Was the North American extreme climate in winter 2013/14 a SST forced response?

Abstract

This study analyzed the possible causes of the atmospheric circulation anomalies responsible for the severe North American (NA) climate in the winter 2013/14. The seasonal (December-January-February) mean circulation anomalies, characterized by an extremely strong ridge off the west coast of NA and an intense trough over the continental US, appeared as a part of wave train across the North Pacific, NA and North Atlantic. The lack of heating-circulation response relationship in the tropics, however, suggested that the wave train might not be a direct response to tropical heating. Meanwhile, dramatic month-to-month variations of the atmospheric circulation over the northeastern Pacific excluded the possibility that the underlying sea surface temperature (SST) anomalies played a dominant role. A further examination of monthly mean circulation patterns suggests that the wave train was initiated by a heating–cooling dipole at the equatorial Pacific in December 2013, and subsequently, was maintained and modified by the extratropical atmospheric internal dynamics in following months. The evolution of tropical heating from December 2013 to February 2014 was associated with the Madden Julian Oscillation (MJO), instead of a quasi-steady response to underlying SST anomalies. Some features of the circulation anomalies in the extratropics for the season can be captured by an atmospheric general circulation model (AGCM) forced with observed SST, but a lack of persistent heating-circulation response relationship in the tropics implied a crucial role of internal variability to extreme NA climate anomalies in the winter 2013/14.