Seasonality in the relationship between the Hadley cell and the eddy-driven jet and their response to climate change forcing

Intriguing seasonality was found in the Hadley cell (HC) and eddy-driven jet (EDJ) response in the Southern Hemisphere to climate change forcings like ozone depletion and/or increase of greenhouse gas concentrations. The response of both factors reaches maximum during austral summer, with an intriguing ratio of ~2° degrees of shift of EDJ per 1° expansion of the HC. Moreover, the 1:2 slope between the HC and the EDJ also hold in the ENSO-related interannual variability as well as in the interannual variability in the absence of the any variation of SST or climate forcing.

We set out to investigate the intrinsic relationship between the width of the HC and the position of the EDJ and its dependence on season using an idealized aqua-planet AGCM with the SST forcing constructed based on the seasonality of the atmospheric boundary conditions. It turned out nontrivial for the idealized experimental setting to reproduce the tight relationship between the HC and the EDJ. In addition, idealized simulations with the SSTs encompassing a range of seasonality suggest that the extent to which the mid-latitude activity may influence the HC scales with the persistence of the leading mode of the variability of the eddy-driven wind.