Global Warming Weakens the Ocean Front and Phytoplankton Blooms in the Luzon Strait Over the Past 40 years

The Luzon Strait is a channel where warmer Kuroshio water from the Pacific Ocean intrudes into the South China Sea (SCS). Under climate change impact, the temperature in marginal seas, including the SCS, rises faster than in open oceans. We speculated that the variation of frontal intensity and its eco-environmental impact in the Luzon Strait may be different from coastal waters, whose frontal intensity is increasing, thus stimulating phytoplankton growth. To confirm this speculation, 40-year satellite, and multiple data sources were analyzed in the Luzon Strait. The results showed that strong frontal intensity (front coverage of over 60%) and higher Chlorophyll a content occurred simultaneously in the Luzon Strait during the winter monsoon period. Phytoplankton blooms were enhanced during El Nino years because the stronger Kuroshio intrusion generated stronger fronts and intensified local upwelling in the Luzon Strait. On an interannual scale, the frontal intensity and phytoplankton growth exhibited a significantly decreasing trend in the Luzon Strait over the past 40 years, since the faster warming in the SCS reduced the temperature difference between the Pacific Ocean and the SCS. Warming and weakening fronts reduced the mixed layer depth to the oligotrophic layer, thus limiting the phytoplankton growth. This study confirmed that faster temperature rises in marginal seas reduced the frontal intensity and phytoplankton growth in the strait between oceans and marginal seas. Ocean fronts are the boundaries between two sides with distinct water mass properties and play an extremely important role in marine eco-environment and global climate change. Studies have shown that thermal fronts are enhancing, thus stimulating chlorophyll growth in the coastal waters due to the intensification of global warming. However, the variation of frontal intensity in the open oceans may be different because warming in marginal seas rises faster than in oceans. To confirm this speculation, 40-year satellite, and multiple data sources were analyzed in the Luzon Strait, a deep conduit between the Pacific Ocean and the South China Sea (SCS). This study found that the frontal intensity and phytoplankton growth exhibited a significantly decreasing trend in the Luzon Strait over the past 40 years, since the faster warming in the SCS reduced the temperature difference between the Pacific Ocean and the SCS. Warming and weakening fronts reduced the mixed layer depth to the oligotrophic layer, thus limiting the phytoplankton growth. Strong ocean front and higher Chlorophyll a (Chl-a) content occurred simultaneously in winterPhytoplankton blooms enhanced in El Nino years due to stronger Kuroshio intrusionGlobal warming decreased the frontal intensity and Chl-a level over the past 40 years