Counteracting effects on ENSO induced by ocean chlorophyll interannual variability and tropical instability wave-scale perturbations in the tropical Pacific

Large perturbations in chlorophyll (Chl) are observed to coexist at interannual and tropical instability wave (TIW) scales in the tropical Pacific; at present, their combined effects on El Niño-Southern Oscillation (ENSO) through ocean biology-induced heating (OBH) feedbacks are not understood well. Here, a hybrid coupled model (HCM) for the atmosphere and ocean physics-biogeochemistry (AOPB) in the tropical Pacific is adopted to quantify how ENSO can be modulated by Chl perturbations at interannual and TIW scales, individually or collectively, respectively. The HCM-based sensitivity experiments demonstrate a counteracting effect on ENSO: the bio-climate feedback due to large-scale Chl interannual variability acts to damp ENSO through its impact on upper-ocean stratification and vertical mixing, whereas that due to TIW-scale Chl perturbations tends to amplify ENSO. Because ENSO simulations are sensitively dependent on the ways Chl effects are represented at these different scales, it is necessary to adequately take into account these related differential Chl effects in climate modeling. A bias source for ENSO simulations is illustrated that is related with the Chl effects in the tropical Pacific, adding in a new insight into interactions between the climate system and ocean ecosystem on different scales in the region. These results reveal a level of complexity of ENSO modulations resulting from Chl effects at interannual and TIW scales, which are associated with ocean biogeochemical processes and their interactions with physical processes in the tropical Pacific.