ENSO teleconnection versus NAO variability in late winter

An observational and multi-model approach is used to investigate the ENSO signal over the North Atlantic-European sector sector in late winter (JFM). The ENSO influence on the North Atlantic-European (NAE) region is still under debate, concerning both the amplitude and the underlying dynamics. The difficulties in detecting the ENSO-related signal in the North Atlantic are mainly due to the large internal variability of the region, and to the tendency of the ENSO signature to project on a dipolar pattern in sea-level pressure (SLP) that resembles the North Atlantic Oscillation (NAO). The relationship between this "NAO-like" ENSO signal and the actual NAO is controversial.

First, the linear response to SST anomalies in the Nino3.4 region is assessed in reanalysis data extending back to the beginning of the 20th century (NOAA- 20CR, ERA-20CR) for variables at both the surface and upper levels. The resulting ENSO-related patterns are compared with the corresponding NAO signatures, computed analogously via linear regression onto the PC-based NAO index. While the similarity in SLP is evident, the analysis of anomalous upper-tropospheric geopotential height, zonal wind, transient-eddy momentum flux, as well as precipitation and zonal-eddy heat flux, suggests that there is no dynamical link between the phenomena.

Atmosphere-only simulations are used to further assess the distinct dynamics of the two teleconnections: a control experiment with climatological SSTs is contrasted with a sensitivity experiment where a canonical El Niño event is superimposed on the seasonal cycle; the former is analyzed to characterize the internally-generated NAO dynamics, while the latter provides the ENSO-forced atmospheric circulation. Finally, a similar protocol is followed to study the impact of La Niña, i.e. the same cycle but with flipped-sign SST anomalies is prescribed in a new sensitivity experiment. The multi-model assessment, with 50 members for each experiment, contributes to the ERA4CS-funded MEDSCOPE project and includes: EC-EARTH/IFS (L91, 0.01hPa), CNRM/ARPEGE (L91, 0.01hPa), CMCC/CAM (L46, 0.3hPa).