Physics of the Northern Annular Mode – Part 2: connection to meridional vorticity transfer

A new perspective on the nature and cause of the Northern Annular Mode (NAM) is offered, by adopting isentropic coordinates, making use of the impermeability to vorticity fluxes of isentropic surfaces, and by avoiding the quasi-geostrophic approximation. The NAM is identified with the zonal-mean circumpolar flow, also called the “primary circulation”. The positive (negative) NAM-phase is characterized by a weaker (more intense) than average zonalmean subtropical jet and also a more poleward (equatorward) than average position of the maximum zonal-mean surface westerlies. The negative NAM-phase is maintained by an intense residual circulation of mass, as explained in part 1 of this paper. The positive NAM-phase is maintained by up-gradient, therefore self-perpetuating, poleward eddy vorticity fluxes in the middle latitudes, which intensify the primary circulation in middle latitudes, while driving it out of gradient wind balance. As a response to the imbalance of the primary circulation, a zonal-mean meridional overturning circulation, or “secondary circulation”, is generated in the middle latitudes. Vorticity fluxes due to the secondary circulation oppose eddy vorticity fluxes. On the monthly time-scale this opposition is nearly exact, implying a stationary state, were it not for the fact that the secondary circulation also transports mass, which, by opposing the eddy mass-flux, reduces the intensity of the residual (net) circulation of mass, and so prevents a transition to the negative NAM-phase. A transition from one extreme NAM-phase to the opposite NAM-phase takes at least several days during which either the reservoir of mass (transition to negative NAM-phase) or the reservoir of vorticity (transition to positive NAM-phase) at high latitudes in the lowermost stratosphere and upper troposphere is replenished.