Inter‐model comparison of subseasonal tropical variability in aquaplanet experiments: Effect of a warm pool

Abstract

Abstract This study compares the simulation of subseasonal tropical variability by a set of six state‐of‐the‐art AGCMs in two experiments in aquaplanet configuration: a zonally symmetric experiment, and an experiment with a warm pool centered on the equator. In all six models, the presence of the warm pool generates zonal asymmetries in the simulated mean states in the form of a “Gill‐type” response, made more complex by feedbacks between moisture, convective heating and circulation. Noticeable differences appear from one model to another. Only half the models simulate mean low‐level equatorial westerlies over the warm pool area. The presence of the warm pool can also favor the development of large‐scale variability consistent with observed Madden‐Julian Oscillation (MJO) characteristics, but this happens only in half the models. Our results do not support the idea that the presence of the warm pool and/or of mean low‐level equatorial westerlies are sufficient conditions for MJO‐like variability to arise in the models. Comparing spectral characteristics of the simulated Convectively Coupled Equatorial Waves (CCEWs) in the aquaplanet experiments and the corresponding coupled atmosphere‐ocean (i.e., CMIP) and atmosphere‐only (i.e., AMIP) simulations, we also show that there is more consistency for a given model across its configurations, than for a given configuration across the six models. Overall, our results confirm that the simulation of subseasonal variability by given model is significantly influenced by the parameterization of subgrid physical processes (most‐likely cloud processes), both directly and through modulation of the mean state.

Keywords

• aquaplanet
• tropical variability
• inter‐model comparison
• Madden Julian Oscillation
• MJO