Tropical influence on the North Pacific Oscillation drives winter extremes in North America

Since the turn of the twenty-first century, North America has experienced a number of record-breaking warm and cold winters. Thus, determining what causes these extremes is of great interest. Here we show that an eastward shift of the North Pacific Oscillation (NPO) in recent decades has caused its flip in phases to have more influence in causing abnormal warming and cooling over North America. Observations and climate models reveal the zonal displacement on an interdecadal timescale, and it is largely attributable to a Rossby wave response to the La Niña-like mean state of the tropical Pacific. This tropical influence affects the atmospheric mean baroclinicity over the extratropical North Pacific, which regulates the rate of available potential energy conversion that feeds the NPO. These results suggest that, as long as the NPO remains in the east, North America may continue to experience prolonged winter extremes.