Role of the Pacific-North American (PNA) pattern in the 2007 Arctic sea ice decline

The extreme loss of sea ice over the western Arctic during the boreal summer of 2007 (July–August–September) was accompanied by a very unusual atmospheric circulation pattern. Here we show that the anomalous circulation was linked to a leading climate mode of the Northern Hemisphere, the Pacific‐North American (PNA) pattern. The PNA index was three standard deviations above the 1950–2007 mean, and its primary signal in the atmospheric circulation is a strong anomalous anticyclone that was collocated with the location of the greatest Arctic sea ice decline. Therefore, the record‐strength PNA along with recent climate trends in the Arctic help to explain the sudden and extreme sea ice melt during the summer of 2007. While the observed PNA was unique, the large decline in sea ice may be further evidence of increased vulnerability to natural atmospheric variability due to a changing climate system.