Using pressure-sensor equipped inverted echo sounder (PIES) to estimate variation in hydrographic field ─ Application to the variability off the east of Taiwan and north east of Luzon

本研究使用位在臺灣東部及呂宋東北部海域的兩條倒置式迴聲測深儀(pressure-sensor equipped echo sounder, PIES)測線觀測得到的聲傳時間資料，探討由東而西的海表面高度異常系統接觸黑潮後的相互影響與宿命。PIES觀測資料聲傳時間的變化與衛星測高資料海表面高度異常值在0時間差時通常為負相關，海面高度下降伴隨水下密度躍層抬升，反之海面高度上升伴隨水下密度躍層下降。但是兩條測線聲傳時間的變化與海表面高度異常值的負相關係數在進入黑潮主要流幅後逐漸下降，顯示海表面高度異常系統對密度躍層深度直接的影響在橫越黑潮的過程中逐漸減弱，另方面海表面高度異常系統也被黑潮的平流作用帶往下游離開PIES觀測斷面。由PIES聲傳時間資料與衛星遙測海表面高度異常值的相關分析結果發現，在臺灣東部北緯23.75°的測線觀測資料中，黑潮東西側海表面高度與等密面變化為反相位，當海表面高度異常系統的相對渦度和科氏參數為同一量級時，黑潮可能受到海表面高度異常系統所影響產生流徑和通量的變化，進而調整其海表面高度與等密面梯度，此外，當海表面高度異常系統的相對渦度為科氏參數的1/10時，海表面高度異常系統無法引起黑潮顯著的改變，但海表面高度異常系統本身受黑潮流域有效科氏參數(effective Coriolis parameter)的影響也可能造成水位梯度與等密面梯度產生調整。在臺灣及呂宋測線最東邊的PIES測站分別觀測到7個與9個密度躍層深度變動事件，進一步分析每個密度躍層深度變動事件與海表面高度異常系統時空變化的關係顯示，觀測海域的密度躍層可能同時受到來自東邊及南邊的海表面高度異常系統影響，且為海表面高度異常系統大小、強度、形狀和位置等因素綜合影響下之結果，遠比數值模擬理想情境下的結果複雜許多。在兩測線觀測重疊的半年內，亦只觀測到一次高壓海表面高度異常系統在影響呂宋測線密度躍層深度21天後影響臺灣東部測線上的密度躍層深度。The influence and fate of westward-propagating eddies that impinge on the Kuroshio were observed with pressure-sensor equipped inverted echo sounders (PIESs) deployed east of Taiwan and northeast of Luzon. Zero-lag correlations observed between PIES-measured acoustic travel times and satellite-measured sea surface height anomalies are negative, consistent with the pycnocline shoaling and deepening beneath sea surface lows and highs, respectively. Correlations have lower magnitude towards the west, suggesting eddy-influence weakens across the Kuroshio. Anticyclones intensify sea surface and pycnocline slopes across the Kuroshio, while cyclones weaken these slopes. During the 6 month period of overlap between the PIES arrays, one anticyclone affected the depth of the pycnocline first at the array northeast of Luzon and 21 days later off Taiwan. Kuroshio response to eddies is complicated due to the variations in eddy strength and shape and due to the simultaneous influences of multiple eddies arriving from different directions