Tropospheric pathways of the late-winter ENSO teleconnection to Europe

The late-winter signal associated with the El Niño-Southern Oscillation (ENSO) over the European continent is unsettled. Two main anomalous patterns of sea-level pressure (SLP) can be identified: a “wave-like” pattern with two opposite-signed anomalies over Europe, and a pattern showing a single anomaly (“semi-isolated”). In this work, potential paths of the tropospheric ENSO teleconnection to Europe and their role in favoring a more wave-like or semi-isolated pattern are explored. Outputs from historical runs (1850-2005, 3 members) of two versions of the MPI-ESM coupled model, which simulate these two types of patterns, are examined. The two versions of MPI-ESM have same horizontal resolution in the atmosphere (T63/1.9º), same top (0.01 hPa), but different vertical resolution: 47 levels in the low-resolution (LR) version and 95 in the mixed-resolution (MR) one. Results from the models are compared with observational ones using the NOAA-20CR reanalysis. A novel ray-tracing approach that accounts for zonal asymmetries in the background flow is used to test potential propagation paths in these simulations and in observations; three source regions are considered: the tropical Pacific, the North America/North Atlantic, and the tropical Atlantic. The semi-isolated pattern is suggested to be related to the well-known Rossby wave train emanating from the tropical Pacific, either via a split over northern North America or via reflection due to inhomogeneities in the background flow. The wave-like pattern, in turn, appears to be related to a secondary wave train emerging from the tropical Atlantic. The competition between these two pathways contributes to determining the actual surface response.