Using CESM2 to isolate radiative effects of clouds on tropical subseasonal variability

Interactions among radiation, moist convection, and circulation are thought to play a key role in the character of tropical subseasonal variability. For example, several global climate model (GCM) studies using column-integrated moist static energy budget analyses demonstrate the importance of cloud-radiative effects (CRE) on sustaining the Madden-Julian oscillation (MJO). While such results are qualitatively consistent with observational estimates, the magnitude and phase of radiative heating can differ considerably among the models. GCM mechanism-denial studies in which CRE are artificially disabled report substantial weakening of MJO disturbances, although some of these studies use techniques that make it difficult to disentangle cloud-radiative impacts from mean state dependencies. Other studies, however, indicate more robust intraseasonal variability or little impact on the MJO when CRE are removed. Limited attention has been paid to the effects of modifying CRE on Kelvin waves, despite strong interaction of this wave type with the MJO. The potential importance of CRE on the organization of tropical disturbances is also apparent in high-resolution modeling studies of convective aggregation.

Our project examines the impact of feedbacks among shortwave and longwave radiation, clouds, and circulation on tropical subseasonal variability, with a focus on the MJO and Kelvin waves. We conduct a suite of simulations using the latest version of the Community Earth System Model (CESM2) with both a fully dynamic ocean and prescribed-SST forcing. Within the model, we use the so-called "cloud locking" approach to disable CRE, but all other atmospheric processes evolve freely and the mean states between control and experimental simulations remain similar. Disabling CRE has profound, disturbance-specific impacts: Kelvin waves and other high-frequency disturbances are strongly enhanced, equatorial Rossby waves and the MJO are substantially weakened, and the MJO shifts toward higher frequencies. We review the theoretical arguments and physical mechanisms that explain these results and provide commentary on their implications and the relevant model biases within CESM2.