Rectified Effects of Interannual Chlorophyll Variability on the Tropical Pacific Climate Revealed by a Hybrid Coupled Physics‐Biology Model
Feng Tian, Rong‐Hua Zhang, Xiujun Wang, Hai Zhi
Abstract
Abstract El Niño‐Southern Oscillation (ENSO) induces large interannual chlorophyll variability (ICV) in the tropical Pacific, which modulates shortwave penetration in the upper ocean. However, the extent to which ICV can have rectified effect (higher‐frequency process impacts low‐frequency climate variability (i.e., ENSO) and mean state) in the tropical Pacific remains unclear. Here, a hybrid coupled physics‐biogeochemistry model with tunable coupling ocean‐atmosphere intensity ( α ) is used to examine this rectified effect. The simulations consist of a control run in which the ICV effect is represented in the model and the other with the ICV effect being purposely excluded by only reserving the climatological chlorophyll field. Results show that the ICV effect depends on α ; ENSO amplitude is reduced by 12%–40% with decreased annual mean sea surface temperature by 0.05–0.4°C in the eastern equatorial Pacific when taking α = 0.8–1.15. A stability analysis reveals that the ICV effect on ENSO exhibits two feedback loops. (a) Positive (negative) chlorophyll anomalies during La Niña (El Niño) act to increase (decrease) shortwave absorption within the mixed layer, which enhances thermodynamic damping (slightly weakening ENSO amplitude). (b) Chlorophyll anomalies modulate vertical redistribution of penetrative shortwave radiation between the mixed layer and subsurface layers; this differential heating affects ocean stratification and vertical mixing, which weakens the thermocline and Ekman feedback (predominantly weakening ENSO amplitude). Additionally, ICV exhibits asymmetric effects on ENSO evolution, with the El Niño amplitude being damped stronger than La Nina; this residual effect leads to a net cooling condition in the eastern tropical Pacific.