Diabatic damping of zonal index variations

Zonal index variations, or north-south shift of the eddy-driven westerly jet, is the leading mode of variability of the mid-latitude tropospheric zonal mean circulation. Previous studies have shown that significant positive feedback via changes in the eddy-momentum flux acts to prolong the e-folding time-scale of this mode. In this study, a new diabatic feedback has been discovered that can potentially lead to damping of the jet variations. This feedback is found in reanalysis data, observations, as well as in IPCC-AR4 GCM simulations.

The impact of this diabatic feedback on zonal index variations is confirmed by imposing diabatic heating anomalies similar to those derived from reanalysis and GCM data in a dry global circulation model. Results of these experiments confirm that diabatic feedback associated with a poleward shift of the jet acts to drive the jet equatorward and vice versa. Imposing a time varying diabatic feedback as a response to jet variations acts to significantly reduce the zonal index time-scale in the dry model, potentially alleviating the problem of unrealistically long zonal index time-scales commonly found in idealized dry global circulation model simulations.