Sea surface temperature (SST) plays an important role in maintaining marine stratocumuli – higher SST can reduce the lower-troposphere stability, deepen the marine boundary layer, and promote the stratus-to-cumulus transition, leading to reduced cloud cover. Lower cloud cover of cumulus clouds allows more incoming solar radiation at sea surface, and further increases SST. This positive SST-low cloud feedback has been previously studied in the context of greenhouse gas warming based on observational data and modeling tools. In this study, we focus on the SST-low cloud feedback over the Southeast Atlantic (SEA) triggered by biomass burning (BB) aerosols emitted from Southern African wildfires. As one of the semi-permanent stratocumulus decks on the Earth, the stratocumulus cloud deck over SEA is unique because it is often under the influence of BB aerosols released from fires over Southern Africa. BB aerosols exert a strong surface cooling effect over the SEA via aerosol-radiation and aerosol-cloud interactions. The question that we would like to ask is: can the surface cooling effect induced by the BB aerosols be amplified through the SST-cloud feedback over SEA?
Marine stratocumulus is extremely important for the global climatic system, because of its large coverage and strong net cooling effect. This study furthers our understanding of the positive SST-cloud feedback, which is one of the key processes controlling the magnitude of global climate change. For a certain region, like SEA in our study, which is under the influence of fire smoke constantly, our study demonstrates that the cooling effect caused by BB aerosols is smaller when the SST-low cloud feedback is considered. Our simulation results also highlight the importance of accounting for the ocean current feedbacks when studying the aerosol-induced cloud radiative effect and SST-cloud feedback.
In this study, we conduct both full-coupled and atmosphere-only simulations with and without BB aerosol emissions over southern Africa using the NCAR Community Earth System Model version 2 (CESM2) model. By contrasting BB aerosol-induced changes in clouds, radiation budgets, and ocean thermodynamic conditions between fully-coupled and atmosphere-only simulations, we elucidated the role of SST-cloud feedback in the BB aerosol-induced changes. The model results from CESM2 demonstrate that counterintuitively the cloud radiative effect caused by the BB aerosols is weaker if SST-low cloud feedback is considered compared to fixed-SST simulation (-2.99 W m-2 vs. -4.79 W m-2). This is caused by 1) stronger sea breeze due to larger sea-land temperature contrast causing less smoke transport over SEA and 2) less moisture supply from the surface due to colder SST. Changes in SST also lead to counterclockwise rotation of ocean circulation anomalies. Consequently, the excess heat transport from the equator reverses the direction of SST- cloud feedback in this region.