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Seasonally variant low cloud adjustment over cool oceans

Kamae, Youichi, Chadwick, Robin, Ackerley, Duncan, Ringer, Mark, Ogura, Tomoo
Climate dynamics 2019 v.52 no.9-10 pp. 5801-5817
General Circulation Models, atmospheric circulation, carbon dioxide, cloud cover, cooling, monsoon season, oceans, radiative forcing, reflectance, seasonal variation, subtropics, summer, surface temperature, troposphere
The Earth’s solar reflectance is reduced through rapid climate adjustments to increasing CO₂, via a decrease in total cloud cover over ocean. Perturbations to marine boundary-layer clouds are essentially important for the global radiative balance at the top of the atmosphere. However, the physical robustness of low cloud adjustments to increasing CO₂ has not been assessed systematically. Here we show that low cloud adjustment is distinct from that in total cloud and is seasonally variant. Among multiple climate models, marine boundary-layer clouds over the subtropics and extratropics (especially over the Northern Hemisphere) are consistently increased in the rapid adjustment, while middle and high clouds are greatly reduced. The increase in low cloud cover is only found during summer, associated with a summertime enhancement of lower tropospheric stability. We further examine mechanisms behind the rapid adjustments of low cloud and inversion strength of the boundary layer, using land surface temperature prescribing experiments in an atmospheric general circulation model (AGCM). Summertime increases in low cloud and enhanced inversion strength over the ocean simulated in this AGCM are attributed to (1) CO₂-induced land warming; and (2) reduced radiative cooling in the lower troposphere due to increased CO₂. The seasonality in the cloud adjustment implies an importance of seasonal variations in background cloud and atmospheric circulation related to the Hadley and monsoon circulations for radiative forcing, feedback and climate sensitivity.