Distribution And Variations Of The Thermal Fronts In The Eastern China Seas

Oceanic fronts are results and indicators of the physical processes, such as currents, upwellings and vertical mixing. Oceanic fronts can influence the circulation pattern, the exchange in water masses and heat, the interaction of air-sea, the bio-chemical processes and the propagation characteristics of the sound. Thus, oceanic front becomes one of the key points in the physical oceanography and also in the inter-disciplinary studies.Based on the AVHRR Pathfinder satellite SST data from 1985 through 2002 and simulation results from POM, the dissertation focuses on the distribution, variation, dynamical mechamism and the environmental effects of thermal fronts in the eastern China Seas (eCS, including the Bohai Sea, the Yellow Sea (YS), the East China Sea (ECS) and the northern South China Sea). The dissertation firstly describes the distribution and variations of fronts in the eCS as a whole, and then investigates 3 typical fronts, say, a coastal front in the western South YS, a continental shelf front in the Yellow Sea Warm Current (YSWC) origin area and a shelf-break front (the Kuroshio front) in the ECS.Fronts in the eCS show clear seasonal variations: in wintertime, 14 main fronts including warm current fronts and coastal current fronts are clearly distinguished; in spring, the eCS shows irregular frontal zones with high intensity and gradual shift from the Kuroshio domain to middle YS; in summertime, the upwelling fronts and the tidal fronts occur in coastal areas. In fall, few fronts can be distinguished. For fronts in wintertime, those generated mainly by warm currents or less influenced by the bathymetric topography show large interannual variations while those generated mainly by coastal currents or controlled by the bathymetric topography show weak interannual variations.The N-shape front in the western YS in winter composes of a west and an east wings roughly along the northeast-southwestward isobaths with a southeastward middle segment across isobaths of 20~50 meters. Numerical simulation reveals that a cold coastal jet flowing anticyclonicaly below the 10 m depth after bypassing the Shandong Peninsula and the cold water penetrates southward into the western South YS. On the south, warmer water from a northwestward YSWC branch and from the northward YSWC also intrude into the western South YS. The N-shape front forms between the cold water and the surrounded warm water, and then it extends upwards to the surface through vertical advection and mixing.The SST climatology in winter clearly shows a westward shifted warm tongue originating from the YSWC origin area. Strong fronts occur on both northern (hereafter YF-N) and southern (YF-S) sides of the warm tongue. The strong interannual variation of the westward shift shows positive relation with the SST in the entire YSWC domain, indicating the shift originates from the YSWC origin area. The westward shift of the warm tongue and the meridional location of the YF-N show strong relationship: the more southward or stronger the Y-NF, the more westward of the warm tongue, indicating the later blockes the YSWC from flowing northward along the YS trough and induces the YSWC to flow northwestward. Significant relation is found between the meridional location of YF-N and cold water in the South YS interior while weak relations are shown between the location of YF-N and the local wind, the cold water in the YS interior as well as the strength of the YSWC. The cold water in the South YS interior may influence the westward shift through influencing the YF-N. Thus, we believe that the location and strength of the YF-N are important factors in generating and controlling the westward shift of the warm tongue. SST and altimetry data also show a possible connection between the Taiwan Warm Current and the variations of the YSWC thermal structures.The Kuroshio Front shows anti-phase interannual variations between areas of northeast of Taiwan and southwest of Kyushu. The anti-phase variations show good relationship with the observational data of the Kuroshio transport and the wind stress magnitude. When the Kuroshio transport decreases along the PN section or when wind stress increases in the middle ECS, the amplitude of the interannual variation increases. Numerical simulation results from case tests further demonstrate these relations. The balance between the transport of potential vorticity, the wind stress curl and the joint effect of baroclinic and bottom release may control the intrusion degrees of the Kuroshio onto the shelf. Thus, differences between the balance patterns in the two areas may be the possible reasons for the anti-phase interannual variation.