Are changes in atmospheric circulation important for black carbon aerosol impacts on clouds, precipitation, and radiation?

Are changes in atmospheric circulation important for black carbon aerosol impacts on clouds, precipitation, and radiation?

Black carbon (BC) aerosols strongly absorb solar radiation, but their effective radiative forcingand impacts on regional climate remain highly uncertain owing to strong feedbacks of BC heating on clouds,convection, and precipitation. This study investigates the role of large‐scale circulation changes in governingsuch feedbacks. In the HadGEM3 climate model BC emissions were increased to 10 times present‐day valueswhile keeping sea surface temperatures fixed, to assess the rapid adjustments to increased BC absorption.The BC perturbation led to an effective radiative forcing of 2.7 W/m2and a 0.13‐mm/day reduction in globalprecipitation. There were also large shifts in the spatial distribution of tropical convection, increased lowcloud over oceans, and a weakening and poleward shift of midlatitude storm tracks, especially in theNorthern Hemisphere. In a parallel experiment, horizontal winds were nudged toward meteorologicalreanalyses to deliberately suppress circulation responses while allowing changes to the thermodynamicstructure of the atmosphere. Surprisingly, BC had approximately the same impact on global‐mean radiationand global precipitation in the nudged experiment, even though regional changes in clouds and convectionwere not fully captured. The results show that large‐scale dynamical responses to BC are important forregional impacts but have a limited role in determining the effective radiative forcing and global‐meanclimate response. The rapid adjustments of clouds, radiation, and global precipitation were primarily aresponse to increased radiative absorption and atmospheric stability. This implies that short nudgedsimulations may be sufficient to assess absorbing aerosol impacts on global‐ mean radiationand precipitation.

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