While the dry feedback mechanisms for the extended memory of the Southern Annular Mode (SAM) beyond a typical weather scale are well understood, the role of diabatic feedbacks, especially that due to the radiative effect of the cloud, remains largely elusive. A cloud locking technique is applied to disable the interactive cloud radiative feedback in DOE’s E3SM atmospheric model by replacing the cloud optical properties taken from a separate run. The result reveals the positive feedback from the cloud radiative effects (CRE) to the standing dipole wind structure of Southern Annual Mode (SAM). This is further confirmed by an annular mode feedback analysis, that explicitly accounts for the contributions to the zonal momentum budget from the diabatic wave sources.

Since SAM is essentially a propagation mode, as recently discovered by Lubis and Hassanzadeh (2021), block-DMD, a data-driven algorithm capable of discriminating between feedback and feedforward connections is developed to elucidate the feedback mechanisms of the propagating SAM. Treating both the zonal wind and its momentum/wave activity sources as the system variables, the block-DMD analysis reveals that wave breaking and the resulting wave activity dissipation are responsible for initiating the poleward propagation in the Southern Hemisphere (SH) zonal wind, while the diabatic heating-induced wave source acts as an impediment to propagation. A comparison between the cloud locking run and the control run in which CRE is active shows that CRE, as a part of the diabatic feedback, acts to dampen propagation, reducing the predictability of the leading mode of the SH zonal wind variability in the control run. These results are further verified by analyzing MERRA-II reanalysis data.