10 October 2016

Confronting the 'Indian Summer Monsoon Response to Black Carbon Aerosols’ with the Uncertainty in its Radiative Forcing and Beyond

Indian monsoon response to a range of black carbon aerosol forcing

Science

Assessed the range of Indian Monsoon response within the bounds of uncertainty of black carbon radiative forcing and beyond - into nuclear winter realms - in a suite of climate model experiments.

Impact

There is a large uncertainty in global black carbon induced radiative forcing. Within those bounds of uncertainty, the Indian summer monsoon strength increases linearly in response to increasing black carbon aerosol forcing. Our results emphasize the need to reduce uncertainty in BC forcing to reduce the uncertainty in its climate impacts. The linearity of response largely holds even as BC forcing enters nuclear winter scenarios, despite large land surface cooling, due to increased BC induced middle/upper troposphere heating - the dominant monsoonal circulation driver - bringing in more moisture from the oceans. This large increase in precipitation is in contrast to previously held belief of a monsoonal breakdown in such scenarios. 

Summary

While black carbon aerosols (BC) are believed to modulate the Indian monsoons, the radiative forcing estimate of BC suffers from large uncertainties globally. We analyze a suite of idealized experiments forced with a range of BC concentrations that span a large swath of the latest estimates of its global radiative forcing. Within those bounds of uncertainty, summer precipitation over the Indian region increases nearly linearly with the increase in BC burden. The linearity holds even as the BC concentration is increased to levels resembling those hypothesized in nuclear winter scenarios, despite large surface cooling over India and adjoining regions. The enhanced monsoonal circulation is associated with a linear increase in the large-scale meridional tropospheric temperature gradient. The precipitable water over the region also increases linearly with an increase in BC burden, due to increased moisture transport from the Arabian sea to the land areas. The wide range of Indian monsoon response elicited in these experiments emphasizes the need to reduce the uncertainty in BC estimates to accurately quantify their role in modulating the Indian monsoons. The increase in monsoonal circulation in response to large BC concentrations contrasts earlier findings that the Indian summer monsoon may break down following a nuclear war.

Contact
Salil Mahajan
Oak Ridge National Laboratory (ORNL)