The asymmetry between El Niño and La Niña is a key aspect of ENSO, and needs to be simulated well by models in order to fully capture the role of ENSO in the climate system. Here we evaluate the asymmetry between the two phases of ENSO in five successive versions of Community Climate System Model (CCSM1, CCSM2, CCSM3 at T42 resolution, CCSM3 at T85 resolution, and the latest CCSM3+NR with Neale and Richter convection scheme). Different from the previous studies, we not only examine the surface signature of ENSO asymmetry, but also its subsurface signature. We attempt to understand the causes of the ENSO asymmetry by comparing the differences among these models as well as the differences between models and the observations.
An underestimate of the ENSO asymmetry is noted in all the models, but the latest version with the Neale and Richter scheme (CCSM3+NR) is getting closer to the observations than the earlier versions. The net surface heat flux is found to damp the asymmetry in the SST field in both models and observations, but the damping effect in the models is weaker than in observations, thus excluding a role of the surface heat flux in contributing to the weaker asymmetry in the SST anomalies associated with ENSO. Examining the subsurface signatures of ENSO—the thermocline depth and the associated subsurface temperature for the western and eastern Pacific—reveals the same bias—the asymmetry in the models is weaker than in the observations.
The analysis of the corresponding AMIP runs in conjunction with the coupled runs suggests that the weaker asymmetry in the subsurface signatures in the models is related to the lack of asymmetry in the zonal wind stress over the central Pacific which is in turn due to a lack of sufficient asymmetry in deep convection (i.e., the nonlinear dependence of the deep convection on SST). In particular, the lack of westward shift in the deep convection in the models in response to a cold phase SST anomaly is found as a common factor that is responsible for the weak asymmetry in the models. We also suggest that a more eastward extension of the deep convection in response to a warm phase SST anomaly may also help to increase the asymmetry of ENSO. The better performance of CCSM3+NR is apparently linked to an enhanced convection over the eastern Pacific during the warm phase of ENSO. Apparently, either a westward shift of deep convection in response to a cold phase SST anomaly or an increase of convection over the eastern Pacific in response to a warm phase SST anomaly leads to an increase in the asymmetry of zonal wind stress and therefore an increase in the asymmetry of subsurface signal, favoring an increase in ENSO asymmetry.