ENSO Asymmetry in the CAMS-CSM
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This study presents an overview of El Niño-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.
KeywordsENSO asymmetry Shortwave radiation flux Oceanic internal dynamic advection
This work was funded by the National Natural Science Foundation of China (No. 41606011), the National Key Research and Development Program of China (No. 2016YFE0102400 and No. 2016YFA0600602), and the Basic Scientific Research and Operation Foundation of CAMS (No. 2017Y007), the Major Program of National Natural Science Foundation of China (No. 91637210), the Startup Foundation for Introducing Talent of NUIST, the open fund of State Key Laboratory of Loess and Quartary Geology (SKLLQG1802), and the LASG open project.
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