Subtropical shallow cumulus convection is shown to play an important role in tropical climate dynamics through a chain of indirect feedbacks. It is found that the presence of shallow convection in the subtropics contributes to setting the width and intensity of oceanic intertropical convergence zones (ITCZs). These conclusions are reached after investigation using a tropical climate model of intermediate complexity, with sufficient vertical degrees of freedom to capture i) boundary layer physics and ii) feedbacks between tropospheric humidity and deep convection. Using this modeling framework, the mechanisms of interaction between small-scale shallow cumulus convection and large-scale tropical and subtropical climate dynamics are identified and quantified. A simple first-order parameterization for shallow cumulus convection is used in which the time scale of shallow cumulus adjustment can be varied to assess sensitivity. A moist static energy budget analysis is then performed which reveals the chains of relevant feedback mechanisms acting in the tropics.

The first direct impact of increasing the time scale of shallow cumulus convection is to keep the boundary layer close to saturation in the subtropics. This locally reduces surface evaporation and surpresses tropospheric humidity. Horizontal transport of this drier air by humidity transients and dry intrusions significantly reduces the precipitating deep convection at the edges of the oceanic ITCZ. As a result, the temperature is reduced by approximately a degree Kelvin throughout the tropics. In turn, the combination of this temperature change and the subtropical drop in water vapor path triggers a compensating increase in net longwave warming and surface heat flux. This increases large-scale convergence in the central axis of the oceanic ITCZs, producing more intense precipitation.