Remote sensing observations of phytoplankton increases triggered by successive typhoons
Phytoplankton blooms in the Western North Pacific, triggered by two successive typhoons with different intensities and translation speeds under different pre-existing oceanic conditions, were observed and analyzed using remotely sensed chlorophyll-a (Chl-a), sea surface temperature (SST), and sea surface height anomaly (SSHA) data, as well as typhoon parameters and CTD (conductivity, temperature, and depth) profiles. Typhoon Sinlaku, with relatively weaker intensity and slower translation speed, induced a stronger phytoplankton bloom than Jangmi with stronger intensity and faster translation speed (Chl-a>0.18 mg·m‒3 versus Chl-a<0.15 mg·m‒3) east of Taiwan Island. Translation speed may be one of the important mechanisms that affect phytoplankton blooms in the study area. Pre-existing cyclonic circulations provided a relatively unstable thermodynamic structure for Sinlaku, and therefore cold water with rich nutrients could be brought up easily. The mixed-layer deepening caused by Typhoon Sinlaku, which occurred first, could have triggered an unfavorable condition for the phytoplankton bloom induced by Typhoon Jangmi which followed afterwards. The sea surface temperature cooling by Jangmi was suppressed due to the presence of the thick upper-ocean mixed-layer, which prevented the deeper cold water from being entrained into the upper-ocean mixed layer, leading to a weaker phytoplankton augment. The present study suggests that both wind (including typhoon translation speed and intensity) and pre-existing conditions (e.g., mixed-layer depths, eddies, and nutrients) play important roles in the strong phytoplankton bloom, and are responsible for the stronger phytoplankton bloom after Sinlaku’s passage than that after Jangmi’s passage. A new typhoon-influencing parameter is introduced that combines the effects of the typhoon forcing (including the typhoon intensity and translation speed) and the oceanic pre-condition. This parameter shows that the forcing effect of Sinlaku was stronger than that of Jangmi.
Keywordstyphoon mixed-layer depth phytoplankton bloom Northwest Pacific Ocean upwelling
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The present research is supported by 1) the Foundation for Distinguished Young Teacher in Higher Education of Guangdong (YQ2013092), 2) the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA11020305, CDA11010301), 3) Project of Enhancing School With Innovation of Guangdong Ocean University, and 4) the National Natural Science Foundation of China (Grant Nos. 41376125, 41006070, and 41376035). This work is also supported by the General Research Fund of Hong Kong Research Grants Council (RGC) under grants CUHK 402912 and 403113, the Hong Kong Innovation and Technology Fund under the grants of ITS/321/13, and the direct grants of the Chinese University of Hong Kong. We thank GlobColor’s Working Group for providing merged Chlorophyll-a data, Remote Sensing Systems for TMIAMSRE sea-surface temperature and QuikScat wind vector data, the Colorado Center for Astrodynamics Research (CCAR) Altimeter Data Research Group for sea-level anomaly data. The authors are very grateful to the anonymous reviewers for their valuable comments and suggestions.
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