Dynamics of the ENSO Teleconnection and NAO Variability in the North Atlantic-European Late Winter

The winter extratropical teleconnection of El Nino-Southern Oscillation (ENSO) in the North Atlantic-European (NAE) sector remains controversial, concerning both the amplitude of its impacts and the underlying dynamics. However, a well-established response is a late-winter (January-March) signal in sea level pressure (SLP) consisting of a dipolar pattern that resembles the North Atlantic Oscillation (NAO). Clarifying the relationship between this NAO-like ENSO signal and the actual NAO is the focus of this study. The ENSO-NAE teleconnection and NAO signature are diagnosed by means of linear regression onto the sea surface temperature (SST) Nino-3.4 index and an EOF-based NAO index, respectively, using long-term reanalysis data (NOAA-20CR, ERA-20CR). While the similarity in SLP is evident, the analysis of anomalous upper-tropospheric geopotential height, zonal wind, and transient-eddy momentum flux, as well as precipitation and meridional eddy heat flux, suggests that there is no dynamical link between the phenomena. The observational results are further confirmed by analyzing two 10-member ensembles of atmosphere-only simulations (using an intermediate-complexity and a state-of-the-art model) with prescribed SSTs over the twentieth century. The SST-forced variability in the Northern Hemisphere is dominated by the extratropical ENSO teleconnection, which provides modest but significant SLP skill in the NAE midlatitudes. The regional internally generated variability, estimated from residuals around the ensemble mean, corresponds to the NAO pattern. It is concluded that distinct dynamics are at play in the ENSO-NAE teleconnection and NAO variability, and caution is advised when interpreting the former in terms of the latter.