10 October 2016

Black Carbon Induced Northern Hemisphere Tropical Expansion

Science

In a suite of experiments forced with a range of Black Carbon aerosols (BC) within estimated uncertainty bounds, we have analyzed the impact of BC on the expansion of the Tropics using a variety of metrics that quantify the extent of Tropics. These experiments suggest that the tropical expansion increases nearly linearly with increasing BC forcing due to the relative warming of the mid-latitudes by increased absorption of solar radiation by BC.

Impact

Climate models severely under-estimate the observed trend in tropical expansion over the last few decades. BC radiative forcing, second only to that of carbon dioxide, is not constrained well and exhibits a large range of uncertainty. Climate models simulate the BC radiative forcing near the lower end of that range. Previous studies have shown that BC plays a role in tropical expansion. We have clearly demonstrated that improving representation of BC aerosol distribution in climate models could indeed improve the simulation of the observed trend in tropical expansion.

Summary

Global Climate Models (GCMs) underestimate the observed trend in tropical expansion. Recent studies partly attribute it to black carbon aerosols (BC), which are poorly represented in GCMs. We conduct a suite of idealized experiments with the Community Atmosphere Model (CAM4) coupled to a slab ocean model forced with increasing BC concentrations covering a large swath of the estimated range of current BC radiative forcing while maintaining their spatial distribution. The Northern Hemisphere (NH) tropics expand polewards nearly linearly as BC radiative forcing increases (0.70 W-1 m2), indicating that a realistic representation of BC could reduce GCM biases. We find support for the mechanism where BC induced midlatitude tropospheric heating shifts the maximum meridional tropospheric temperature gradient polewards resulting in tropical expansion. We also find that the NH poleward tropical edge is nearly linearly correlated with the location of the inter- tropical convergence zone (ITCZ), which shifts northwards in response to increasing BC.

Contact
Salil Mahajan
Oak Ridge National Laboratory (ORNL)