Nonlinearity and Asymmetry of the ENSO Stratospheric Pathway to North Atlantic and Europe, Revisited

Abstract

Nonlinearities and asymmetries of El Niño Southern Oscillation (ENSO) stratospheric pathway to the North Atlantic and Europe are examined in large ensembles conducted with fully coupled climate models during wintertime. The analysis is centered on historical experiments of the Max Planck Institute Grand Ensemble (MPI-GE, 95 members) and expanded to six other ensembles of more limited size. In MPI-GE, significant responses are obtained for each ENSO phase and three different intensities (weak, moderate and strong). Overall, linear relationships are found for either El Niño or La Niña key diagnostics that characterize the pathway. These relationships are generally weaker for the cold La Niña than for the warm El Niño so that asymmetries between them develop as the events intensify. Specifically for strong events, the extra-tropical North Pacific and stratospheric responses are asymmetric, with larger responses for El Niño. In addition, the stratospheric asymmetry in strong events seems to contribute to the asymmetry in strong events in the North Atlantic—Europe response in the troposphere in late winter. The extra-tropical North Pacific response shows general agreement between MPI-GE and the other large ensembles. However, this agreement is not as large when other parts of the pathway are compared. Relatively high inter-model response spread confirms the typical model uncertainty found when examining atmospheric circulation responses which include the stratosphere in state-of-the-art climate models.