Comparing North Pacific Wintertime Blocking and Jet Retractions: Identification, Structure, and Mechanisms

The north Pacific jet stream is characterized by two prominent modes of variability on intraseasonal timescales, the leading mode being manifest as a zonal extension or retraction of the jet. A retracted jet is associated with a persistent, sprawling midlatitude anticyclone and anomalous extratropical cyclone activity over the Hawaiian Islands, which, in concert, lead to weaker westerlies in the central Pacific. The importance of a persistent anticyclonic anomaly suggests a possible relationship between jet retractions and midlatitude blocking. Understanding of the evolution of the circulation in the north Pacific is compromised by the fact that the frequency and characteristics of regional blocking depend highly upon the methodology used to identify blocks. We present a direct comparison of retractions and blocking, with the goal of further disentangling coherent modes of variability in the north Pacific.

The composite circulation associated with jet retractions, emphasizing the structural evolution of the dynamic tropopause, is contrasted with that associated with blocks. Though a mid-basin dipole composed of a potent midlatitude anticyclone with a cyclonic anomaly to its south is observed during both retractions and blocks, the broader upper-tropospheric north Pacific circulation differs dramatically between the two. Retractions are characterized by a diffuse tropopause-level potential vorticity (PV) gradient throughout the basin, consistent with weaker westerly winds, while blocks exhibit a sharper-than-average PV gradient to the south of the dipole. Mechanistic differences between these phenomena are considered via application of local Eliassen-Palm flux diagnostics to one retraction and one block, revealing rather different eddy-mean interactions. Finally, use of a linear inverse model to identify the optimal structures that develop into retractions and blocks suggest different precursors for each type of event.