Decadal variability in weather regimes and teleconnections in reanalysis datasets and century long hindcasts.

Atmospheric intraseasonal variability, especially in the extratropics during the cold season, is characterized by preferred large-scale quasi-stationary flow patterns that have non-Gaussian characteristics. They are known as weather regimes and can be associated with significant temperature and precipitation anomalies. Several observation- based and model studies have shown that El Nino-Southern Oscillation (ENSO) and Madden and Julian Oscillation (MJO) forcing affects the relative frequency of occurrence of circulation regimes. The ENSO forcing is particularly important for the Pacific North-American (PNA) region, whose intraseasonal variability is directly linked to tropical Pacific SST anomalies. The MJO forcing affects the occurrence (and sub-seasonal predictability) of the North Atlantic Oscillation (NAO) and, to a lesser extent, the Euro-Atlantic Blocking. On the other hand, several recent studies have shown that wintertime large scale extratropical circulation over both the Pacific North American and the Euro-Atlantic sectors exhibits a non-negligible interdecadal variability, which seems to be related to a decadal modulation of the tropical-extratropical teleconnections and can strongly impact on seasonal predictability (see for example Weisheimer et al. 2016, O'Reilly et al. 2017, O'Reilly 2017).