臺灣東南海域夏季期間水文及生地化特性變動

Abstract

臺灣東南海域位處西北太平洋內的副熱帶反流區(Subtropical Countercurrent, STCC)，常見由斜壓不穩定(Baroclinic instability)引起的中尺度渦旋(Mesoscale Eddy)；除此以外，西北太平洋亦為颱風生成的熱區，兩者在上層海洋的物理現象和生地化作用上扮演重要角色。自2018至2020年期間的夏季，國立臺灣大學海洋研究所佈放於西北太平洋的兩組海氣象錨碇浮標NTU1及NTU2，觀測到一個氣旋式渦旋(Cyclonic Eddy, CE)和一個反氣旋式渦旋(Anti-Cyclonic Eddy, ACE)及數個颱風，渦旋和颱風顯著地改變上層海洋水文結構。掛載於浮標錨串上的水文、氣象、溶氧、螢光和pH各式探針，提供了寶貴的時間序列觀測資料，有助於更深入了解西北太平洋地區海洋生地化環境。本論文科學目標為探討中尺度渦旋與颱風經過時，上層海洋水文與生地化參數伴隨的變化，以及海氣之間氣體通量的交換。此外，透過一維溶氧和總無機碳的質量平衡模式，在本研究中亦嘗試推估臺灣東南海域夏季時上層海洋的生物群集淨生產力(Net Community Production, NCP)。 分析結果顯示，溶氧濃度和葉綠素濃度在浮標測站上層海洋，各存在一個極大值，分別位於水深約70和90公尺處，其極大值約為200 μM和0.2 μg L-1。在近表層處，溶氧濃度變化受太陽短波輻射主導，但可能還受中尺度渦旋等機制影響。在2019年六至八月氣旋式中尺度渦旋通過期間，溶氧濃度和溫度在水深50公尺處的全日潮和半日潮頻帶上，分別有高達30 μM和5 ℃的振幅，其變化量隨深度增加而逐漸減小；2018年八月反氣旋式中尺度渦旋通過期間，振幅則分別縮小至5 μM和1 ℃。此現象可能是由於中尺度渦旋通過後影響上層海洋溫度結構，使內潮垂直速度、節點(Node point)位置和背景濃度梯度的垂直結構發生變化所致。 颱風經過對研究海域的影響，本論文以2019年強度為C1的玲玲颱風為實例，其34節風暴風圈半徑(R34)通過浮標期間，溶氧濃度極大值深度上層和下層分別有顯著上升及下降的現象。根據Price-Weller-Pinkel (PWP) 3D模式模擬結果，此段期間上層海洋主要受到垂直對流主導，其在水深約70公尺處造成約30公尺的等密度線垂直向上位移，使水深50和100公尺處溶氧濃度分別上升和下降。在2020年閃電颱風通過NTU1測站期間，分別約有700 mmol m-2的氧氣及640 mmol m-2的二氧化碳從大氣被帶入海洋，其中氧氣通量約佔表水層一公尺溶氧總量的13%，二氧化碳通量則約為非颱風時期的三至四倍。另藉由一維溶氧濃度質量守恆模式，在2020年夏季期間NTU1測站定義與海表溫相差攝氏0.8度的混合層中，群集淨生產力平均約為45 mmol m-2 d-1。

In the northwestern subtropical Pacific Ocean (NWSTP), the region of the Subtropical Countercurrent (STCC) is known for its abundant mesoscale eddies, which are primarily caused by baroclinic instability. In addition, the NWSTP serves as a hotspot for typhoon formation. Both the mesoscale eddies and typhoons play a crucial role in the physical and biogeochemical processes of the upper ocean. During the summer of 2018 to 2020, we deployed two metocean moored buoys, NTU1 and NTU2, in the NWSTP. These buoys observed one cyclonic eddy (CE), one anti-cyclonic eddy (ACE) and several typhoons passing by, which resulted in significant change of the hydrographic profile. The data buoys could provide a continuous dataset of in-situ meteorology, hydrography, dissolved oxygen (DO) concentration, chlorophyll-a and pH in the upper ocean, which helped us better understand physical dynamics, variation of biogeochemical parameters and air-sea gas fluxes under the influence of mesoscale eddies and typhoons. Besides, using the one-dimension calculation of the mass conservation of oxygen and dissolved inorganic carbon, we estimated the net community production (NCP) in the upper ocean of the NWSTP during the summer. The result showed that DO concentration and chlorophyll-a had their maximum values of approximately 200 μM and 0.2 μg L-1 at depth of 70 and 90 meters, respectively. In addition to the diurnal irradiance, DO concentration near the surface layer may also be affected by mesoscale eddy. The thermocline deepened (uplifted) during the periods of ACE and CE from June to August 2019 and in August 2018, respectively. Additionally, spectrum analyses showed that the amplitude of diurnal and semi-diurnal bands on DO concentration and temperature had significant fluctuations up to 30 μM and 5 ℃, respectively, at 50 meters depth, whereas they narrowed to 5 μM and 1 ℃ during the ACE period. The CE and ACE could modulate the thermal structure through upwelling and downwelling, respectively, which changed the amplitude and node point of internal tide, as well as the vertical structure of background concentration gradient. Influenced by typhoon Lingling in 2019, with an intensity classified as C1, DO concentration increased (decreased) above (below) depth of its maximum value during the transit period of its 34-knot wind radius (R34), respectively. Simulated by Price-Weller-Pinkel (PWP) 3D numerical model, we found that upper ocean was dominated by vertical advection, which caused upward vertical displacement of the isopycnal by 30 meters at depth of 70 meters. This could result in an increase in DO concentration at a depth of 50 meters and a decrease at a depth of 100 meters. Besides, during the passage of typhoon Atsani in 2020, about 700 mmol m-2 of oxygen and 640 mmol m-2 of carbon dioxide entered the ocean from the atmosphere at NTU1 buoy, which accounted for about 13% of total DO in the surface layer and was about three to four times larger than non-typhoon period, respectively. Finally, based on the observation of NTU1 in 2020, the average of NCP was about 45 mmol-C m-2 d-1 in the mixed layer, with SST–TMLD = 0.8℃.