Asymmetry and nonlinearity of the influence of ENSO on the northern winter stratosphere: 1. Observations

The El Nino-Southern Oscillation (ENSO) has significant effects on the extratropical stratosphere. This study explores the nonlinearity and the asymmetry of these influences by distinguishing the different effects of four types of ENSO: moderate El Nino, strong El Nino, moderate La Nina, and strong La Nina. It is revealed that the moderate El Nino and the strong La Nina are much more efficient than the strong El Nino and the moderate La Nina, respectively, in modulating the northern winter stratospheric variability, resulting in significant nonlinearity and asymmetry. The tropical rainfall anomalies induced by a moderate El Nino or a strong La Nina are centered over the central equatorial Pacific region near the dateline, while the convection responses to a strong El Nino or a moderate La Nina are centered farther eastward. Accordingly, the anomalous Pacific-North America wave train pattern is modulated by ENSO in a nonlinear and asymmetric way, which leads to the large nonlinear and asymmetric components of the vertical Eliassen-Palm (E-P) flux responses to ENSO. The increase of planetary wave activity in the extratropical stratosphere in moderate El Nino winters is thus greater than in strong El Nino winters, whereas a strong La Nina gives a larger decrease in propagation than a moderate La Nina. The relatively strong (weak) E-P flux responses to moderate (strong), El Nino and strong (moderate) La Nina explains the remarkable nonlinearity and asymmetry in the response of the extratropical stratosphere to ENSO.