Effects of multiple dynamic processes on chlorophyll variation in the Luzon Strait in summer 2019 based on glider observation.

Luzon Strait is the main channel connecting the South China Sea (SCS) and the western Pacific, with complex atmospheric and oceanic dynamic processes. Based on 44 days of glider measurements and satellite observations, we investigated the temporal and vertical variations of chlorophyll-a (Chl-a) concentration in the Luzon Strait from July 25 to September 6, 2019. The Chl a was mainly distributed above 200 m and concentrated in the subsurface chlorophyll maximum (SCM) layer. The depth of SCM ranged between 50 m and 110 m, and the magnitude of SCM varied from 0.42 mg/m³ to 1.12 mg/m³. The variation of Chl a was identified with three stages responding to different dynamic processes. Under the influence of Kuroshio intrusion, the SCM depth sharply deepened, and its magnitude decreased in Stage 1. Afterward, a prominent Chl-a bloom was observed in the SCM layer from August 6 to August 16. The Chl-a bloom in Stage 2 was related to the influence of a cyclonic eddy, which uplifted of the thermocline and thus the deep nutrients. During Stage 3, prolonged heavy rainfall in the northeastern SCS resulted in a significant salinity decrease in the upper ocean. The convergence of upper water deepened the thermocline and the mixed layer. Thus, the Chl a decreased in the SCM layer but increased in the surface layer. In particular, a typhoon passed through the Luzon Strait on August 24, which induced the Chl a increase in the upper 50 m. However, there was little change in the depth-integrated Chl a (0–200 m), indicating that the Chl a increase in the surface layer was likely associated with physical entrainment of SCM caused by strong mixing, rather than the phytoplankton bloom in the upper water column. Underwater gliders provide frequent autonomous observations that help us understand the regional ocean's complex dynamic processes and biological responses. [ABSTRACT FROM AUTHOR]