Upscale energy transfer from mesoscale convective systems to African easterly waves
arXiv:2611.16941
Ben Maybee, Elliot McKinnon-Gray, Dan Shipley, John Methven
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West African storms do not only ride the larger easterly waves. In a kilometer-scale model, the storms feed kinetic energy up to those waves, especially inside the anvil, and that upward transfer leads the trough.
Read free explainer →African easterly waves (AEWs) drive the synoptic variability of West African monsoon rainfall via intricate two-way interactions with mesoscale convective systems (MCSs). The composite MCS-AEW system is a canonical example of multiscale interactions in the tropical atmosphere, with upscale feedbacks from deep convection driving the evolution of a larger-scale circulation. Here we support this perspective through application of techniques from turbulence theory, calculating for the first time patterns of interscale kinetic energy transfer, $\mathcal{D}_L$, across different spatial scales $L$ around MCSs and AEW lifecycle phases. We study AEW and MCS dynamics in two pan-tropical 40 day Met Office Unified Model simulations with $\sim$5km grid spacings, for which $\mathcal{D}_L$ is calculated with the LoSSETT software package. We find explicitly-simulated MCSs show a distinctive vertical dipole structure of $\mathcal{D}_L$ at scales below 1000km, with strong upscale energy transfer located in the upper troposphere within storm anvils. Regionally, upscale energy transfer occurs most strongly where organised convection is active. We find 200hPa upscale transfer consistently leads AEW troughs, and that growing continental AEWs in the explicit simulation are characterised by strengthened upscale transfer of energy from MCS to AEW scales. Case-study analysis shows this transfer can contribute to the eddy kinetic energy growth of strengthening AEW troughs. These signals are weak in a simulation with parameterised convection: here, zonal mean energetics show enhanced barotropic conversion, caused by a stronger African Easterly Jet, provides a stronger AEW population. Our analysis of upscale energy transfers complements previous analysis of AEW potential vorticity dynamics, providing a route towards building new theoretical understanding of tropical dynamics from km-scale models.