The Tropical Pacific and Sub-Arctic Weather in North America

Abstract:

In addition to the classical El Niño, other structures in tropical Pacific SST anomalies have a strong influence on extratropical weather. Since 1979 the second EOF of global SST is a mode with warm SST in the western equatorial Pacific and cool SST in the eastern equatorial Pacific. This pattern has a strong signal in the extratropical North Pacific associated with the Rossby wave anomalies that are forced in the tropics and propagate over North America, and has been termed the North Pacific Mode. This mode is associated with a ridge on the west coast of North American and a trough downstream of the Rocky Mountains. This mode of variability had a strong influence on the winters of 2014 and 2015, and appears to have played a strong role in the cold winter of 2014 in central and eastern North America. This pattern persisted from the middle of 2013 through at least July of 2015, and also seems to have an effect on summer weather anomalies of the same nature. In the past the strength of the North Pacific Mode has preceded strong El Niño events, after which point it went into a strongly negative phase. This history suggests a transition to a more classical El Niño anomaly pattern for the winter of 2016, which would produce a warmer winter in central and eastern North America and cool and moist conditions over the Gulf of Mexico.