以GRACE衛星偵測東海陸棚沉積物及黑潮質量變化

Abstract

本研究主要以CSR (Center for Space Research at University of Texas)、GFZ (GeoForschungsZentrum Potsdam)及JPL (Jet Propulsion Laboratory)提供GRACE (Gravity Recovery and Climate Experiment)最新Release-05時變重力場的各月球諧係數，偵測東海陸棚沉積物及臺灣附近黑潮海域的質量變化。東海陸棚因其海底地形寬且淺，並受到季節性北向的臺灣暖流與南向的東海沿岸流交互作用影響，形成一有利於長江沉積物堆積於此的天然環境。黑潮為北太平洋副熱帶西部邊界流，黑潮起源於北赤道洋流向北流的分支，其主軸流經呂宋島與臺灣東岸，其分支通過呂宋海峽入侵南海北部。GRACE計算過程中，考量本研究區位處海陸交界地帶，將大氣及海洋負載回加至GSM (GRACE Satellite-only Model)重力場，以避免模式影響有關沉積物及黑潮質量的訊號。另使用去條帶(destriping)濾波與高斯平滑濾波以提升GRACE訊號，並採用GLDAS (Global Land Data Assimilation System)水文模式的土壤濕度去除洩漏效應(leakage effect)，重力變化可由GRACE計算的等效水高(equivalent water height, EWH)表示。在探討東海陸棚沉積物質量累積速率及其時變性方面，以GRACE推算2002年4月至2015年3月的各月等效水高及其線性速率，等效水高線性速率為6.25 ± 0.54 mm/yr，其空間分布與實測資料相近，並以小波分析東海陸棚沉積物質量於半年至跨年間的時變性。為檢測GRACE可否觀測全球主要河口的沉積物質量累積速率，另估算亞馬遜河、剛果河、印度河、密西西比河、萊茵河及珠江等六個主要河口的質量累積速率。在探討臺灣附近黑潮海域質量變化方面，以GRACE推算2002年4月至2016年8月的各月等效水高及其線性速率。臺灣東北部海域自2003年至2012年等效水高線性速率為1.59 ± 0.43 mm/yr，南海北部自2003年至2012年等效水高線性速率為-1.77 ± 0.40 mm/yr，顯示因黑潮沿呂宋島及臺灣東部主軸強化，使黑潮慣性逐漸增加及入侵南海北部現象減弱，另以SODA (Simple Ocean Data Assimilation)海洋模式計算海水單位面積之質量驗證GRACE計算結果。自副熱帶反流(Subtropical Countercurrent, STCC)向西移動的渦漩至臺灣東部海岸將與黑潮產生交互作用。臺灣目前設置兩部超導重力儀：SG-T048於2006年設置於國家重力基準站，SG-T049則於2012年設置於陽明山擎天崗衛星追蹤站。由於SG-T049高度為759.6 m，較有可能偵測臺灣周圍的海水質量變化，考量黑潮及渦漩引起的重力訊號較小，SG-T049原始重力需經環境重力效應改正以獲得包含此兩效應的剩餘重力，並以小波分析SG-T049自2013年1月至2016年6月剩餘重力訊號，以期偵測黑潮及渦漩引起的重力訊號。模擬由不同尺度的渦漩於SG-T048及SG-T049引起的牛頓吸引力與彈性負載變化，結果顯示重力變化可達至數百ngal。惟真實黑潮及渦漩重力訊號混合，需長期且穩定的超導重力訊號以及精進的環境重力效應才能有效將黑潮及渦漩引起的重力變化分離。

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.