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The Arctic Oscillation describes a relationship between pressure fields in the Arctic which governs atmospheric mass distribution around the hemisphere. Central to it is its Polar Vortex, mostly in the stratosphere, that is usually manifested as strong easterly winds around the North Pole. In the negative phase of the AO, the vortex is weakened and this allows a more meridional flow - meaning more cold Arctic air can slide south, invading the lower latitudes. For different (dynamical) reasons, this more high-amplitude pattern is more conducive to the development and amplification of cyclones in Eastern North America and Western Europe. That's why it tends to be colder and snowier in these phases of the AO in these locations.

Check out the related link for the standard diagram of the negative vs. positive phases.

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14y ago

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