The positive polarity of the Arctic Oscillation (top) is characterized by a strengthening of the polar vortex from the surface to the lower stratosphere. Cool winds sweep across eastern Canada while North Atlantic storms bring rain and mild temperatures to Northern Europe. Drought conditions prevail over the Mediterranean region. During the negative polarity of the Arctic Oscillation (bottom), cool continental air plunges into the Midwestern United States and Western Europe while storms bring rainfall to the Mediterranean region.