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Tropical instability waves (TIWs) induce large physical and biological perturbations, which have a feedback onto the tropical Pacific climate and ecosystem. However, the extent to which TIW-induced chlorophyll perturbations (Chl(TIW)) can influence El Nirio-Southern Oscillation (ENSO) remains unknown. Here we used a hybrid-coupled model to investigate the Chl(TIW) effect on ENSO. Two experiments are conducted, one with the Chl(TIW) effect being represented in the control run (CTRL) and other with the Chl(TIW) effect being purposely excluded by filtering out Chl(TIW) signals (FILT). Results show that the amplitude of ENSO is increased by 27% in CTRL compared to FILT. Chl(TIW) tends to modulate the penetrative solar radiation in the upper ocean, acting to weaken the intensity of TIWs. Then, the weakened TIWs lead to a reduction in the equatorward meridional heat transport and consequently less warming effect on the cold tongue. Therefore, La Nina conditions tend to be intensified, and ENSO amplitude is increased.

Plain Language Summary Tropical instability waves (TIWs) are prominent cusp-like waves in the tropical Pacific and Atlantic, with the period of 20-40 days and wavelength of 600-1,000 km. TIWs can induce large perturbations in chlorophyll (Chl(TIW)) and sea surface temperature. Given that chlorophyll absorbs solar radiation in the upper ocean and alters the upper-ocean temperature structure, it is natural to ask the following question: Can TIW-related mesoscale biology processes likeChl(TIW) influence the tropical climate? Here we examine the Chl(TIW) effect on El Nino-Southern Oscillation (ENSO) using a coupled physics-biogeochemistry model, with the Chl(TIW) effect taken into account or not. Results suggest that the Chl(TIW) effects tend to increase ENSO amplitude by 27%. This is because the Chl(TIW) effect weakens the intensity of TIW itself, which inevitably decreases the meridional heat transport onto the equator and consequently less warming effect on the cold tongue in the eastern equatorial Pacific. As a result, La Nina conditions tend to be intensified and ENSO amplitude is increased. Our study contributes to resolving the ongoing debate on the role of biological processes in modulating ENSO amplitude (i.e., increase or decrease), and improves current understanding of ENSO modulation.