The Role of Convective Moistening in the Madden–Julian Oscillation

Abstract

This study compares two models that differ primarily in their cloud parameterizations and produce extremely different simulations of the Madden?Julian oscillation (MJO). The Community Atmosphere Model (CAM) version 3.0 from NCAR uses the Zhang?McFarlane scheme for deep convection and does not produce an MJO. The ?superparameterized? version of the CAM (SP-CAM) replaces the cloud parameterizations with a two-dimensional cloud-resolving model (CRM) in each grid column and produces an extremely vigorous MJO. This analysis shows that the CAM is unable to produce high-humidity regions in the mid- to lower troposphere because of a lack of coupling between parameterized convection and environmental relative humidity. The SP-CAM produces an overly moist column due in part to excessive near-surface winds and evaporation during strong convective events. In the real tropics and the SP-CAM, convection within a high-humidity environment produces intense latent heating, which excites the large-scale circulation that is the signature of the MJO. The authors suggest that a model must realistically represent convective processes that moisten the entire tropical troposphere in order to produce a simulation of the MJO.