The Arctic Oscillation and circulation - positive and negative phase
The Arctic Oscillation is characterized by different air pressure over the High Arctic relative to lower northern latitudes. In the positive phase, low pressure over the High Arctic pulls warmer, wetter air northwards from lower latitudes. This pattern predominated during the 1980s and early 1990s, and could have partly explained rising temperatures over the Arctic during that time. The warming was strongest over the Eurasian Arctic, consistent with wind patterns associated with the positive phase of the Arctic Oscillation.
Since 1997, the Arctic Oscillation has frequently switched between positive and negative phases. In the negative phase, pressure is high in the far North, keeping the Arctic cold and dry, and pushing frigid air southwards over North America and Eurasia. Despite frequent occurrences of this pattern, overall Arctic temperatures have continued to rise. In December 2009 and January 2010, and again in December 2010 and January 2011, the Arctic Oscillation was in a very negative mode, indicating higher pressure in the High Arctic. Central Europe was extremely cold, but the High Arctic stayed relatively warm. So increasingly high Arctic temperatures observed since 2005 cannot be explained by the Arctic Oscillation.