Contrasting climate oscillations impacts on phytoplankton in the western and eastern tropical Pacific

Climate oscillations like the El Niño-Southern Oscillation and Pacific Decadal Oscillation significantly impact marine ecosystems. Traditionally, warm events are thought to suppress nutrient availability, reduce phytoplankton biomass, and change community structure. However, using two decades of field observations, remote sensing, Copernicus reanalysis, and Earth system modeling, we find very different responses in the western tropical Pacific. During El Niño, chlorophyll concentrations and diatom contributions increase, while La Niña reverses that trend. The further amplification of these El Niño-Southern Oscillation-driven contrasts by the Pacific Decadal Oscillation can be attributed to its effects on regional circulation, including the enhancement of eddy activity that facilitates upwelling of nutrients and stimulates phytoplankton production. These findings go beyond the simple narrative of community structure shifts and suggest more complex physical-biological responses in the tropical Pacific. Consideration of these mechanisms will be crucial for improvement of climate change predictions and development of strategies for sustainable management.