The Importance of Zonal Asymmetry in the Southern Hemisphere Circulation

Abstract:

The atmospheric circulation in the Southern Hemisphere is considered a good example of a zonally symmetric jet, but recent papers have questioned the accuracy of this approximation. Numerous studies of the southern annular mode and the budget of zonal-mean momentum have sought to understand the timescales and causes of variability, as well as trends associated with climate change. This raises a question: when does taking the zonal mean give you the wrong answer?

To address this question we employ the CESM-Large Ensemble, a 30-member, fully coupled set of simulations, suitable for assessing both the forced response and natural variability of the climate system. We find that in DJF, 20^th century trends associated with the ozone hole are uniform with longitude and height, and far exceed the natural variability of the system. In contrast, the JJA circulation and its trends have a longitudinal structure that is obscured by zonal averaging. The consequence is that a significant regional deceleration of the jet stream predicted for the coming century, presumably driven by increasing concentrations of greenhouse gases, is completely eliminated in the zonal mean.