Change from aerosol-driven to cloud-feedback-driven trend in short-wave radiative flux over the North Atlantic

• Published in: Atmospheric Chemistry and Physics
• Main Authors: D. P. Grosvenor, K. S. Carslaw
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2023
• Subjects: Physics; QC1-999; Chemistry; QD1-999; North Atlantic
• Online Access: https://doi.org/10.5194/acp-23-6743-2023; https://doaj.org/article/07e463fac5ae4cc9a422dcc3c51cf278

Aerosol radiative forcing and cloud–climate feedbacks each have a large effect on climate, mainly through modification of solar short-wave radiative fluxes. Here we determine what causes the long-term trends in the upwelling short-wave (SW) top-of-the-atmosphere (TOA) fluxes ( F SW↑ ) over the North Atlantic region. Coupled atmosphere–ocean simulations from the UK Earth System Model (UKESM1) and the Hadley Centre General Environment Model (HadGEM3-GC3.1) show a positive F SW↑ trend between 1850 and 1970 (increasing SW reflection) and a negative trend between 1970 and 2014. We find that the 1850–1970 positive F SW↑ trend is mainly driven by an increase in cloud droplet number concentration due to increases in aerosol, while the 1970–2014 trend is mainly driven by a decrease in cloud fraction, which we attribute mainly to cloud feedbacks caused by greenhouse gas-induced warming. In the 1850–1970 period, aerosol-induced cooling and greenhouse gas warming roughly counteract each other, so the temperature-driven cloud feedback effect on the F SW↑ trend is weak (contributing to only 23 % of the Δ F SW↑ ), and aerosol forcing is the dominant effect (77 % of Δ F SW↑ ). However, in the 1970–2014 period the warming from greenhouse gases intensifies, and the cooling from aerosol radiative forcing reduces, resulting in a large overall warming and a reduction in F SW↑ that is mainly driven by cloud feedbacks (87 % of Δ F SW↑ ). The results suggest that it is difficult to use satellite observations in the post-1970 period to evaluate and constrain the magnitude of the aerosol–cloud interaction forcing but that cloud feedbacks might be evaluated. Comparisons with observations between 1985 and 2014 show that the simulated reduction in F SW↑ and the increase in temperature are too strong. However, the temperature discrepancy can account for only part of the F SW↑ discrepancy given the estimated model feedback strength ( λ = ∂ F SW ∂ T <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="18pt" ...