Chlorophyll-a along the northern coastal of Luzon Island affected by the Kuroshio in 2015–2016 winter

The Luzon Strait serves as an entrance of the seawater from the Pacific Ocean to the South China Sea (SCS). When the oligotrophic Kuroshio intrusion into the Luzon Strait encounters with the sea surface chlorophyll-a (SChl-a) discharged from the river estuary in the northern Luzon Island, this patch of SChl-a may be transported by the Kuroshio water from the coastal region westward to the northern SCS. During the strong 2015–2016 El Niño event, the Kuroshio intruded into the SCS through the Luzon Strait on 25 and 26 December 2015. Simultaneously, a patch of SChl-a was directed from the northern coast of Luzon Island to the northern SCS along the rotational trajectory of a series of oceanic mesoscale eddies, ultimately resulting in its diffusion. However, measurements from geostrophic currents from satellite altimeter data cannot accurately depict the current fields near the coasts, where the targeted SChl-a patch generated, due to the shallow topography of the Babuyan Channel and complicated Babuyan Islands acting as barriers. The Maximum Cross-Correlation (MCC) method is thereby applied to address this issue by capturing the moving pattern of the target SChl-a patch from the concurrent sea surface temperature (SST) from satellite observations (MODIS-Aqua Level 3) and analysis data (GHRSST Level 4), as well as SChl-a images from satellite observations (MODIS-Aqua Level 3) and merged datasets (OC-CCI and GlobColour). This study investigated the average magnitude of MCC-derived westward current vectors in the Babuyan Channel (18.25°N–18.75°N, 121°E–122°E) after the Kuroshio intrusion occurred in a looping path, V_west, based on daily GHRSST data (0.01°×0.01°). The results reveal that V_west reached 18.84 cm s⁻¹ during December 2015, which is 57.1% and 14.3% faster than other winters in November 2012 (a neutral year, V_west=11.99 cm s⁻¹) and during November 2022, La Niña event (V_west=16.48 cm s⁻¹), respectively. Consequently, the stronger Kuroshio intrusion increases the westward currents in the Babuyan Channel and may further influence the ecosystem in the SCS, which are modulated by various phases of El Niño–Southern Oscillation (ENSO).