| Summary: | 博士 === 國立交通大學 === 土木工程系所 === 105 === This study aims to detect the mass variations associated with sedimentation in the East China Sea (ECS) and transport change of the Kuroshio Current (KC) around Taiwan using temporal gravity fields, in the form of monthly spheirical harmonic coefficients (SHCs), from the latest GRACE Release-05, provided by Center for Space Research at University of Texas (CSR), the Jet Propulsion Laboratory (JPL) and GeoForschungsZentrum Potsdam (GFZ). The ECS is a region with wide and shallow continental shelf, where the interaction of seasonal northward-flowing Taiwan Warm Current and southward-flowing China Coastal Current allow sediments from the Yangtze River to deposit on its inner shelf. The KC is a subtropical western boundary current in the North Pacific, and it originates from the north branch of the North Equatorial Current and flows northward along the east coasts of Luzon and Taiwan. The KC intrudes into the northern South China Sea through the Luzon Strait. Since our targeted area is adjacent to land-ocean boundary, the atmospheric and oceanic effects are restored to GSM (GRACE Satellite-only Model) gravity field to avoid model contaminations on gravity signals associated with sediment and Kuroshio masses. The destriping and Gaussian smoothing filters are applied in order to improve the gravity signals from GRACE. Soil moistures from the Global Land Data Assimilation System (GLDAS) are used to reduce land leakage effect. The gravity variations can be expressed as GRACE-derived equivalent water height (EWH). The sediment mass accumulation rates (MARs) and variabilities on the ECS inner shelf are evaluated. The mean GRACE-derived MAR on the inner ECS shelf is 6.25 ± 0.54 mm/year from April 2002 to March 2015, which is consistent with in situ measurements. A wavelet analysis is used to detect the oscillations at the semi-annual to interannual time scales. We examine whether GRACE can be used to detect MAR signals in six major estuaries. i.e., Amazon, Congo, Indus, Mississippi, Pear and Rhine River. To evaluate the mass change of Kuroshio around Taiwan, the monthly EWHs and their linear rates are derived from GRACE from April 2002 to August 2016. The EWH linear rate in sea area off northeastern Taiwan is 1.59 ± 0.43 mm/yr (2003-2012), while the EWH linear rate in the northern South China Sea is -1.77 ± 0.40 mm/yr (2003-2012). As the Kuroshio main current strengthens along the east coasts of Luzon and Taiwan, the inertia of Kuroshio’s flow is enhanced to weaken the intrusion of the Kuroshio into the northern South China Sea. This weakening is detected by GRACE and is validated by SODA-derived oceanic mass per unit area variations. Westward-propagating oceanic eddies around Taiwan in the region of Subtropical Countercurrent interact with the Kuroshio east of Taiwan. Two superconducting gravimeters have been installed in Taiwan: SG-T048 was installed at the National Gravity Datum Service (NGDS) laboratory in 2006 and SG-T049 was installed at the Mt. Yangming satellite tracking station in 2012. Since the elevation of the SG-T049 station is 759.6 m, making it possible to detect oceanic mass variations around Taiwan. Since gravity changes induced by Kuroshio and eddies are small, raw gravity records of SG-T049 must be corrected for environmental gravity effects to obtain the residual gravity containing gravity signals from the two sources. A wavelet analysis is used to detect such signals from the SG-T049 residual gravity over January 2013 to June 2016. Eddy-induced Newtonian and elastic gravity effects at SG-T048 and SG-T049 stations simulated induced by of various dimensions are simulated and the simulations show that, such gravity changes can be up to several hundreds of ngal. How well the Kuroshio and eddy-induce gravity changes can be separated depends on the long-term stability of SG records and the sophistication of the models of the environmental gravity effects.
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