Effects on Ocean Biology Induced by El Niño‐Accompanied Positive Freshwater Flux Anomalies in the Tropical Pacific.

The El Niño and Southern Oscillation (ENSO) can induce large perturbations in freshwater flux (FWF, defined as precipitation minus evaporation) and ocean ecosystem in the tropical Pacific. However, how El Niño‐induced FWF can affect the tropical Pacific ecosystem (e.g., chlorophyll) is still unknown. Here, a series of ocean‐only experiments are performed using a coupled ocean‐physical biogeochemistry model forced by prescribed climatological wind stress. Interannual FWF anomalies observed during the 1997–1998 El Niño event are imposed onto the ocean model in the tropical Pacific, and the related positive FWF anomalies are specified to be varying in its intensity, and the corresponding sensitivity experiments are performed to examine the ocean ecosystem response. In general, chlorophyll increases with the intensities of the El Niño‐induced positive FWF anomalies at a certain strength but decreases when the positive FWF forcing is underrepresented (e.g., specified to be its half intensity). Additionally, chlorophyll in the eastern tropical Pacific keeps almost steady during the increasing stage of the El Niño‐induced interannual FWF forcing and then rapidly increases after the peak stage of the FWF forcing. The phytoplankton budget and diagnostic analyses are conducted to understand the behavior of chlorophyll in response to varying intensities of FWF forcing. Chlorophyll response to varying intensities of FWF forcing depends on the iron concentration in the mixed layer, suggesting that the ocean ecosystem response may be shifted from one regime to another when FWF forcing reaches a certain intensity. These results offer insights into biophysical interactions and chlorophyll‐induced feedback effects on ENSO. Plain Language Summary: Chlorophyll is used to represent changes in phytoplankton biomass, and its variability is significantly modulated by multiscale physical processes. In the tropical Pacific, ENSO can induce large perturbations in freshwater flux (FWF) and ocean ecosystem. Given that FWF forcing affects sea surface salinity and buoyancy flux, it is reasonable to ask the following question: Can El Niño‐induced FWF forcing affect the tropical Pacific ecosystem (e.g., phytoplankton and chlorophyll)? Here we examine the effect of FWF forcing on ocean biology using a coupled physics‐biogeochemistry model, in which FWF forcing is explicitly represented with varying intensity. Results suggest that chlorophyll concentration in the eastern equatorial Pacific tends to increase in response to El Niño‐accompanied positive FWF anomalies. However, a regime shift can occur when the FWF forcing intensity is underrepresented (e.g., specified to be its half intensity), with chlorophyll concentration tending to decrease instead. The behavior of chlorophyll in response to varying FWF forcing intensities is regulated by the iron concentration in the mixed layer, suggesting that the ocean ecosystem response may be shifted from one regime to another when FWF forcing reaches a certain intensity. These results offer insights into biophysical interactions in the tropical Pacific and chlorophyll‐induced feedback effects on ENSO. Key Points: Chlorophyll in the eastern equatorial Pacific tends to increase in response to El Niño‐accompanied positive freshwater flux (FWF) anomaliesA regime shift can occur when FWF forcing is weakly represented (e.g., its half intensity), with chlorophyll tending to decrease insteadThe behavior of chlorophyll response to different FWF forcing intensity is regulated by iron concentration in the mixed layer [ABSTRACT FROM AUTHOR]