Rectified Effects of Interannual Chlorophyll Variability on the Tropical Pacific Climate Revealed by a Hybrid Coupled Physics-Biology Model

El Nino-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 (alpha) 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 alpha; ENSO amplitude is reduced by 12%-40% with decreased annual mean sea surface temperature by 0.05-0.4 degrees C in the eastern equatorial Pacific when taking alpha = 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 Nina (El Nino) 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 Nino amplitude being damped stronger than La Nina; this residual effect leads to a net cooling condition in the eastern tropical Pacific. Plain Language Summary As a proxy of marine phytoplankton biomass, ocean chlorophyll not only responds to El Nino-Southern Oscillation (ENSO) phase change but also exerts a significant influence on ENSO and mean state through modulating the absorbed shortwave radiation in the upper ocean. However, the extent to which interannual chlorophyll variability (ICV) can have rectified effect (higher-frequency process impacts low-frequency climate variability and mean state) in the tropical Pacific remains unclear. In this study, we examine the rectified effect of ICV on the tropical Pacific climate by performing a series of numerical simulations. By isolating the ICV effect from the climatological mean chlorophyll field, we found that ICV acts to weaken ENSO amplitude and induces a cooling effect in the eastern equatorial Pacific, characterized by a La Nina-like pattern. A stability analysis reveals that the weakened responses of the thermocline or upwelling to the zonal wind stress (the thermocline and Ekman feedbacks) due to the ICV effect dominantly reduce ENSO amplitude. Furthermore, the stronger damping effect on El Nino due to ICV leads to a net cooling mean-state. Given the pronounced rectified effect of ICV on the tropical Pacific climate, further efforts are needed to adequately represent multi-scale ocean physics-biology interactions for improving earth system modeling.

Automated summary

This study investigates the rectified effect of interannual chlorophyll variability (ICV) on the tropical Pacific climate, finding that ICV weakens ENSO amplitude and induces a cooling effect in the eastern equatorial Pacific. Stability analysis reveals two feedback loops through which ICV impacts ENSO dynamics, highlighting the significant role of marine phytoplankton biomass in modulating low-frequency climate variability.