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Distribution and Modification of North Pacific Intermediate Water around the Subarctic Frontal Zone East of 150°E

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  • 1 National Research Institute of Far Seas Fisheries, Fisheries Research Agency, Yokohama, Japan
  • | 2 Ocean Research Institute, University of Tokyo, Tokyo, Japan
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Abstract

Modification and distribution of North Pacific Intermediate Water (NPIW) in the area north of 30°N and east of 150°E are discussed based on an isopycnal particle tracking experiment with the velocity field estimated from a β-spiral inverse method based on climatological data, annual climatological data analysis, and isopycnal-tracking profiling float data. NPIW emanating from 149.5°E between 35.5° and 37.5°N retains its salinity minimum structure along the eastward flow of the northern subtropical gyre, and these pathways occupy the northern part of the NPIW distribution. On the other hand, NPIW from 149.5°E between 37.5° and 40.5°N loses its salinity minimum as it flows eastward to the “transition domain.” The extinction of the salinity minimum structure is caused by a salinity decrease in the upper layer (26.5–26.6 σθ) in the region at 40°–45°N, 155°–165°E where a winter mixed layer is developed to a depth of 300 m and a density of 26.7 σθ. This extinction was observed by a profiling float tracking a 26.7-σθ isopycnal density surface. Low-salinity subarctic water advected southward across the Subarctic Front by wind-induced Ekman drift from strong westerlies in winter is suggested as the source of the low-salinity mixed layer. After the extinction of the salinity minimum structure, a state without salinity minimum is maintained during transport eastward in the transition domain because the winter mixed layer in the area east of the deep mixed layer region reaches a density surface shallower than 26.5 σθ.

Corresponding author address: Masachika Masujima, National Research Institute of Far Seas Fisheries, Fisheries Research Agency, 2–12–4 Fuku-ura, Kanazawa-ku, Yokohama-si, Kanagawa 236–8648, Japan. Email: masujima@affrc.go.jp

Abstract

Modification and distribution of North Pacific Intermediate Water (NPIW) in the area north of 30°N and east of 150°E are discussed based on an isopycnal particle tracking experiment with the velocity field estimated from a β-spiral inverse method based on climatological data, annual climatological data analysis, and isopycnal-tracking profiling float data. NPIW emanating from 149.5°E between 35.5° and 37.5°N retains its salinity minimum structure along the eastward flow of the northern subtropical gyre, and these pathways occupy the northern part of the NPIW distribution. On the other hand, NPIW from 149.5°E between 37.5° and 40.5°N loses its salinity minimum as it flows eastward to the “transition domain.” The extinction of the salinity minimum structure is caused by a salinity decrease in the upper layer (26.5–26.6 σθ) in the region at 40°–45°N, 155°–165°E where a winter mixed layer is developed to a depth of 300 m and a density of 26.7 σθ. This extinction was observed by a profiling float tracking a 26.7-σθ isopycnal density surface. Low-salinity subarctic water advected southward across the Subarctic Front by wind-induced Ekman drift from strong westerlies in winter is suggested as the source of the low-salinity mixed layer. After the extinction of the salinity minimum structure, a state without salinity minimum is maintained during transport eastward in the transition domain because the winter mixed layer in the area east of the deep mixed layer region reaches a density surface shallower than 26.5 σθ.

Corresponding author address: Masachika Masujima, National Research Institute of Far Seas Fisheries, Fisheries Research Agency, 2–12–4 Fuku-ura, Kanazawa-ku, Yokohama-si, Kanagawa 236–8648, Japan. Email: masujima@affrc.go.jp

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