LAUSR

Funding information National Science Foundation, 1265182. An unusually prolonged retraction of the tropopause-level jet transformed the circulation in the North Pacific for weeks in mid-February and March 2006 and preceded the development of a long-lived negative Pacific–North American pattern. The initiation of this jet retraction, associated with a series of anticyclonic/LC1 wave-breaking events at high latitude, is investigated through the lens of quasi-geostrophic (QG) piecewise tendency analysis. Two key anticyclonic anomalies divert and retract the jet through serial LC1 wave-breaking events, mostly confined to the 315–330 K isentropic layer and occurring in regions of strong deformation. The rapid development of the first anomaly coinciding with the beginning of retraction is diagnosed in a QG framework, in order to quantify contributions to QG height tendencies from various processes. The resultant analysis reveals that growth was induced by a deep potential vorticity intrusion that perturbed the jet, while the remainder of the ridge’s life cycle was largely governed by upper-level deformation in the jet exit region. Deformation facilitated both growth and decay, as the phasing between the ridge and deformation changed. Baroclinic development also contributed to growth but, unlike prior published analyses of anticyclones in the North Pacific, was not the dominant term.