| Summary: | El Ni˜no-Southern Oscillation (ENSO) is a natural phenomenon in the tropical Pacific and the dominant mode of climate variability on interannual timescales. The first term, El Ni˜no, refers to a recurring warming of the tropical Pacific Ocean (every 2-7 years), while the opposite phase, an anomalous cooling, is called La Ni˜na. These variations in sea surface temperature (SST) are accompanied by changes in the tropical atmospheric circulation (Southern Oscillation), thus making ENSO a coupled phenomenon involving ocean-atmosphere interactions. Furthermore, ENSO can affect climate in regions far from the tropical Pacific, producing a cascade of global impacts through so-called ‘teleconnections’. Understanding the extra-tropical impacts of ENSO is important to improve seasonal forecasts, for which it represents the most important source of predictability. In the North Atlantic-European (NAE) sector, the ENSO teleconnection is still controversial in several aspects. A first cornerstone was set in a review by Br¨onnimann (2007) [1], who concluded that a robust, ‘canonical’ ENSO signal exists over the NAE region in late winter (January to March, JFM): a dipole in sea-level pressure (SLP) with centers over the midlatitude and high-latitude North Atlantic. While Br¨onnimann described this pattern as “close to symmetric” for El Ni˜no and La Ni˜na, recent studies deliver contradictory results, with some reporting a symmetric signal (e.g. [2] [3] [4]) and others claiming asymmetry (e.g. [5] [6] [7]). The actual linearity of the ENSO-NAE teleconnection thus remains unresolved, and addressing this issue is the primary objective of this study. We will also investigate another key aspect of the ENSONAE teleconnection that is nothing but settled: the dynamical mechanism leading to the ‘canonical’ SLP dipole. The underlying idea of this study is to use idealized experiments with atmospheric models forced by symmetric anomalous SST patterns representing El Ni˜no and La Ni˜na to diagnose symmetries and asymmetries in the ...
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