The Self-Regulated Induction of United States Cold Air Outbreaks by the North Atlantic Oscillation

Wintertime cold air outbreaks (CAOs) over the eastern United States (US) are often linked to the negative phase of the North Atlantic Oscillation (NAO) and accompanied by enhanced northerly flow from Canada. However, this relationship is not one-to-one and the physical mechanism responsible for this upstream influence has not been previously determined. Here we perform a near-surface heat budget analysis in conjunction with piecewise potential vorticity inversions to identify the underlying physical mechanism. For the events of interest, several days prior to CAO onset, the NAO pattern intensifies and retrogresses westward from its canonical position. During this time, the NAO pattern induces lower-tropospheric easterly flow extending from the North Atlantic into Canada, leading to the formation of a prominent near-surface warm air anomaly over northeast Canada. The newly created warm air anomaly, in turn, induces enhanced northwesterly flow over the eastern US seaboard and, counterintuitively, contributes to the establishment of cold air anomalies and CAO onset over the eastern US. Thereafter, however, the Canadian warm air anomaly invades southeastern and central Canada, thereby eliminating the cold air within the original source region and effectively shutting off the CAO event. Thus, the lower-tropospheric warm anomaly over Canada serves to (a) physically connect the NAO to the US CAO events and (b) regulate the duration of the CAO event.