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Direct Measurements of the Luzon Undercurrent

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  • 1 * Institute of Oceanology, Chinese Academy of Sciences, and Key Laboratory of Ocean Circulation and Wave, Chinese Academy of Sciences, Qingdao, China
  • | 2 Institute of Oceanology, Chinese Academy of Sciences, and Key Laboratory of Ocean Circulation and Wave, Chinese Academy of Sciences, Qingdao, and University of Chinese Academy of Sciences, Beijing, China
  • | 3 Physical Oceanography Laboratory, Ocean University of China, Qingdao, China
  • | 4 Institute of Oceanology, Chinese Academy of Sciences, and R&D Center of Marine Environmental Engineering and Technology, Chinese Academy of Sciences, Qingdao, China
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Abstract

The Luzon Undercurrent (LUC) was discovered about 20 years ago by geostrophic calculation from conductivity–temperature–depth (CTD) data. But it was not directly measured until 2010. From November 2010 to July 2011, the LUC was first directly measured by acoustic Doppler current profiler (ADCP) from a subsurface mooring at 18.0°N, 122.7°E to the east of Luzon Island. A number of new features of the LUC were identified from the measurements of the current. Its depth covers a range from 400 m to deeper than 700 m. The observed maximum velocity of the LUC, centered at about 650 m, could exceed 27.5 cm s−1, four times stronger than the one derived from previous geostrophic calculation with hydrographic data. According to the time series available, the seasonality of the LUC strength is in winter > summer > spring. Significant intraseasonal variability (ISV; 70–80 days) of the LUC is exposed. Evidence exists to suggest that a large portion of the intraseasonal variability in the LUC is related to the westward propagation of mesoscale eddies from the east of the mooring site.

Corresponding author address: Dunxin Hu, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China. E-mail: dxhu@qdio.ac.cn

Abstract

The Luzon Undercurrent (LUC) was discovered about 20 years ago by geostrophic calculation from conductivity–temperature–depth (CTD) data. But it was not directly measured until 2010. From November 2010 to July 2011, the LUC was first directly measured by acoustic Doppler current profiler (ADCP) from a subsurface mooring at 18.0°N, 122.7°E to the east of Luzon Island. A number of new features of the LUC were identified from the measurements of the current. Its depth covers a range from 400 m to deeper than 700 m. The observed maximum velocity of the LUC, centered at about 650 m, could exceed 27.5 cm s−1, four times stronger than the one derived from previous geostrophic calculation with hydrographic data. According to the time series available, the seasonality of the LUC strength is in winter > summer > spring. Significant intraseasonal variability (ISV; 70–80 days) of the LUC is exposed. Evidence exists to suggest that a large portion of the intraseasonal variability in the LUC is related to the westward propagation of mesoscale eddies from the east of the mooring site.

Corresponding author address: Dunxin Hu, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China. E-mail: dxhu@qdio.ac.cn
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