Asymmetric responses of phytoplankton size structure across the track of Typhoon Yagi and the underlying mechanisms

Abstract

Typhoons disrupt upper-ocean dynamics through wind-driven mixing and upwelling, often producing cross-track asymmetry in the Northern Hemisphere. However, how such asymmetric physical forcing reshapes phytoplankton size structure remains poorly understood. This study integrates multi-sensor satellite observations with in situ size-fractionated chlorophyll a (Chl a) and nutrient measurements to examine the impacts of Typhoon Yagi (2024) in the northwestern South China Sea. Post-typhoon satellite observations showed cross-track physical contrasts: the right-of-track side exhibited stronger surface cooling (~ 3–4°C). In situ sampling on September 10 revealed strong biogeochemical contrasts across the track. Mean surface ${\mathrm{NO}}_3^{-}$ and ${\mathrm{SiO}}_3^{2-}$ concentrations were higher on the right side, with the most robust cross-track contrast observed for ${\mathrm{SiO}}_3^{2-}$. Total surface Chl a was approximately 2.7-fold higher on the right side when all three left-side stations were included. At T6, elevated Chl a co-occurred with an independently identified frontal and hydrographic anomaly. Size-fractionated Chl a indicated that fractions > 2 μm dominated on the right side, whereas the 0.2–2 μm fraction contributed approximately 70% at left-side Stas. T2 and T4. Notably, near-bottom total Chl a on the left side peaked at T6, coincident with an intensified offshore sea surface temperature front. Satellite imagery on September 11–12 captured a delayed surface Chl a enhancement. These results are consistent with cross-track differences in nutrient availability and upper-ocean physical structure associated with asymmetric typhoon forcing, while lateral advection and frontal convergence may have further modulated the observed Chl a distribution. These findings provide new insight into post-typhoon ecosystem restructuring.