The south–north equatorial asymmetrical distribution of Chlorophyll in El Niño events in the observations and CMIP6 models

Abstract

The interannual variability of chlorophyll (Chl) in the tropical Pacific Ocean is influenced by El Niño. When El Niño occurs, insufficient nutrients (suppressed upwelling), reduced shortwave radiation reaching the surface (increased precipitation), and eastward zonal current (U) anomalies, which transport low Chl from western Pacific warm pool water to the east, can cause a reduction in tropical Pacific Chl. The El Niño-induced Chl anomaly is asymmetrically distributed across the equator between 10°S and 10°N, and is particularly significant in extreme El Niño events. This asymmetric phenomenon is mainly caused by asymmetrical anomalies of the eastward U and precipitation. During El Niño events, U and precipitation anomalies are much stronger north of the equator, which can cause a greater reduction in Chl, which is asymmetrical. Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) are used to evaluate the simulation of the south–north asymmetry of Chl under extreme El Niño events. It finds that the simulation of U and precipitation anomalies is the key parameter to the simulation of this asymmetry. Among 15 CMIP6 models that provided Chl data, only 5 of them (MIROC-ES2 L, MPI-ESM1-2-HR, MPI-ESM1-2-LR, NorESM2-LM, and NorESM2-MM) can be used to investigate the asymmetrical distribution of Chl during extreme El Niño after comparison with observational Chl. Among the 5 selected models, only 2 (NorESM2-LM, NorESM2-MM) can show asymmetry during extreme El Niño events, and the other three models (MIROC-ES2 L, MPI-ESM1- (MIROC-ES2 L, MPI-ESM1-2-HR, MPI-ESM1-2-LR, NorESM2-LM, and NorESM2-MM) 2-HR, MPI-ESM1-2-LR) show symmetrical distributions. From the composite distribution of U and precipitation for these five models, it can be seen that the key reasons for whether a model can simulate the asymmetry lies in the simulation of U and precipitation.