Dynamic vs. diabatic controls on atmospheric variability in a tropical aquachannel

Synoptic- to planetary-scale atmospheric variability in the tropics is a potential source of predictability worldwide. However, current weather prediction models hardly capture this variability. Here, we conduct tropical aquachannel simulations using the ICOsahedral Nonhydrostatic model with horizontal grid spacings of 13 and 5 km, and different representations of deep and shallow convection. The channel is confined between 30◦N and 30◦S with zonally symmetric sea surface temperatures (SSTs) prescribed. All aquachannel runs exhibit prominent Kelvin wave (KW) activity. The 13-km run with parameterized deep and shallow convection is characterized by KWs with propagation speeds varying between 15 and 27 m s−1. Only this run, the background flow of which markedly differs from the others, produces a large-scale disturbance that resembles the Madden-Julian Oscillation (MJO) with off-equatorial Rossby gyres. The other aquachannel runs exhibit wavenumber-one KWs with an almost constant propagation speed of 24 m s−1 superposed on standing or slowly drifting waves with much higher zonal wavenumbers. In runs with explicit deep convection, these standing waves are primarily controlled by the vertical advection of moist static energy, while in the 5-km run with parameterized deep and shallow convection, cloud-radiative effects play a bigger role. The latter are also key to maintaining the MJO-like disturbance in the 13-km run with parameterized deep and shallow convection. This study demonstrates that subtle changes in model configuration can shift the large-scale atmospheric variability in the tropics between standing and propagating types of waves, which would greatly influence possible modes of interaction with the extratropics.