A generalized ω-equation is used to identify the contributions from different processes that force upward motions in the Atlantic Marine ITCZ (AMI) from a numerical mesoscale simulation of June 2010. This ω-equation separates the diabatic heating contributions, which lie at the core of the Weak Temperature Gradient (WTG) framework, from the dynamical terms. Three layers of atmosphere are found with different balance. In the Marine Atmospheric Boundary-Layer (MABL), the upward motions in the AMI are induced by the frontogenesis and buoyancy components, which are regulated by the ageostrophic adjustment due to the presence of thermal-wind imbalance. The balance of these three processes well captures the variability of the vertical velocity and the associated precipitation, meaning that boundary-layer processes play a central role in the AMI dynamics. In the layer [600–2,000 m], a zone of strong vertical wind-shear just above the MABL, the upward motions are induced by the ageostrophic adjustment and radiative components, which are counteracted by evaporation of convective precipitation. Above 2,000 m the ascending motions are driven by the deep convection heating, as expected by the WTG framework, and more surprisingly by the ageostrophic adjustment term within the Tropical Easterly Jet. Thanks to the use of the ω-equation, these results extend the current WTG framework to the boundary layer, where it is not expected to hold. In the free troposphere, the WTG framework only accounts for half of the AMI ascent, the other half being forced by the dynamical terms.

Key Points

Upward motions in the Atlantic Marine ITCZ are not resulting from latent heat release in the deep troposphere only

Buoyancy fluxes, frontogenesis, and ageostrophic circulation are also key factors that force the ascent from the surface to the top

The surface buoyancy flux is found to strongly influence the vertical velocity in the marine atmospheric boundary-layer, unlike the sea surface temperature

Plain Language Summary

Upward motions in the Atlantic Marine ITCZ are generally explained as resulting from latent heat release associated with deep convection in the troposphere. This study shows that this approach is challenged by other processes on the vertical. In the lowest atmospheric layers, upward motions are driven by frontogenetic processes induced by wind convergence and by differential heating associated with the turbulent heat fluxes. In the middle troposphere, upward motions are controlled by ageostrophic adjustment, characterizing an unbalanced system, and radiative heating. In the deep troposphere, ascending motions are controlled by heating induced by the convective latent heat release, as expected by classic theories, but more surprisingly also by ageostrophic adjustment within the Tropical Easterly Jet. These results shows that dynamic forcings have an equivalent role to diabatic forcings in the production of upward motions within the ITCZ.