以水下滑翔觀測儀(Seaglider)資料探討臺灣東邊海域熱帶水團的變化

Abstract

本研究使用水下滑翔觀測儀(Seaglider)在臺灣東部海域6次觀測任務收集的溫鹽水文資料，輔以Argo 浮標水文資料，以前人研究定義之臺灣周圍主要水型的溫鹽曲線為基準，判讀臺灣東部海域熱帶水團(Tropical water mass)並檢視其時空變異性。為解決人工判讀溫鹽曲線耗時且缺乏客觀的問題，本研究提出一個可以量化水團與水型(Water type)間相似性的水團指標(Water mass index, WMI)，能將水團空間分佈以視覺化呈現。分析結果顯示，臺灣東部海域熱帶水水團主要由西菲律賓海熱帶水(West Philippine Sea Tropical Water)構成，但臺灣東部近岸及呂宋海峽周遭經常有南海熱帶水(South China Sea Tropical Water)的成分，離岸的開放海域雖與北太平洋熱帶水(North Pacific Tropical Water)有不同程度的混合，但主要仍以西菲律賓海熱帶水為主，惟其中一次在2016年底至2017年初的觀測任務為例外，當時北太平洋熱帶水顯著向西擴展。分析發現此太平洋熱帶水異常分佈的發生與2015年的強聖嬰現象有關，北太平洋熱帶水的主要源頭是副熱帶次表層水(Subtropical Underwater)，因大氣環境蒸發量大於降雨量使得海表高鹽水下沉所形成，2015年的強聖嬰現象，使整個西太平洋蒸發量大於降雨量的區域，由原本的大洋中與東部大幅向西擴展，連帶使得北太平洋熱帶水範圍向西擴展，此大氣驅動與導致的水團變化約有2年延遲。

The present study examines the water mass off the eastern coast of Taiwan using temperature-salinity (T-S) observations collected from the six Seaglider missions, supplemented by the Argo float measurements. We identify water masses by comparing their T-S distribution with previously defined water types. We then examine the spatiotemporal variability of water masses around Taiwan. We propose the Water Mass Index (WMI) to quantify the similarity between T-S distribution and defined water types. This index can be integrated into geographic information to visualize the spatial distribution of water mass. The analysis results indicate that the West Philippine Sea Tropical Water (WPSTW) is the primary component of the water mass on the eastern coast of Taiwan. The South China Sea Tropical Water (SCSTW) can be found near the coast of Taiwan and in the Luzon Strait. Intermittently, the North Pacific Tropical Water (NPTW) mixes with the WPSTW to varying degrees in the open ocean. The WPSTW remains dominant. The NPTW extended significantly westward during a Seaglider mission from late 2016 to early 2017, resulting in an exception. Analysis indicates the abnormal water mass distribution is related to the strong El Niño effect in 2015. The Subtropical Underwater results from excess evaporation over precipitation, causing saltier water on the surface to sink into the subsurface. The Subtropical Underwater is the origin of NPTW. The area having greater evaporation than precipitation during the 2015 El Niño year extends from the middle and eastern parts of the North Pacific to its western region. Therefore, the prevailing NPTW east of Taiwan is a result of atmospheric forcing, which has a time lag of ~ 2 years.