West Pacific Jet Retraction: A Case Study and Phase Space Analysis

The north Pacific jet stream is characterized by two prominent modes of variability. The leading mode (EOF1) is manifest as a zonal extension or retraction of the jet, while the second (EOF2) is a north/south shifting of the jet exit region. A retracted jet (EOF1-) is associated with a negative Pacific-North American pattern, characterized by anomalous extratropical cyclone activity over the Hawaiian Islands and East Pacific blocking. While the composite evolution of the flow field during jet retractions has been described (Jaffe et al 2010), no detailed case study of an individual jet retraction has yet been undertaken.

A retraction that occurred in February 2006 triggered transition to a negative PNA pattern that lasted for six weeks. Analysis of this event reveals that anticyclonic wave breaking facilitated the transition by systematically redistributing the upper level potential vorticity (PV). Prior to retraction, negative PV anomalies were located upstream and downstream of the West Pacific jet. Evidence is presented that these two features eventually merge to form a high-amplitude ridge that ultimately breaks anticyclonically in the midlatitude East Pacific. The sources of the initial negative PV anomalies associated with this evolution are discussed, with particular consideration given to the phase of the Madden-Julien Oscillation and the role of a Sudden Stratospheric Warming event that occurred three weeks prior to the onset of the retraction. Analysis of the jet's evolution in EOF1/EOF2 phase space reveals that the jet is initially extended and northward-shifted (EOF1+, EOF2+). It proceeds to shift southward (EOF2+ → EOF2~) and then retract westward (EOF1+ → EOF1-) for several days before shifting even further southward (EOF1-, EOF2-). The meridional shifting and zonal retracting occur sequentially rather than simultaneously. Evidence of other retractions in the EOF phase space is presented that suggests this evolution is anomalous.