Evidence for nonlinear processes in fostering a North Pacific jet retraction

Abstract

A diagnostic method for calculating local geostrophic wind tendencies in a piecewise manner within the quasi‐geostrophic framework is introduced. The method is applied to a case‐study of a North Pacific jet retraction that occurred in February 2006, and suggests that nonlinear interactions, which are dependent upon the phasing between potential vorticity anomalies and height anomalies, can lead to a weakening of the jet. The synoptic context in which nonlinear advection weakens the jet is presented, revealing that a positively tilted wave train situated north of the jet is conducive to retraction. This circumstance is consistent with conditions associated with barotropic energy extraction in which the growth of eddies occurs at the expense of the kinetic energy of the mean state. The relationship between this new method and existing methods of assessing geostrophic wind tendencies is explored and, though broad consistency is found, importance differences are identified and considered.

Composite Vertical-Motion Patterns near North American Polar–Subtropical Jet Superposition Events

“…4a-c reveal that the distributions of 300-hPa QGPV [scaled by 2g(›Q r /›p)] and 300-hPa Ertel PV are strongly correlated 1 and qualitatively similar across all event type composites at the time of jet superposition. Prior applications of QGPV inversion to the synoptic-scale flow pattern (e.g., Holopainen and Kaurola 1991;Black and Dole 1993;Hakim et al 1996;Nielsen-Gammon and Lefevre 1996;Wandishin et al 2000;Breeden andMartin 2018, 2019) provide additional confidence in the utility of QGPV inversion for investigating jet superposition environments.…”

“…In turn, the vertical-motion pattern associated with the calculated mass and wind fields can be determined using an v equation that is consistent with the balance condition used to perform the aforementioned PV inversion. The application of PV inversion has shown considerable utility for investigating a variety of characteristics of the atmospheric flow pattern, such as flow in the middle atmosphere (e.g., Robinson 1988), the planetary-scale tropospheric flow (e.g., Holopainen and Kaurola 1991), surface cyclogenesis (e.g., Davis and Emanuel 1991;Black and Dole 1993;Hakim et al 1996;Nielsen-Gammon and Lefevre 1996), tropopause folding (e.g., Wandishin et al 2000), and upper-tropospheric blocking events (e.g., Breeden andMartin 2018, 2019).…”