Suppressed eddy driving during southward excursions of the North Atlantic jet on synoptic to seasonal time scales

Abstract

Jet streams shape midlatitude weather and climate. The North Atlantic jet is mainly eddy‐driven, with frequent north–south excursions on synoptic time scales arising from eddy forcings and feedbacks. There are, however, special periods during which the underlying dynamics appear to change—for example, winter 2009/2010, when the jet was persistently southward‐shifted, extremely zonal, and more thermally driven. This study shows evidence that the southern jet configuration exhibits altered dynamical behavior involving a shift in the balance of thermal and eddy‐driving processes, independent of timescale. Specifically, southern jets exhibit weaker eddy feedbacks and are associated with enhanced heating in the tropical Pacific. During winter 2009/2010, a remarkably frequent (66 days out of the 90‐day winter season) and persistent southern jet shaped the unusual seasonal signature. These results bridge the synoptic and climate perspectives of jet variability, with potential to help understand and reduce biases in regional climate variability as simulated by models.