Large-Scale Tropical Circulation Intensification by Aerosol Effects on Clouds

This study addresses a critical gap in understanding anthropogenic influences on tropical climate dynamics by investigating the impact of aerosol-cloud interactions on large-scale circulation. Despite extensive research on greenhouse gas-induced warming and its effects on tropical circulation, the impact of aerosols, particularly their interactions with clouds, on large-scale circulation remains understudied. Utilizing large-domain radiative convective equilibrium cloud-resolving simulations, this research demonstrates that increasing aerosol concentration intensifies tropical overturning circulation, evaluated at the mid-troposphere (I) $(\mathcal{I})$, strongly correlating with domain mean cloud and radiative properties. Employing a weak temperature gradient approximation, I attribute variations in I $\mathcal{I}$ to changes in clear-sky radiative cooling rather than stability. These radiative cooling changes are linked to humidity changes driven by warm rain suppression by aerosols. This study's findings underscore the need to take into account microphysical changes, particularly aerosol concentrations, when studying anthropogenic effects on tropical circulation. Clouds intricately couple with the large-scale overturning circulation in the tropics. Aerosols, small particles suspended in the atmosphere, interact with clouds. Thus, changes in aerosols concentration (Na) have the potential to affect cloud properties and by that the large-scale circulation, with important implications for tropical climate and climate change. Despite this crucial relationship, the subject has been insufficiently explored. In this paper, I use a simplified, yet realistic, representation of the tropical atmosphere-radiative convective equilibrium large-domain cloud-resolving simulations-to investigate the effect of aerosol-cloud interaction on overturning tropical circulation. By varying Na under diverse sea surface temperatures, I demonstrate that the strength of the overturning circulation intensifies with increasing Na. Utilizing a semi-analytical expression for subsidence strength in clear-sky tropical regions, I show that the intensified overturning circulation with increased Na can be attributed to heightened humidity in the mid-troposphere, leading to intense radiative cooling. This mid-tropospheric humidification can be explained by warm rain suppression by aerosols, which fosters stronger humidity mixing between clouds and their surroundings. These findings underscore the need to consider aerosol concentration changes when examining anthropogenic impacts on tropical circulation. The overturning tropical circulation intensifies with aerosol concentration in a radiative convective equilibrium model Changes in circulation strength can be explained by changes in diabatic subsidence driven by radiative changes Radiative changes can be explained by increased humidity at the mid-troposphere due to warm rain suppression