Decoupling of Convectively Coupled Kelvin Waves: Super Cloud Clusters versus Moist Kelvin Waves

Several theories have been proposed to explain the dynamics of Convectively Coupled Equatorial Waves, such as wave-CISK, WISHE, and stratiform instability. Another framework to study these fundamental processes in tropical variability uses simplified numerical models, such as two-dimensional cloud resolving models and aquaplanet simulations. We conduct WRF simulations with an aquachannel domain (flat, zonally-periodic, with meridional walls at ± 60° latitude) to investigate the life cycles of Convectively Coupled Kelvin Waves (CCKWs). The use of power spectra, filtering, tracking and compositing is combined with a subjective method to assess the structure and phase speed propagation of CCKWs for the strengthening, mature, and decaying phases. It is found that there is a late response of the upper level zonal wind to organized convective activity and that for most variables the time-mean structures are a dynamical response to the convection, with the exception of pressure and thermodynamic variables in the boundary layer. Additional inspection of the time evolution of pressure and low-level moist static energy finds that these variables propagate eastward as a “dry” Kelvin wave, faster than the envelope of organized convection or Super Cloud Clusters (SCCs). When the separation is sufficiently large, the SCCs are cut off from heat and moisture supply from lower levels, leading to their dissipation. We revisit the concept itself of the “coupling” between convection and dynamics, and we also propose a conceptual model for the life cycle of CCKWs, with clear distinctions between both components.