Influence of Long and Short Planetary Waves on the Separation of the Eddy-Driven and Subtropical Jets

Abstract

The interaction of the subtropical and eddy-driven jets is associated with both the internal low-frequency variability of the midlatitudes and the atmospheric response to anthropologically forced climate change. Here we examine the mechanisms that lead to a merger and separation of the jets in time varying and the statistically steady state of numerous experiments using two idealized models: a barotropic b-plane and the dry dynamical core on a sphere. We specifically focus on the interaction of the jets due to changes in the meridional propagation of planetary waves of varying length scales.

In both models we find that the eddy momentum flux convergence waves are bound in zonal phase speed by a wavelength-dependent minimum phase speed associated with wave reflection and turning latitudes. In mean flow regime with two distinct jets, these turning latitudes are located within the interjet region and inhibit the equatorward propagation of short planetary waves. Short waves are largely trapped in the eddy-driven jet waveguide. Long waves, on the other hand, interact with both the subtropical and eddy-driven jet. We find that short waves tend to sharpen the eddy-driven jet and long waves, displaying eddy momentum flux convergence patterns similar that that as would be expect from barotropic instability, act to widen and merge the eddy-driven and subtropical jets.

We additionally note that these idealized models contain significant low frequency variability similar to that of the observed atmosphere, namely the poleward propagation of anomalies to the zonal mean flow. We hypothesize that these features of low frequency variability are a product of wave-mean flow interaction and the migration of critical lines due to wave breaking.