Asymmetry and nonlinearity of the influence of ENSO on the northern winter stratosphere: 2. model study with WACCM

Abstract

Long-term simulations from the Community Earth System Model with the Whole Atmosphere Community Climate Model (CESM-WACCM) as its atmospheric component are used to investigate the asymmetry and the nonlinearity of the influences of El Niño-Southern Oscillation (ENSO) on the northern winter stratosphere. As in Part 1 of this study, four different types of ENSO are considered. The composite CESM-WACCM results first confirm the conclusions drawn from the observations, that the stratospheric polar jet responses to “moderate El Niño” and “strong La Niña” are stronger than those to “strong El Niño” and “moderate La Niña”. In association with the ENSO sea surface temperature (SST) patterns that are reproduced well in the model, the tropical rainfall response centers exhibit an asymmetric east-west shift between El Niño and La Niña, which directly leads to the nonlinear and asymmetric Pacific-North America responses in the extratropics. Accordingly, the strengthening (weakening) planetary wave response in the stratosphere during warm (cold) ENSO also exhibits nonlinear and asymmetric features. When the ENSO SST forcing is prescribed to be linear and symmetric in WACCM, the nonlinearity and asymmetry of the stratospheric responses to moderate ENSO reveal the dominant role of the inherent properties of the atmosphere. However, the absence of asymmetry and nonlinearity in the stratospheric responses to strong ENSO in our sensitivity experiments indicates that the asymmetry in SST forcing between strong El Niño and La Niña still plays an important role in the asymmetric and nonlinear influences of ENSO on the extratropics.