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

Abstract

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.