Multiple processes are involved in modulating the El Niño‐Southern Oscillation (ENSO) in the tropical Pacific, and these processes are neither well‐represented nor well‐understood in climate models. A new hybrid coupled model (HCM) of atmosphere, ocean physics, and ocean biogeochemistry (AOPB) is developed to represent the feedback from ocean biogeochemistry onto ocean physics via modulating the penetration of shortwave radiation in the upper ocean. An ocean biogeochemistry model is coupled with a simplified ocean‐atmosphere system consisting of an ocean general circulation model (OGCM) and a statistical atmospheric model for interannual anomalies of wind stress (τ). The HCM AOPB serves as a simple Earth system for the tropical Pacific to represent the coupling among the atmospheric and physical and biogeochemical ocean components. Model experiments are performed to illustrate this new model's ability to depict the mean ocean state and interannual variability associated with the ENSO. The relationships among anomaly fields are analyzed to illustrate the ocean biogeochemistry‐induced heating feedback and its modulating effects on the ENSO, which is characterized by a negative feedback. The underlying processes and mechanisms are analyzed and can be attributed to dominant modulation of the penetrative solar radiation through the base of the mixed layer (ML). It is demonstrated that the ocean biogeochemistry‐induced negative feedback is mainly driven by more solar radiation penetrating out of the ML during El Nino and less penetrating during La Nina. Further model applications to studies on these processes and biogeochemical‐physical interactions are discussed.

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 Niño–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 Niña conditions tend to be intensified, and ENSO amplitude is increased.

Tropical instability waves (TIWs) play an important role in modulating the physical variability (e.g., sea surface temperature [SST] and heat flux) and biological activity (e.g., phytoplankton and chlorophyll (Chl)) in the eastern tropical Pacific. The impacts of TIW‐induced Chl perturbations (Chl_TIW) are examined using a coupled ocean physics‐biology model in the tropical Pacific. TIW‐scale biological and physical perturbations are extracted using a zonal high‐passed filter. The TIW‐induced perturbations of biological and physical fields (e.g., Chl, nitrate, iron, and SST.) exhibit their close relationships, with negative correlation between SST and Chl at TIW scale. Processes responsible for the impacts of Chl_TIW are diagnosed. Chl_TIW acts to modulate the incoming solar radiation that penetrates the bottom of the mixed layer (Q_pen). In addition, two experiments are conducted with the impacts of Chl_TIW on the ocean included or not. The results show that the Chl_TIW effects tend to substantially increase seasonal variability of eddy kinetic energy (EKE) and slightly decrease SST in the eastern equatorial Pacific. At large scale, Chl_TIW leads to an increase in mean Chl concentration, which induces a SST cooling and an increase in EKE in the boreal winter. At TIW scale, Chl_TIW leads to the change in Q_pen and a decrease in baroclinic conversion term and eventually decreases the EKE in the boreal spring and summer. So the Chl_TIW acts to have influences both on the intensity of TIW and large‐scale SST in the tropical Pacific, involving multiscale interactions and feedbacks between ocean physics and biology.

During El Niño to La Niña transitions in 1998 and 2010, satellite observations revealed a sharp increase in surface chlorophyll in the eastern equatorial Pacific (EEP), exceeding the interannual amplitude by threefold; however, the causes of such super phytoplankton blooms (SPBs) remain unclear. Here, observational data, climate model simulations, and coupled ocean-biogeochemical modeling experiments are adopted to show that Indian Ocean (IO) warming plays an active role in remotely triggering SPBs in the EEP. During the previous boreal winter in an El Niño year, IO warming generates anomalous easterlies over the western edge of the tropical Pacific, which excite upwelling Kelvin waves propagating into the EEP during the following boreal spring, remotely causing an uplift of the nutricline in the EEP. Seasonally, the mixed layer deepens and the upper ocean warms during the following late spring, and large amounts of nutrient-rich cold subsurface waters entrain into the mixed layer; interannually, the local grazing pressure is low after the peak of El Niño. These remote and local factors jointly promote SPBs in the EEP.

Previous studies have revealed the potential importance of salinity in El Niño‐Southern Oscillation (ENSO) asymmetry, but the responsible mechanisms remain unclear. Here, we investigate how the zonal structure of surface salinity anomalies affects ENSO intensity and asymmetry by modifying freshwater flux in ocean general circulation model simulations. Results show that the amplitudes of El Niño and La Niña are both highly sensitive to zonal patterns of salinity anomalies, with the strongest sea surface temperature change occurring when maximum salinity anomalies are located near 170°W. Furthermore, the effect of salinity anomalies in the central Pacific on El Niño warming is larger than those in the western Pacific on La Niña cooling. Thus, the asymmetric salinity anomalies strengthen the temperature asymmetry between El Niño and La Niña. Temperature budget analysis shows that vertical mixing and entrainment due to salinity‐induced changes in stratification are a major response to different zonal patterns of freshwater flux forcing.