臺灣東南海域綠島海脊上海流的變化

Abstract

黑潮是流經臺灣東部海域重要的洋流，源於北緯10度至20度由東向西流的北赤道洋流，當北赤道洋流向西流至菲律賓東邊附近海域後分為南北兩支流，北向分支即為黑潮，沿菲東岸往北流經呂宋海峽流向臺灣東部海域。然而過去研究中，臺灣東南海域長時間的現場觀測資料較為稀少，為了對流經此區域的黑潮以及此區域的潮汐有更詳細的了解，有必要進一步的研究討論。本研究主要利用五組位於綠島西北面海域的錨碇式都卜勒流速剖面儀於2011年5月到2012年8月期間所測得的海流資料，來探討臺灣東南海域海流的變化。 觀測流速資料顯示較靠近綠島的S1站海流流向為北北東，北向流速最快可達2.26 m/s，靠近臺灣的S5站海流流向為東北，平行於臺灣東海岸線，北向流速可達1.77 m/s。流速資料經頻譜分析的結果顯示此區域潮流運動以O1潮、K1潮、M2潮為主，各分潮的潮流橢圓垂直結構也顯示本區域潮流包含了正壓以及斜壓潮流運動，本研究利用深度平均法將正壓潮與斜壓潮分解，結果顯示斜壓潮呈現一個節點以及兩個節點為主的情形。進一步將觀測海流資料經過16天的低通濾波濾除相對高頻的潮汐訊號後，從流速時序資料可看到數個季節內低頻的變化，本文定義北向流速在兩個月內變化超過0.1 m/s 為變動事件，歸納總共連續15個月橫跨夏季以及冬季的觀測資料，共有5次流速增強的事件以及3次流速減弱的事件。利用衛星觀測海面高度資料研究海表面高度異常的分佈以及了解海流於空間上的變化，結果顯示5次增強事件中可能有4次與正的海表面高度異常有關，而3次減弱事件中有2次可能與負的海表面高度異常有關。此外，有些流速的增減事件也與上游黑潮路徑向太平洋方向偏移或者入侵南海等現象有關。而海表面高度異常的分佈可能會改變觀測區域的流速，亦可能改變本研究區域的水文垂直結構進而改變斜壓潮流流場。

Kuroshio is the major current flowing through the east of Taiwan, which is originated from the North Equatorial current between 10 degrees to 20 degrees north latitude.When the North Equatorial current flows westward to the east of Philippines, it is divided into north and south branch.The north branch is the Kuroshio,and it flows along the east coast of Philippines and then truns northward to the Luzon Strait to the east of Taiwan. There were few mooring observations of the current in the southeast of Taiwan in previous studies. In this study, we used five Acoustic Doppler Current Profiler which set up on the northwest of Green Island from May 2011 to August 2012,to explore the variation of this current in the southeast of Taiwan. The current velocity data showed that the current flow is north north east at the S1 station which is near to Green Island. The velocity of the northward current can reach 2.26 m/s. Similarly, the current flow is northeast at the S5 station which is close to Taiwan, and it is parallel to the coastline of east Taiwan, and the velocity of the northward current can reach 1.77 m/s . Using Spectrum analysis to study the velocity data, the results showed of O1、K1、M2 tidal currents in this area significantly. The vertical structure of each tidal currents also showed that the tidal currents in this area are containing the barotropic tide and baroclinic tide.This study discussed the barotropic tide and baroclinic tide by depth-average separating. The results showed that the 1st baroclinic mode and 2nd baroclinic mode dominated. Further this study analyzed the current by using the sixteen days low pass filter to filter the tidal signals which are relative high frequency. After low pass filter, the current displays many of the intraseasonal variations of the low frequency.This study defined the north velocity changes more than 0.1 m/s for the event during two months. Sumed up the observation in a total of 15 consecutive months which across the summer and winter, there are 5 velocity enhancing events and 3 velocity decreasing events. Using the satellite altimetry data sea level anomaly to distinguish eddies and to study the currents on the horizontal space. The results showed that there were four times events contacted with the warm eddy within five enhancing events and two times events contacted with the cold eddy within three decreasing events. In addition, the velocity enchancing events and the decreasing events were also contacted with the path of Kuroshio shifted to the Pacific Ocean or the invasion of the South China Sea. The eddies not only changed the velocity but also change the hydrology of the vertical structure and then changed the baroclinic tidal currents.