Diabatic Damping of Zonal Index Variations

Abstract

onal index variations, or north?south shifts of the midlatitude jet, are the dominant mode of zonal wind variability in the Southern Hemisphere. Previous studies have shown that synoptic-time-scale eddy momentum flux provides a positive feedback and acts to increase the persistence and low-frequency variance of the zonal index. However, the impact of diabatic heating due to the precipitation associated with these eddies has not been investigated. In this study, regression analyses have been conducted to demonstrate that a robust precipitation anomaly can be found to accompany the jet and eddy momentum flux anomalies associated with a poleward shift of the jet, with enhanced precipitation on the poleward flank of the jet and reduced precipitation on the equatorward flank. Diabatic heating associated with such a precipitation anomaly is expected to reduce the temperature gradient across the jet anomaly, thus decreasing eddy generation and damping the anomaly. This expectation is confirmed by three sets of mechanistic model experiments, using three different ways to mimic the impact of moist heating in a dry model. Results of this study suggest that diabatic heating provides a negative feedback to zonal index variations, partially offsetting the positive feedback provided by eddy momentum flux. These results could partially explain why zonal index variations have been found to be very persistent in dry mechanistic model experiments since this negative diabatic feedback is absent in dry models. These results suggest that these models may be overly sensitive to climate forcings that produce a jet shift response.