Phytoplankton Increases Triggered By Tropical Cyclones In The Western North Pacific

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 temperature 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 > 0.3 mg m-3versus Chl-a < 0.2 mg m-3) east of the Taiwan Island. Translation speed may be one of the important factors 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 the first typhoon of Sinlaku could facilitate an unfavorable condition for phytoplankton bloom induced by the second typhoon of Jangmi. The sea surface temperature cooling by Jangmi was suppressed owing 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, nutrients) play important roles in the strong phytoplankton bloom, responsible for the stronger phytoplankton bloom after Sinlaku passage than that after Jangmi’s passage. A typhoon affecting parameter is introduced that combines the effects of the typhoon forcing(including the typhoon intensity and translation speed) and the oceanic pre-condition. The typhoon affecting factor shows that the forcing effect of Sinlaku was stronger than that of Jangmi.