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

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Published: 2024-12-16
DOI: https://doi.org/10.4401/ag-9013

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Aarnout J. van Delden
Institute for Marine and Atmospheric Research Utrecht (IMAU), Physics Department Utrecht University, Utrecht, the Netherlands
https://orcid.org/0000-0002-4980-5016

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.

Article Details

Issue

Vol. 67 No. 5 (2024)

Section

Paleoclimatology
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How to Cite

(1)
van Delden, A. J. Physics of the Northern Annular Mode – Part 1: Connection to Meridional Mass Transfer. Ann. Geophys. 2024, 67 (5), GC545. https://doi.org/10.4401/ag-9013.