Nonlinear response of Equatorial Western Pacific phytoplankton blooms to ‘double-dip’ La Niña events

Phytoplankton in the equatorial western Pacific tends to bloom during consecutive (‘double-dip’) La Niña events with nonlinear characteristics: extremely high chlorophyll-a (Chl-a) concentrations typically occur during the second-year La Niña events even when the associated SST anomalies are significantly weakened. Photosynthetically available radiation is found to have the strongest correlation with the equatorial western Pacific Chl-a fluctuations. However, barrier layer variation is critical in driving the strong bloom events seen in the second-year La Niña, which can be further explained by the nonlinear heat advection within the isothermal layer. To improve the current climate models’ performance in simulating the western Pacific phytoplankton bloom events, it is recommended that the influence of barrier layer should be better considered.