The North Atlantic Oscillation (NAO) is a weather phenomenon that is defined by fluctuations in atmospheric pressure differences between the Icelandic Low and the Azores High in the North Atlantic region. This oscillation significantly influences weather patterns in Europe and North America, affecting precipitation, temperature, and storm tracks. The NAO has two phases: 1. **Positive NAO Phase**: During this phase, the pressure difference between the Icelandic Low and the Azores High is greater than normal.
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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.[ref]
The North Atlantic Oscillation by Met Office. Definition, north-south pressure contrast and positive/negative circulation regimes
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.