Case Study of a Cold-Season, north Pacific Jet Retraction Event

The north Pacific jet stream is characterized by two prominent modes of variability. The leading mode is manifest as a zonal extension or retraction of the jet core. A retracted jet is associated with a negative Pacific-North American pattern and anomalous extratropical cyclone activity over the Hawaiian Islands. While the composite evolution of the flow field during jet retractions has been described (Jaffe et al 2010), a detailed case study of an individual jet retraction has, as yet, not been undertaken.

We present the results of a case study of an unusually robust jet retraction that occurred in February 2006 and was linked to very heavy flooding on Oahu (Jayawardena & Chen 2011). An extended jet in early February decelerated for several days culminating in the robust retraction. Prior to the onset of the retraction, a strong, zonally elongated jet extended well past the dateline. An upper level trough north of the jet was bookended by two high-amplitude ridges. The upstream ridge was centered over the Tibetan plateau with predominantly northerly flow on its eastern edge. The extremely high-amplitude downstream ridge was centered near the Aleutian Islands and extended far into the Arctic north of Alaska. As the jet retraction began, the upstream ridge weakened and the northerly flow on its eastern flank veered to westerly. Meanwhile, the downstream ridge became positively tilted, ultimately breaking anticyclonically, placing a mid-tropospheric ridge in the eastern Pacific. Our analysis links this sequence of events to a change in the upper-level Tibetan high circulation, which at lower levels is linked to the East Asian Winter Monsoon (EAWM). This leads us to the suggestion that Pacific jet retraction is modulated by the EAWM.