Amplifying effects of multiyear La Niña on phytoplankton biomass in the tropical Pacific

Traditional views suggest that overall phytoplankton biomass in the tropical Pacific should be proportional to La Niña intensity because of enhanced nutrient supplies from equatorial upwelling associated with La Niña intensity. Here, composite analysis of satellite-derived chlorophyll a (Chl a, a proxy for biomass) data across five multiyear La Niña (M-LN) events (1997–2023) showed a double increase in the tropical Pacific average Chl a in the 2^nd/3^rd years [La Niña(2)] of M-LN compared to the 1^st year [La Niña(1)], despite declining La Niña intensities. This unexpected rise is attributed to a ∼ 1-yr lagged response of the off-equatorial phytoplankton to La Niña/El Niño. The lagged response of off-equatorial Chl a to La Niña(1) superimposed with the immediate response of equatorial Chl a to La Niña(2) amplifies overall Chl a levels during La Niña(2). Simulations from Earth System Models support this mechanism. Our findings imply that increased M-LN events may mitigate warming-induced biomass decline in the tropical Pacific.