Clouds impeding the propagation of the Southern Annular Mode

The Southern Annular Mode (SAM), characterized by an iconic bipolar zonal wind pattern around the storm track axis, is the most dominant mode of variability regulating weather and winds from the subtropics to the poles. Its origin, maintenance, and feedbacks are among the central themes in atmospheric dynamics. However, while it has long been observed that SAM-associated wind patterns organize the distribution of clouds in the storm track, until now the role of clouds in the annular mode’s life cycle has remained poorly understood.

This research was published in the Journal of Geophysical Research: Atmospheres.

In this study, we implemented a cloud-locking technique in the atmospheric component of the US Department of Energy’s (DOE) Energy Exascale Earth System Model (E3SM), substituting the radiatively active cloud field at every time step where the radiation module is invoked, turning off the interactions of clouds with other dynamical and thermodynamic quantities.A careful feedback analysis reveals that SAM-organized cloud variations radiatively act as a brake on the propagation of AM.

Clouds and their radiative effects are one of the processes that determine the energy balance of the Earth’s climate. They are also the most difficult processes to simulate in climate models and therefore represent a significant source of model bias and model uncertainty. The identification of the braking effect of clouds on the propagation of the SAM suggests that the inaccurate representation of clouds is an important source of model bias and uncertainty in the simulated spatiotemporal characteristics of the AM. Capturing the correct sign of the cloud feedback on the AM provides a valuable metric to measure model fidelity.

The SAM is the most dominant natural variability mode in the Southern Hemisphere (SH) mid-latitudes. However, the sign and magnitude of feedbacks to the SAM from non-diabatic processes, especially cloud-related processes, remain unclear. In this study, we apply a cloud-locked technique to the E3SM atmospheric model to separate interacting cloud-radiative effects (CREs) and analyze the associated feedbacks to the SAM in both single empirical orthogonal function (EOF) and cross-EOF frameworks.

Although the magnitude of the CRE feedback appears to be secondary compared to feedbacks from drying and other non-adiabatic processes, indirect CRE effects through interactions with other dynamic and thermodynamic processes appear to play as important a role as direct CRE in the SAM life cycle.

Cross-EOF analysis further reveals that the interactive CRE plays an inhibitory role on the propagation mode of the Southern Hemispheric zonal wind, both directly through the CRE wave source and indirectly by modulating other non-diabatic processes. As a result, when the interactive CRE is disabled, the propagation mode becomes more persistent and the SAM it represents becomes more predictable.