台灣東北海域之黑潮

Abstract

本研究使用一個海洋數值模式、衛星遙測資料以及實測航次資料完成以下研究，台灣東北海域數值模擬研究包含以下兩個部份：南東海陸棚湧升流之季節變化；納莉颱風與黑潮的海氣交互作用。 南東海陸棚湧升流為全年湧升的現象，不同季節受到不同機制影響呈現的季節變化趨勢也不相同，由表層積分至100米的平均垂直速度主要受黑潮擺動影響，夏天當黑潮遠離台灣東岸時，湧升較強，冬天黑潮入侵東海陸棚抑制了湧升流發展，於是湧升較弱，由表層至30米的垂直速度受當地風場的影響，冬天湧升較夏天強。 納莉颱風與黑潮之間存在著海氣交互作用，納莉颱風數度穿越黑潮使得其強度多次變化，當颱風行經黑潮北邊時，海洋形成了一個渦漩，在黑潮上與黑潮南邊則沒有觀察到此現象，原因來自受到黑潮強勁流速影響以及地形限制。當颱風行進速度緩慢時，能影響的海洋深度也較大，此外颱風也在海洋留下了震盪的現象，此現象由周期判定為為近慣性震盪。在海表面溫度圖當中所看到的冷水海域則是颱風過後所造成的湧升現象，由衛星測葉綠素甲圖以及海表面溫度圖得知，此湧升現象確實將次表層較冷與富營養鹽的海水帶至表層。Kuroshio off northeast Taiwan has been studied using a numerical ocean model and available observations. Particularly, the seasonal variability of upwelling near the southern East China Sea shelf and the interaction between typhoon Nari and Kuroshio have been extensively investigated. Based on model simulation, the vertical velocity in the upwelling region is positive in all months, indicating the upwelling is a year-round phenomenon. Furthermore, opposite seasonal tendencies of upwelling are found for different depth range. The vertical velocity integrated from surface to 100 m is weaker in winter than in summer, whereas it reaches its minimum during summertime in the top 30 m. Different mechanisms are responsible for this seasonal discrepancy. Seasonality in the upper 100 m is influenced by the Kuroshio migration. In winter, Kuroshio intrudes onto the shelf and prevents the upwelling developing, and therefore vertical velocity is weaker. As Kuroshio retreats from the shelf in summer, vertical velocity becomes stronger. On the other hand, seasonal tendency in the top 30 m is attributed to the local wind speed that is generally weaker in summer but becomes much stronger during winter. We also use the model results and satellite data to discuss the interaction between Typhoon Nari and Kuroshio. Nari ambled around and crossed Kuroshio several times, which causes variations in typhoon intensity. Model simulation suggests that a cyclonic eddy triggered by Nari has occurred in regions north of Kuroshio, as a result, the cold SST patch is only visible to the north of the Kuroshio axis. The cyclonic circulation penetrates much deeper for a slowly-moving storm, regardless of the typhoon intensity. Near-inertial frequency oscillations after typhoon departure are simulated by the model in terms of the vertical displacement of isotherms. The SST cooling caused by upwelling and vertical mixing is effective in cooling the upper ocean several days after the storm has passed. At certain locations, surface chlorophyll a concentrations increase significantly after Nari’s departure. Upwelling and mixing bring nutrient-rich subsurface water to the sea surface, causing enhancement of phytoplankton bloom.