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Zusammenfassung:
Nonlinearities in El Niño (EN) and La Niña (LN) teleconnections can arise for many reasons, involving nonlinearities in the two phases of El Niño Southern Oscillation (ENSO) itself and those occurring along the pathway of an atmospheric response. Here we search for nonlinearity in the ENSO stratospheric pathway. The ENSO stratospheric pathway is a teleconnection linking Equatorial Pacific SST anomalies to the North Atlantic and European regions, mediated by the stratospheric polar vortex. To address the question of nonlinear responses to ENSO, we use Large Ensembles (LEs, ~30 members) of historical simulations performed with six CMIP6 models. In addition, we consider the MPI-GE, a set of 95 members. In doing so, we assess the role of internal variability on ENSO related nonlinearities, with consistent atmosphere – ocean coupling. For the models with a stratospheric pathway, three key pathway metrics (tropospheric North Pacific low, polar night jet and North Atlantic pressure dipole) tend to show linearity in the response to moderate to strong EN events, but not so for LN events. Common to these models is also the tendency for asymmetry of strong events, with the EN response larger in amplitude. Instead, all metrics tend to be symmetric for weak and moderate events. The largest variety of outcomes across the models is found in the North Atlantic sea level pressure response, possibly because of the contribution of other factors, such as tropospheric processes, affecting the response.
