= North Atlantic Oscillation
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The <North Atlantic Oscillation> is variability in the large-scale North Atlantic atmospheric <pressure gradient>, commonly summarized by an index comparing the subtropical high-pressure region with the subpolar low-pressure region. A stronger-than-usual contrast characterizes a positive phase and is associated with stronger Atlantic westerly <winds>; a weaker contrast changes the preferred paths of weather systems.<The North Atlantic Oscillation by Met Office>[Definition, north-south pressure contrast and positive/negative circulation regimes]{ref}
The index is useful because it connects a broad circulation pattern to regional <temperature> and <rainfall> variability. It does not imply one repeating fixed-period oscillation, nor does its sign uniquely determine the weather at every location. A station-based index and an index fitted to a spatial pattern need not be numerically identical.
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