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 heating, especially that due to the radiative effect of the cloud, remains unclear, and even the sign of the diabatic feedback is debatable. A cloud locking technique is applied to disable the interactive cloud radiative feedback in DOE’s E3SM atmospheric model by overriding the cloud optical properties taken from a separate run. The result reveals a positive feedback from the cloud radiative effects (CRE) to the Southern Annual Mode (SAM) when it is defined as a standing dipole pattern. 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, an algorithm based on the Dynamical Mode Decomposition (DMD) and 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 mainly responsible for 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. Most of the results discussed here have been verified by analysis of a separate reanalysis data set.