Climate models generally underrepresent the warming by Central Africa biomass-burning aerosols over the Southeast Atlantic.

Marc Mallet,Pierre Nabat,Martine Michou,Jim M. Haywood,Cheng Chen,Oleg Dubovik,Ben Johnson

doi:10.1126/sciadv.abg9998

Abstract

The radiative budget, cloud properties, and precipitation over tropical Africa are influenced by solar absorption by biomass-burning aerosols (BBA) from Central Africa. Recent field campaigns, reinforced by new remote-sensing and aerosol climatology datasets, have highlighted the absorbing nature of the elevated BBA layers over the South-East Atlantic (SEA), indicating that the absorption could be stronger than previously thought. We show that most of the latest generation of general circulation models (GCMs) from the sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6) underestimates the absorption of BBA over the SEA. This underlines why many (~75%) CMIP6 models do not fully capture the intense positive (warming) direct radiative forcing at the top of the atmosphere observed over this region. In addition, underestimating the magnitude of the BBA-induced solar heating could lead to misrepresentations of the low-level cloud responses and fast precipitation feedbacks that are induced by BBA in tropical regions.

Highlights

The South-East Atlantic (SEA) region has been the center of focused international attention through the deployment of large-scale measurement campaigns [Layered Atlantic Smoke Interactions with Clouds (LASIC) [1], Dynamics-Aerosol-Chemistry- Cloud Interactions in West Africa (DACCIWA) [2], ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) [3], CLoud-Aerosol-Radiation interaction and forcing (CLARIFY-2017) [4], and Aerosols, Radiation and Clouds in Southern Africa (AEROCLO-sA) [5]] aiming to understand the role of biomass- burning aerosols (BBA) emitted over Central Africa on the radiative balance and climate of this region
We argue that the underrepresentation of BBA- induced heating in general circulation models (GCMs) would lead to insufficient increases in low-level stratocumulus clouds, which are known to be extremely important for the radiation/climatic balance [22]
We evaluate the solar absorption of smoke aerosols in the SEA region simulated by the Coupled Model Intercomparison Project 6 (CMIP6) models by comparing the mean single- scattering albedo (SSA) obtained during the July-August-September (JAS) season against that derived by PARASOL/Generalized Retrieval of Aerosol and Surface Properties (GRASP) [30] and MACv2 data (Fig. 1)

Summary

Introduction

The South-East Atlantic (SEA) region has been the center of focused international attention through the deployment of large-scale measurement campaigns [Layered Atlantic Smoke Interactions with Clouds (LASIC) [1], Dynamics-Aerosol-Chemistry- Cloud Interactions in West Africa (DACCIWA) [2], ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) [3], CLoud-Aerosol-Radiation interaction and forcing (CLARIFY-2017) [4], and Aerosols, Radiation and Clouds in Southern Africa (AEROCLO-sA) [5]] aiming to understand the role of biomass- burning aerosols (BBA) emitted over Central Africa (mainly in Congo, Angola, and Zambia) on the radiative balance and climate of this region. By overlying the quasi-permanent stratocumulus clouds over the SEA, these absorbing smoke aerosols that have been transported over the ocean are known to produce an intense (warming) positive direct effect [monthly mean between ~0 up to +8 W m−2 [7,8,9]] at the top of the atmosphere. This is a unique region in terms of the sign and magnitude of the aerosol direct radiative effect, making it a prominent feature of aerosol forcing at the global scale. Quantifying the warming induced by the BBA over the SEA is very important for the radiative

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Conclusion