The Year-Round Upwelling at the Shelf Break Near the Northern Tip of Taiwan as Evidenced by Chemical Hydrography
Other Title
臺灣背部外海大陸棚緣湧升流之全年化學水文觀測
Journal
Terrestrial, Atmospheric and Oceanic Sciences
Journal Volume
3
Journal Issue
3
Pages
243-275
Date Issued
1992
Author(s)
Abstract
臺灣北部大陸棚邊緣有湧升流，這是黑潮與東海陸棚接觸後所產生的一個重要的物理現象，並造成明顯的化學及生物變化。過去經常觀察到在海面上有一片異常冷之水域，即湧升區。然而，此種溫度的對比並不一定常年存在，因為陸棚水在冬季會變冷，甚至比湧升區之水溫更低。因此，以往並沒有全年的資料顯示湧升流是經常存在的。從1990年8月到1991年7月，本研究執行了十一次的海上探測，由溫度、硝酸鹽及溶氧的分佈顯示，湧升流的確經年存在著。當冬季溫度及硝酸鹽的分佈無法顯示湧升流的全貌時，溶氧是一個特別有用的指標，它顯示冬季時湧升流的丘狀構造與其他季節相似。在十二月至五月間，湧升區的表水都有溶氧不飽和的現象。利用一個盒式模型可估計，在1991年3月間湧升區的平均溶氧速率約為0.034莫耳／平方公尺／天。如要維持表層海水的不飽和，它必須以每天5公尺的速率被下層海水所取代。在面積為2900平方公里的湧升區內，總湧升流量約為0.2Sv，所攜帶的硝酸根為2×10^(9)公克氮／天。
Upwelling of the Kuroshio subsurface water at the shelf break near the northern tip of Taiwan is an important oceanic phenomenon, that results from impingement of the Kuroshio onto the continental shelf and causes profound chemical and biological responses. This upwelling region was frequently observed in the past as a pool of cold water. However, such temperature contrast does not exist throughout the year due to the cooling of the shelf water in winter. Consequently, it has never been established until now that the upwelling is a year-round phenomenon. In this study, monthly cruises were conducted from August 1990 to July 1991, and a one year-long monthly record of up- welling was evidenced from the distribution of temperature, nitrate and dissolved oxygen. The last, which revealed the structure of the upwelling dome in winter similar to those in other months, was an especially useful indicator of upwelling when temperature or nitrate concentration failed to show the whole picture of upwelling. Undersaturation of oxygen in the surface water at the upwelling center was observed during December-May. The average downward flux of oxygen in the upwelling area was computed to be 0.034 mollm2/day in March 1991. According to a box model for thermal and mass balance, an upwelling speed of about 5 m/day was required to maintain the oxygen-deficiency in the surface layer. The upwelling velocity yielded a total volume transport of 0.2 Sv and nitrate transport of 2xlO9gN/day to the top 60m in the upwelling area of 2900km2.
Type
journal article
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