ENSO Asymmetry in the CAMS-CSM

Abstract

This study presents an overview of El Nino-Southern Oscillation (ENSO) asymmetry using the Chinese Academy of Meteorological Sciences climate system model (CAMS-CSM). We discover that the coupled run of the CAMS-CSM has an obvious bias of ENSO opposite-sign asymmetry compared to observation, mainly in the eastern Pacific. Further analysis shows that the spatial distributions of sea surface temperature anomalies (SSTA) during both phases of the ENSO present individual biases, consisting of a warmer field during the warm phase and a colder field during the cold phase, in comparison with observation. The bias of ENSO asymmetry during both phases is partly due to the unrealistic simulation of shortwave (SW) radiation flux and the corresponding total cloud cover (TCC). The Atmospheric Model Intercomparison Project (AMIP) run demonstrates that biases of the SW radiation flux and the associated TCC originate in the atmospheric component of the model, which could be attributed to its unrealistic cloud microphysical scheme. Through air-sea interaction, these biases are amplified significantly during both ENSO phases of the coupled run. Moreover, another cause for the bias of ENSO asymmetry during the warm phase is the relatively slow decay of the ENSO in the simulation, with the thermocline anomalies propagating eastward more slowly. The bias of ENSO asymmetry in the cold phase is attributed to oceanic internal dynamic advection, mainly associated with zonal and meridional terms. Further analysis also highlights the essential role of reasonably representing the climatological mean state in ENSO model simulation.