Multiyear La Niña Impacts on Surface Chlorophyll in the Equatorial Pacific: Insights from Ocean Dynamics

Abstract The frequency of multiyear La Niña (MLN) events is increasing under global warming, exerting significant impacts on marine ecosystems through various ocean dynamic processes. However, the characteristics and physical mechanisms underlying the response of ocean chlorophyll to MLN events remain poorly understood. In this study, using observational and reanalysis data, we show that surface chlorophyll in the eastern and western equatorial Pacific exhibits distinct responses in individual years during the MLN events. In the first year, enhanced vertical mixing induced by intensified trade winds facilitates a rapid increase in both large and small phytoplankton in the eastern and western equatorial Pacific, respectively, leading to an overall increase in surface chlorophyll. In the second year, the zonal advection process plays a key role in determining the decrease and increase in chlorophyll during the boreal spring and winter in the western equatorial Pacific, respectively. In contrast, a notable decrease in chlorophyll in the eastern equatorial Pacific is associated with ocean wave adjustments during the boreal spring. Sensitivity experiments using an ocean general circulation model confirm that the reduction in chlorophyll in the eastern equatorial Pacific is driven by easterly wind anomalies over the northwestern equatorial Pacific, along with westerly wind anomalies associated with the negative phase of the North Pacific meridional mode. These anomalies generate eastward downwelling Kelvin waves along the equator, which deepen the thermocline and nutricline, further contributing to the reduction in chlorophyll through weakened upwelling in the eastern equatorial Pacific. These findings suggest that marine ecosystems exhibit complex regional responses to MLN events, which are closely associated with ocean dynamic processes. Significance Statement Observations indicate that El Niño events typically terminate rapidly after reaching their peak phase during the boreal winter. In contrast, some La Niña events can persist or even reintensify over the subsequent 1–2 years, referred to as double or triple La Niña events. However, the responses of surface chlorophyll—an essential indicator of marine primary productivity—to multiyear La Niña (MLN) events remain unclear, and the identification of key oceanic dynamical processes involved is important to the interannual prediction of the marine ecosystem in the Pacific. This study reveals different response characteristics in the tropical Pacific with different ocean dynamic processes. In the western equatorial Pacific, chlorophyll concentrations exhibit two rapid increases driven by vertical mixing and zonal advection, respectively. Conversely, in the eastern equatorial Pacific, there is an initial increase, which is followed by a decrease primarily influenced by adjustments in ocean waves originating from the western equatorial Pacific. These findings suggest that marine ecosystems exhibit complex regional responses to MLN events.