Kuroshio thermal front under the surface caused by the cooperation of them in this area is the strongest in summer. It is found through the diagnostic analysis that the contribution of the upwelling to this thermal front is three times than that of the Kuroshio warm advection, that is to say, the upwelling is the main mechanism for the frontogenesis and maintenance of this thermal front. The upwelling velocity reachs its maximum at 200m level and from this level to surface reduces to almost zero. Therefore, the surface thermal front to the east of Taiwan is not obvious while the subsurface frontal intensity is stronger. The presence of vortex may influence this thermal front. The result indicates that cold vortex (warm vortex) can reduce (raise) the water temperature to the right of the Kuroshio thermal front to the east of Taiwan, and weaken (strengthen) the bilateral temperature difference of the front, thereby weaken (strengthen) the strength of the Kuroshio thermal front to the east of Taiwan and narrow (widen) the width of this front.The frontal wave in the reanalysis result is studied in this paper. It is found through the diagnostic analysis of the energy source of the frontal wave that the contribution of barotropic instability or that of baroclinic instability is more than that of K-H instability by 1-2 order of magnitude, and the contribution of the baroclinic instability is 5 times than that of the barotropic instability, thereby the frontal wave is basically driven by the baroclinic instability.