Phytoplankton bloom during the northeast monsoon in the Luzon Strait bordering the Kuroshio

Several studies have reported intensive phytoplankton blooms during the northeast monsoon season in the Luzon Strait and its vicinity (the “LZS bloom”); the primary mechanism underlying this phenomenon is however not yet clear. Here we investigated the January 2010 LZS bloom and climatological state using in situ and remote sensing data, including the MODIS phytoplankton absorption coefficient at 443 nm (Aph) as a preferable pigmentation index. Blooming patches of Aph ≥ 0.02 m− 1 overlaid on a background level of 0.01 m− 1 were discernible for both situations. A twin-core structure of the bloom was identified; the major core lay far offshore in the deep basin to the northwest of Luzon, with a maximum horizontal scale of ~ 250 km, and the other core was nearshore in the Babuyan Channel, with a maximum scale of ~ 100 km. Variations in climatological monthly mean Aph indicate that the LZS bloom as a whole persists from November to February, peaking in December. Further investigations with satellite altimetry and sea surface temperature suggest that the bloom is primarily associated with the Kuroshio and its intrusion, which generate various meso-scale processes and thus induce enhanced nutrient pumping/entrainment from depth when interacting with the topography of the Luzon Strait and the circulation of the northeastern South China Sea (i.e. the Luzon Gyre). Meanwhile, upwelling at the center of the Luzon Gyre contributes to the offshore bloom. The nearshore bloom is substantially enhanced by nutrient input from the Cagayan River.