Intermodel spread in Walker circulation responses linked to spread in moist stability and radiation responses

15 The response of the Pacific Walker circulation (WC) to long-term warming remains un- 16 certain. Here, we diagnose contributions to the WC response in comprehensive and ide- 17 alized general circulation model (GCM) simulations. We find that the spread in WC re- 18 sponse is substantial across both the Coupled Model Intercomparison Project (CMIP6) 19 and the Atmospheric Model Intercomparison Project (AMIP) models, implicating dif- 20 ferences in atmospheric models in the spread in projected WC strength. Using a moist 21 static energy (MSE) budget, we evaluate the contributions to changes in the WC strength 22 related to changes in gross moist stability (GMS), horizontal MSE advection, radiation, 23 and surface fluxes. We find that the multimodel mean WC weakening is mostly related 24 to changes in GMS and radiation. Furthermore, the spread in WC response is related 25 to the spread in GMS and radiation responses. The GMS response is potentially sen- 26 sitive to parameterized convective entrainment which can affect lapse rates and the depth 27 of convection. We thus investigate the role of entrainment in setting the GMS response 28 by varying the entrainment rate in an idealized GCM. The idealized GCM is run with 29 a simplified Betts-Miller convection scheme, modified to represent entrainment. The weak- 30 ening of the WC with warming in the idealized GCM is dampened when higher entrain- 31 ment rates are used. However, the spread in GMS responses due to differing entrainment 32 rates is much smaller than the spread in GMS responses across CMIP6 models. There- 33 fore, further work is needed to understand the large spread in GMS responses across CMIP6 34 and AMIP models. 35