Jet Regimes Induced by Stratification Changes in a Dry Dynamical Core Model

The tropical circulation is typically not well represented in idealized models used to study jet dynamics. 
We implement a convective relaxation algorithm into a dry dynamical core model following Schneider and Walker (2006) to improve the representation of the driving mechanism behind the subtropical jet: the tropical meridional overturning.
We study the dependence of the general circulation on the vertical stratification set by a convective relaxation scheme.
Varying tropospheric lapse rates produces two jet regimes that are characterized by the distance between the subtropical jet and the eddy-driven jet.
The separated jet state features distances of more than 12° latitude between subtropical and eddy-driven jet and is dominant in simulations with higher tropospheric static stability.
Both jets approximately coincide in the joined jet regime, which is dominant in lower stability simulations.

In addition to a steady state analysis, transitions from one regime to the other are induced by changes in convective lapse rate.
Regime changes are also observed as events produced by natural variability in some of the model runs.
This time-dependent perspective shows that the structure of net Rossby wave dissipation in the upper troposphere, measured by the Eliassen-Palm flux divergence, is crucial in order to understand the regimes.
Jet merge and split events are mediated by upper tropospheric momentum flux variability.
They are preceded by heat flux variability and are tied to variations in the hemispheric eddy kinetic energy.
The results are also interpreted through the concept of criticality, relating meridional and vertical gradients in potential temperature.
The analysis highlights the importance of static stability and its changes to mid-latitude jet dynamics.