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<p>Typhoons can induce significant changes in the upper ocean, increasing sea surface nutrients via mixing, entrainment, and upwelling, which often leads to a substantial phytoplankton bloom in an oligotrophic region of the South China Sea (SCS), subsequently triggers carbon cycling and ecosystem responses, and enhances local marine primary productivity. Using a coupled physical-biogeochemical model, we analyzed the dynamic and ecological responses of the shelf region near the Dongsha Islands in the SCS caused by the fast-moving Typhoon Hagupit, and investigated the underlying mechanisms of sea surface chlorophyll-<i>a</i> increase. Results indicate that after Hagupit, sea surface temperature along the typhoon path rapidly decreased by maximum of 5.2 °C, and the chlorophyll <i>a</i> along the typhoon path increased. In the shelf region near the Dongsha Islands, the sea surface chlorophyll <i>a</i> increased from 0.07 mg/m<sup>3</sup> (weekly average) to 0.14 mg/m<sup>3</sup> after Hagupit. The maximum increase reached 0.16 mg/m<sup>3</sup>, occurred 6 days after Hagupit, which is more than twice the average concentration before the typhoon. In contrast, the subsurface chlorophyll <i>a</i> decreased from 0.18 to 0.10 mg/m<sup>3</sup>. Power spectral analysis of horizontal velocity indicated that Hagupit triggered strong near-inertial waves (NIWs), which had a period of approximately 31.45 h, slightly greater than the local inertial period, and lasted for about one week. Therefore, we believed that while fast-moving typhoons cannot induce strong Ekman pumping velocity (EPV) as slow-moving ones do, they can trigger NIWs. The NIWs enhanced turbulent mixing in the upper 60 m, causing directly a rapid increase in sea surface chlorophyll <i>a</i> and nutrients after the typhoon. Two to three days later, the nitrates uptake by sea surface phytoplankton increased, promoting phytoplankton growth. Therefore, the variability in upper-ocean chlorophyll <i>a</i> induced by fast-moving typhoons involves not only strong dynamic processes but also significant participation of marine biological processes.</p>

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Mechanisms of upper ocean chlorophyll a response to fast-moving Typhoon Hagupit in South China Sea

  • 期刊论文

  • 发表日期：

    2025年9月15日

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  • Haoyu Wang,
  • Bing Yang,
  • Yuqi Yin,
  • Po Hu

摘要

Typhoons can induce significant changes in the upper ocean, increasing sea surface nutrients via mixing, entrainment, and upwelling, which often leads to a substantial phytoplankton bloom in an oligotrophic region of the South China Sea (SCS), subsequently triggers carbon cycling and ecosystem responses, and enhances local marine primary productivity. Using a coupled physical-biogeochemical model, we analyzed the dynamic and ecological responses of the shelf region near the Dongsha Islands in the SCS caused by the fast-moving Typhoon Hagupit, and investigated the underlying mechanisms of sea surface chlorophyll-a increase. Results indicate that after Hagupit, sea surface temperature along the typhoon path rapidly decreased by maximum of 5.2 °C, and the chlorophyll a along the typhoon path increased. In the shelf region near the Dongsha Islands, the sea surface chlorophyll a increased from 0.07 mg/m3 (weekly average) to 0.14 mg/m3 after Hagupit. The maximum increase reached 0.16 mg/m3, occurred 6 days after Hagupit, which is more than twice the average concentration before the typhoon. In contrast, the subsurface chlorophyll a decreased from 0.18 to 0.10 mg/m3. Power spectral analysis of horizontal velocity indicated that Hagupit triggered strong near-inertial waves (NIWs), which had a period of approximately 31.45 h, slightly greater than the local inertial period, and lasted for about one week. Therefore, we believed that while fast-moving typhoons cannot induce strong Ekman pumping velocity (EPV) as slow-moving ones do, they can trigger NIWs. The NIWs enhanced turbulent mixing in the upper 60 m, causing directly a rapid increase in sea surface chlorophyll a and nutrients after the typhoon. Two to three days later, the nitrates uptake by sea surface phytoplankton increased, promoting phytoplankton growth. Therefore, the variability in upper-ocean chlorophyll a induced by fast-moving typhoons involves not only strong dynamic processes but also significant participation of marine biological processes.