Dynamics of the late-winter ENSO teleconnection to the North Atlantic-European region

The El Niño-Southern Oscillation (ENSO) is a natural mode of climate variability in the tropical Pacific, which not only perturbs the local atmosphere, but whose impacts can also reach remote, extra-tropical regions through atmospheric teleconnections. While the atmospheric response to ENSO and the dynamics of its teleconnections are well assessed in certain locations, such as the North Pacific-American sector, it is not the case for the North Atlantic-European (NAE) sector. In this thesis, the dynamics of the ENSO-NAE teleconnection are assessed in late winter (January-March), the season in which a significant and robust ENSO-related signal has been reported in the literature.
\nPart of this well-known late-winter ENSO signal is a sea-level pressure (SLP) dipole in the North Atlantic. In this thesis, it is assessed that this “canonical” dipole is mostly driven by tropospheric dynamics and is related to the large-scale Rossby wave train triggered from the tropical Pacific by the anomalous upper-level divergence associated with ENSO. 
\nThe “canonical” SLP dipole is known to be reminiscent of the surface signature of the North Atlantic Oscillation (NAO), which is the dominant mode of variability in the North Atlantic. The relationship between the ENSO-forced response in the NAE region and the NAO is examined: it is found that no further similarities are evident apart from the surface SLP signature, and it is suggested that ENSO and the NAO are largely unrelated. 
\nAnother issue tackled in this thesis is the (a)symmetry of the atmospheric signal associated with the two phases of ENSO, El Niño and La Niña, which show roughly opposite patterns of sea surface temperature (SST) anomalies in the tropical Pacific but not necessarily a symmetric atmospheric response. It is noticed that, in a set of sensitivity experiments with three state-of-the-art models, the extra-tropical response in the troposphere is slightly asymmetric in amplitude and longitudinal location. However, it is highlighted that the mechanisms at play, in particular concerning the response in the NAE region, are the same for El Niño and La Niña. 
\nThe ENSO impact on the Northern Hemisphere polar stratosphere is also studied, since a stratospheric pathway of the ENSO-NAE teleconnection has been suggested in the literature, and an ENSO signal in the polar stratosphere has been previously reported. In the same set of experiments, it is assessed that the stratospheric response to La Niña is symmetric in structure to that of El Niño, but with smaller amplitude. This response is found in both the lower and middle-upper stratosphere and is suggested to be related to the upward propagation of the ENSO-forced tropospheric Rossby wave train, and specifically to its center of action located over North America/Canada. In the middle-upper stratosphere, the anomalies project onto a wavenumber-1 pattern and show a westward tilt with height that indicates upward wave propagation rather than wave-breaking, as previously suggested. 
\nFinally, it is noticed that the well-established “canonical” dipole is mostly located over the North Atlantic, while less is known about the ENSO signal over the European continent. A novel ray-tracing approach that considers zonal asymmetries in the background flow is developed to examine potential tropospheric pathways of the ENSO teleconnection to Europe. In some cases, a SLP pattern featuring a single anomaly over Europe is present, which appears to be linked to the ENSO wave train emanated from the tropical Pacific via a split over northern North America or via reflection due to zonal inhomogeneities in the background flow. Alternatively, a wave-like pattern with two opposite-signed SLP anomalies over Europe is observed, which is suggested to be related to a secondary wave train emerging from the tropical Atlantic.

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