An Anomalously Cold 2013–14 North American Winter: Role of the West Pacific Pattern / North Pacific Oscillation

The 2013-14 boreal winter (December 2013 – February 2014) brought extended periods of anomalously cold weather to central and eastern North America. We show that a leading pattern of extratropical circulation variability, the West Pacific pattern, whose footprint in sea-level pressure is the North Pacific Oscillation – together, the WP/NPO – exhibited extreme and persistent amplitude in this winter. Reconstruction of 850-hPa temperature, 200-hPa geopotential height, and precipitation reveals that WP/NPO was the leading contributor to the winter climate anomaly over much of North America. Our analysis, furthermore, indicates that WP/NPO explains the most monthly variance of wintertime temperature over east-central North America since, at least, 1979. Analysis of the WP/NPO related thermal advection provides physical insight on the generation of the temperature response over North America.

Our finding suggests that notable anomalies in boreal winter climate need not originate, directly, from the Tropics. More broadly, the attribution of the severe 2013-14 winter to the flexing of an extratropical variability pattern is cautionary given the propensity to implicate the Tropics, following several decades of focus on El Nino Southern Oscillation and its regional and far-field impacts.