Intraseasonal Variability in a Cloud-Permitting Near-Global Equatorial Aquaplanet Model

Abstract

Recent studies have suggested that the Madden-Julian oscillation is a result of an instability driven mainly by cloud-radiation feedbacks, similar in character to self-aggregation of convection in nonrotating, cloud-permitting simulations of radiative-convective equilibrium (RCE). Here we bolster that inference by simulating radiative-convective equilibrium states on a rotating sphere with constant sea surface temperature, using the cloud-permitting System for Atmospheric Modeling (SAM) with 20-km grid spacing and extending to walls at 46° latitude in each hemisphere. Mechanism-denial experiments reveal that cloud-radiation interaction is the quintessential driving mechanism of the simulated MJO-like disturbances, but wind-induced surface heat exchange (WISHE) feedbacks are the primary driver of its eastward propagation. WISHE may also explain the faster Kelvin-like modes in the simulations. These conclusions are supported by a linear stability analysis of RCE states on an equatorial beta plane. ©2018 American Meteorological Society.