Understanding the ENSO Asymmetry in CCSM4 and CESM1

Abstract:

The asymmetry between El Nino and La Nina events is a fundamental property of ENSO. Understanding the causes and consequences of this property of ENSO may hold the key to understanding decadal variability in the tropics and beyond. Here we present an analysis of the strength of ENSO asymmetry in two latest NCAR models—CCSM4 and CESM1-- which have the same ocean component. This analysis is aimed to extend a previous study based on CMIP5 models to further explore the impact of atmospheric processes on ENSO asymmetry. Our evaluation shows that, in general, the two NCAR models underestimate the observed ENSO asymmetry measured by Nino3 SST skewness. We find that the weak ENSO asymmetry in these models corresponds to a cold bias in the mean SST climatology. The cold bias in mean SST is larger in CESM1 than in CCSM4, despite an overall improvement in the simulation of ENSO asymmetry in CESM1. Corresponding AMIP runs are analyzed to understand the causes of the underestimate of ENSO asymmetry in these coupled models. The analysis reveals that associated with the bias in mean precipitation, CESM1 AMIP runs have a stronger equatorial mean zonal winds than CCSM4 AMIP runs. Both the CAM4 and CAM5 AMIP models are found to have a weaker skewness of zonal wind stress than observations, but CAM5 shows a broad increase in the simulation of zonal winds stress skewness across the equatorial Pacific in contrast to CAM4. Finally, wind-forced ocean GCM experiments demonstrate that the bias in the simulation of ENSO asymmetry in coupled models can be traced back to the bias in the stand-alone atmosphere models. Our results point toward a pathway to reduce the bias in simulation of ENSO asymmetry in coupled models.