Sensitivity of the Walker Circulation to Convective Entrainment in a Changing Climate

Observational evidence suggests that the Walker circulation has strengthened over the past few decades. However, coupled climate-model simulations disagree on the sign of the trend for this period, and it has been suggested that this disagreement is partly due to different changes in static stability across the models. Entrainment is an uncertain process in convective parameterizations that is known to affect the vertical structure of temperature in the tropics. Here, we investigate the effect of convective entrainment on the Walker circulation and its response to climate change in idealized GCM simulations. To vary entrainment, we modify an entrainment limiter applied to the relaxed Arakawa-Schubert convection scheme. Entrainment is found to affect the response of the Walker circulation to climate warming, with little weakening of the Walker circulation when a nonentraining plume is included in the convective parameterization and substantial weakening when the entrainment limiter suppresses parameterized deep convection. We use a zonally-anomalous moist static energy budget to diagnose the differences in the Walker circulation and its response to climate change for a range of entrainment parameters, with an emphasis on the relationship between gross moist stability and circulation strength.