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<p>This study examines the interannual variability in nutrients and phytoplankton in the western North Pacific (WNP) using the eddy-resolving ocean simulation coupled with a biological model. We focus on the tropical and subtropical regions during the August–November (ASON) season, analyzing their relationship with ocean dynamics. Phytoplankton variability in the WNP largely follows heat content fluctuations in the top 100&#xa0;m. In contrast, the seasonal-mean impact of tropical cyclones (TCs) through pumping and mixing is negligible, and a clear relationship between TC activity and phytoplankton enhancement remains uncertain. Further investigation into ecosystem variability reveals distinct mechanisms governing nutrient and phytoplankton changes. In the tropical regions south of 15°N, thermocline shoaling driven by wind-induced 1.5-layer dynamics explains the simulated nutrient and phytoplankton increases. In the subtropics, the influence of subtropical mode water (STMW) is more pronounced, with the uplift of the upper thermocline playing a crucial role. In the subtropical region around 130°E–140°E, 20°N–25°N, variations in mixed-layer depth (MLD) near the MLD front regulate STMW thickness, which in turn modulates nutrient availability and phytoplankton variability through changes in the upper thermocline structure.</p>

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Nutrient and phytoplankton variability in the tropical and subtropical western North Pacific simulated using OFES2

  • 期刊论文

  • 发表日期：

    2025年8月23日

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  • Tomomichi Ogata,
  • Hidenori Aiki,
  • Fumiaki Kobashi,
  • Yoshikazu Sasai

摘要

This study examines the interannual variability in nutrients and phytoplankton in the western North Pacific (WNP) using the eddy-resolving ocean simulation coupled with a biological model. We focus on the tropical and subtropical regions during the August–November (ASON) season, analyzing their relationship with ocean dynamics. Phytoplankton variability in the WNP largely follows heat content fluctuations in the top 100 m. In contrast, the seasonal-mean impact of tropical cyclones (TCs) through pumping and mixing is negligible, and a clear relationship between TC activity and phytoplankton enhancement remains uncertain. Further investigation into ecosystem variability reveals distinct mechanisms governing nutrient and phytoplankton changes. In the tropical regions south of 15°N, thermocline shoaling driven by wind-induced 1.5-layer dynamics explains the simulated nutrient and phytoplankton increases. In the subtropics, the influence of subtropical mode water (STMW) is more pronounced, with the uplift of the upper thermocline playing a crucial role. In the subtropical region around 130°E–140°E, 20°N–25°N, variations in mixed-layer depth (MLD) near the MLD front regulate STMW thickness, which in turn modulates nutrient availability and phytoplankton variability through changes in the upper thermocline structure.