The Arctic Oscillation (AO), also known as the Northern Annular Mode (NAM), is an important element of the variability in the weather and climate of the Northern Hemisphere. It relates fundamentally to the state of the flow at northern latitudes, with the positive phase associated with a strong, axisymmetric polar vortex and the negative phase associated with a weak polar vortex, allowing for more prominent large-scale ridges and troughs for the whole hemisphere.
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