What controls probability distribution of local wave activity in the midlatitudes?

Abstract

<p>We examine probability distributions of <em>local wave activity</em> (LWA), a measure of the jet stream's meander, and factors that control them.  The observed column-mean LWA distributions exhibit significant seasonal, interhemispheric, and regional variations but are always positively skewed in the extratropics, and their tail often involves disruptions of the jet stream.  A previously derived 1D traffic flow model driven by observed spectra of transient eddy forcing qualitatively reproduces the shape of the observed LWA distribution.  It is shown that the skewed distribution emerges from nonlinearity in the zonal advection of LWA even though the eddy forcing is symmetrically distributed.  A slower jet and stronger transient and stationary eddy forcings, when introduced independently, all broaden the LWA distribution and increase the probability of spontaneous jet disruption.  Quasigeostrophic two-layer model also simulates skewed LWA distributions in the upper layer.  However, in the two-layer model both transient eddy forcing and the jet speed increase with an increasing shear (meridional temperature gradient), and their opposing influence leaves the frequency of jet disruptions insensitive to the vertical shear.  When the model's nonlinearity in the zonal flux of potential vorticity is artificially suppressed, it hinders wave-flow interaction and virtually eliminates reversal of the upper-layer zonal wind.  The study underscores the importance of nonlinearity in the zonal transmission of Rossby waves to the frequency of jet disruptions and associated weather anomalies. </p>