Annular modes
Climate Change 2021 – The Physical Science Basis, Annex VII: Glossary, written by the Intergovernmental Panel on Climate Change (IPCC) and published by Cambridge University Press in 2023, defines annular modes as: ‘Hemispheric scale patterns of atmospheric variability characterized by opposing and synchronous fluctuations in sea level pressure between the polar caps and mid-latitudes, with a structure exhibiting a high degree of zonal symmetry, and with no real preferred time scales ranging from days to decades. In each hemisphere, these fluctuations reflect changes in the latitudinal position and strength of the mid-latitude jets and associated storm tracks. Annular modes are defined as the leading mode of variability of extratropical sea level pressure or geopotential heights and are known as the Northern Annular Mode (NAM) and Southern Annular Mode (SAM) in the two hemispheres, respectively.’
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