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.
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