Evidence for Nonlinear Processes in Fostering a North Pacific Jet Retraction

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.