Physics of the Northern Annular Mode – Part 1: connection to meridional mass transfer

The Northern Annular Mode (NAM) of atmospheric variability is manifested and identified foremost as coupled variability of (1) the latitude of the most intense zonal-mean surface westerlies in the Northern Hemisphere, (2) the intensity of the zonal-mean subtropical jet (STJ) in the Northern Hemisphere and (3) the difference in zonal-mean sea-level pressure between high pressure in the subtropics (≈35°N) and low pressure at subpolar latitudes (≈65°N). The extreme phases of the NAM are referred to as the positive NAM-phase (weak STJ, very poleward position of the maximum surface westerlies and large meridional sea-level pressure gradient) and the negative NAM-phase (intense STJ, very equatorward position of the maximum surface westerlies and weak or sometimes reversed meridional sea-level pressure gradient). Because the zonal-mean state of the atmosphere is in close thermal wind balance, zonal-mean mass- and vorticity-distributions are connected by the presence of a positive potential vorticity (PV) anomaly, which defines the lowermost stratosphere and induces the STJ and its associated tropospheric baroclinicity. The amplitude of this PV-anomaly is reduced at high (low) latitudes due to poleward (equatorward) isentropic baroclinic eddy-driven shifts of mass during the negative (positive) NAM-phase. This sharpens (weakens) the meridional gradient of PV in the subtropics. Adjustment to zonal-mean thermal wind balance intensifies (weakens) tropospheric baroclinicity below the PV-anomaly and intensifies (weakens) the STJ during the negative (positive) NAM-phase. A feedback loop is identified, which explains the persistence of the negative NAM-phase. This loop features the zonal-mean baroclinicity at the edge of the subtropics and zonal-mean isentropic eddy-driven mass flux divergence in the lowermost stratosphere just poleward of the subtropics.