Dynamical and Radiative Influence on the Hadley Circulation by Aerosol-Cloud Interactions

[Geoengineering News]

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2025MS005465

Authors: Takanobu Yamaguchi, Ryuji Yoshida, Yao-Sheng Chen, Isabel L. McCoy, Graham Feingold

First published: 03 April 2026

https://doi.org/10.1029/2025MS005465

Abstract

How aerosols modulate large-scale circulation and the cloud radiative effect (CRE) is explored using a two-dimensional Hadley circulation model with near-large eddy simulation resolution. Six 30-day simulations are performed under three aerosol loadings with a prescribed sea surface temperature (SST0) or slab ocean model (SOM). These simulations produce various clouds, although stratocumulus are underrepresented. Aerosol perturbations intensify large-scale circulation and brighten clouds, as previously reported, although the effect is weaker under SOM. Hadley circulation intensification from a large aerosol perturbation in the subsidence region is linked to radiative-dynamical adjustments of the shallow meridional circulation driven by enhanced reflectance of shallow clouds. The CRE response to aerosol perturbations is driven by changes in cloud water path (CWP) and cloud fraction (fc), with the contribution from droplet number concentration (Nd) similar to those from CWP and fc under SST0 but reduced under SOM. The CRE response is dominated by the deep convection region, where aerosol effects amplify CRE along with a slight increase in fc for SST0, whereas under SOM, the weaker aerosol effect coincides with a slight decrease in fc. In the subsidence region, CRE enhancement diminishes at high aerosol concentrations, likely due to a negative microphysical-dynamical feedback identified in shallow clouds. Causal inference analysis applied to the deep convection region reveals that the fc response to Nd differs between SST treatments at high aerosol loadings, leading to a muted CRE response under SOM. These results suggest that the Hadley circulation moderates aerosol effects under a dynamically coupled atmosphere-ocean system.

Plain Language Summary

This study explores how tiny airborne particles called aerosols affect large-scale air circulation, clouds, and how much sunlight these clouds reflect (the cloud radiative effect, or CRE). We use a high-resolution two-dimensional model of the Hadley circulation to simulate different aerosol levels under two ocean conditions: one with a fixed sea surface temperature (SST) and another using a simplified model that allows the ocean to interact with the atmosphere (SOM). We find that an increase in aerosols tends to strengthen the circulation and brighten clouds, but these effects are weaker when the ocean can respond. Changes in cloud water and coverage mainly drive the changes in cloud reflectivity. The effect of droplet number concentration is smaller under SOM but similar to other factors under fixed SST. Most of the CRE response comes from the deep convection. There, cloud coverage increases with more aerosols under fixed SST but slightly decreases under SOM. In regions with sinking air, cloud brightening weakens at high aerosol levels, likely due to a feedback in shallow clouds. Overall, the results suggest that large-scale atmospheric circulation can dampen the effects of aerosol changes when interactions between the atmosphere and ocean are included.

Source: AGU