The Dynamics of the Northern Annular Mode at Weekly Time Scales

There are numerous studies discussing the influence of the stratospheric Northern Annular Mode (NAM) on the tropospheric NAM. However, as the typical time scale of the latter is much smaller than the former, some intrinsic tropospheric variability may lead to the tropospheric NAM, regardless of the stratosphere. The purpose of the present study is to present a tropospheric mechanism explaining the rapid onsets of the NAM phases at weekly time scales and proving the existence of a dynamical link between the North Pacific and North Atlantic anomalies of the NAM. The methodology is based on reanalysis datasets and simulations of a quasi-geostrophic model. A composite analysis shows that during the onset of rapidly-formed NAM events, North Pacific anomalies appear prior to the North Atlantic anomalies. The North Pacific anomalies are shown to modify the propagation of synoptic wave trains across North America. Prior to the positive NAM, the northeastern Pacific jet exhibits a strong deviation which deflects the downstream propagation of the synoptic wave trains in such a way that they mainly propagate equatorward in the Atlantic and break anticyclonically there. This process triggers the North Atlantic Oscillation (NAO) positive anomalies of the positive NAM event. On the contrary, prior to the peak of the negative NAM, the Pacific jet is more zonal, there is less downstream propagation of synoptic wave energy across North America and the propagation is more zonal. It favors cyclonic wave-breaking and the onset of negative NAO anomalies of the negative NAM event. The two scenarios are well reproduced by short-term nonlinear simulations of the QG model. Finally, the mechanism is shown to be at stake during the contrasting boreal winters 2009-2010 and 2013-2014 for which the NAM was negative and positive respectively.