Delayed ENSO impact on phytoplankton variability over the Western-North Pacific Ocean

Abstract The El Niño-Southern Oscillation (ENSO) is a prominent climatological phenomenon in the Pacific Ocean that exerts a significant influence on both physical states and biological activities across the globe through oceanic and atmospheric teleconnections. In this study, we investigate the impact of ENSO on phytoplankton variability in the Western North Pacific (WNP) region, revealing a delayed relationship with a time lag of more than one year. During the boreal early spring after the peak phase of El Niño, we find a significant increase in chlorophyll-a concentrations in the WNP. This delayed ENSO impact is due to the propagation of reflected ocean Rossby waves from the eastern to the western Pacific. As the Rossby waves reach the western Pacific, they initiate continuous northward transports of the nutrients. The increased nutrient supply promotes phytoplankton growth in the WNP region, where phosphate (PO 4 ) - limited conditions prevail. Our finding suggests the role of ENSO in shaping the biological dynamics in the WNP over an extended time scale through the specific mechanisms involved. Long-term climate model simulation supports these observed findings.