Circulation widening in an instantaneously forced climate

The recent poleward shift of the southern hemisphere midlatitude jet, though largely reproducible in climate simulations, is not well understood. To study the underlying mechanisms, we apply idealized, instantaneous forcings in a realistic atmospheric general circulation model (GFDL AM2.1). The prescribed forcings include greenhouse gas increases, stratospheric ozone depletion, and sea surface warming. This blend of idealized forcings and comprehensive model physics allows us to discriminate between several hypothesized mechanisms behind the poleward shifts. While changes in the wavenumber and phase speed of midlatitude eddies may feed back onto existing jet latitude anomalies, markers of the changing wave reflection mechanism recently posited by David J. Lorenz (2014) appear to explain best the poleward shifts seen in the model. This study thus constitutes an early validation of the Lorenz mechanism in an earth-like general circulation model. Our results also echo those from an idealized simulation by Chen et al. (2013), which uses potential vorticity arguments to explain the circulation widening.