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On average, the surface pressure near Iceland is relatively low (L), while the pressure near the Azores Island is relatively high (H). During a negative phase (left), this pressure difference weakens.  During a positive phase (right), the difference becomes even stronger than usual. The variation in pressure patterns influences the strength and location of the jet stream and the path of storms across the North Atlantic.  Schematic adapted from AIRMAP by Ned Gardiner and David Herring, NOAA.