Rapid adjustments to black carbon cause precipitation invigoration in monsoon regions

Atmospheric black carbon (BC) is known to strongly affect precipitation, primarily through rapid adjustments to emissions changes. Globally, studies have found a strong, negative correlation between BC induced atmospheric absorption and precipitation, meaning that the overall effect of BC emissions is a drying. A primary thermodynamic mechanism is that heating aloft, induced by shortwave absorption, competes with latent heat release from condensation, inhibiting droplet formation.

In some regions, however, the modelled precipitation response to an increase in BC emissions is positive. Previous studies indicate that this is a local effect, occurring in tropical regions and close to the source, but as yet there is no full mechanistic explanation.

Using a range of recent BC emission perturbation simulations from global climate models, we show that BC precipitation invigoration primarily occurs in monsoon regions, and is due to a dynamical "chimney effect", or elevated heat pump, overcoming the thermodynamic inhibition. This has previously been discussed for absorbing aerosols over India, but we find similar results across most monsoon regions. Here, there is a clear positive correlation between BC atmospheric absorption and precipitation change, that persists from rapid adjustments through to the full climate response to BC emissions. We also find a shift in precipitation patterns through the monsoon season, with monsoon onset on average coming earlier and becoming more intense.

These results have clear implications for the precipitation related climate hazards arising from BC emission changes, and therefore also for science based policy advice on BC mitigation measures.