资源环境科技发展态势分析平台

The dominant mode of inter-annual variability in the tropical Pacific is El Nino-Southern Oscillation (ENSO). ENSO is not symmetric in the sense that El Nino is generally stronger than La Nina. However, many CMIP5 models, including the Max Planck Institute Earth System Model (MPI-ESM), produce an almost symmetric ENSO. This paper shows that, when resolving the intra-daily air-sea interactions by coupling the atmospheric and oceanic model components once per hour, the simulated ENSO asymmetry is improved. The improvement is closely related to the simulated diurnal cycle of sea surface tempreature (SST). In the central tropical Pacific, the simulated diurnal range of SST is about 0.2 degrees C, up to 10% of SST anomalies of the simulated ENSO events. During El Nino events, the diurnal cycle of SST anomalies enhances the atmospheric moist instability, whereby triggering more convection in the central tropical Pacific. During La Nina and normal years, however, the mean convection is not changed by the included diurnal cycle of SST anomalies. As a result, the anomalies of the trades, which are directly related to the convection, are stronger during El Nino years than that during La Nina years, making El Nino to be stronger than La Nina via Bjerknes feedback. These results obtained with a low resolution MPI-ESM are further confirmed by simulation with the same model at higher spatial resolutions, suggesting that the role of the intra-daily air-sea interactions for the ENSO asymmetry is independent of model resolutions.