Sub‐Tropical Aerosols Enhance Tropical Cloudiness—A Remote Aerosol‐Cloud Lifetime Effect

Abstract

Earth's climate is changing (Masson-Delmotte et al., 2021). Reliable climate predictions are essential for developing climate change mitigation and adaptation strategies. However, climate predictions are still highly uncertain due largely to our limited understanding of the role of clouds in climate change. A major source of uncertainty is related to the cloud response to anthropogenic aerosols (Bellouin et al., 2019). It has been shown that aerosol-cloud-interaction (ACI) mechanisms are cloud-regime-dependent (Altaratz et al., 2014;Christensen et al., 2016;Dagan & Stier, 2020b;Gryspeerdt & Stier, 2012), and most previous studies have evaluated them on a per-regime basis. However, since the different cloud regimes in the atmosphere are connected by the global circulation, aerosol effects on one regime may also affect the properties of different regimes, as well as the transition between the different regimes (Christensen et al., 2020;Dagan & Chemke, 2016;Goren et al., 2019). These inter-cloud regime connections are under-explored and not fully understood.High-resolution, limited-area, cloud-resolving model (CRM) simulations and large-eddy simulations (LES) are the main tools to explore ACI mechanisms. However, traditionally, these tools are unable to directly account for changes in the dynamics and thermodynamics of the large-scale climate system, and hence lack an important component of clouds' response (Abbott & Cronin, 2021;Anber et al., 2019). For example, it was recently shown that the representation of large-scale effects on the CRM domain, that is, the lateral boundary conditions or "large-scale forcing" (LSF), could artificially determine clouds' response to aerosol perturbation (Dagan, Stier, Spill, et al., 2022;Spill et al., 2021). Specifically, it was shown that the recent efforts to couple local cloud response with changes in the large-scale vertical velocity (Abbott & Cronin, 2021) could result in an unrealistic Abstract Clouds' susceptibility to aerosols is considered to be a leading source of uncertainty in climate research. An inability to account simultaneously for the large range of scales involved in cloud-aerosol-climate interactions has hindered the progress of research. In this study, using a novel system of idealized large-eddysimulations that explicitly resolves clouds but also accounts, in an idealized manner, for large-scale changes in the thermodynamic and dynamic conditions and for inter-cloud regime coupling, it is shown that aerosol perturbation in the sub-tropics increases cloudiness in the tropics. Specifically, aerosol-driven sub-tropical rain suppression leads to increased advection of cold and moist air from the sub-tropics to the tropics, thus enhancing tropical cloudiness. The increased tropical cloudiness has a strong cooling effect by reflecting more of the incoming solar radiation. The classical "aerosol-cloud lifetime effect" is shown here to have a small local effect (in the sub-tropics) but a strong remote effect (sub-tropical aerosols increase cloudiness in the tropics)...

Unexpected failure of regional marine cloud brightening in a warmer world

“…Similar to previous findings 31,33 , the initiating mechanism of this teleconnection between the mid-latitude North Pacific and the tropics is mean wind advection (Fig. 1c,d).…”

Radiative forcing from aerosol–cloud interactions enhanced by large-scale circulation adjustments

“…In conclusion, our study contributes to a growing body of research that seeks to understand the complex interactions between aerosols, clouds and the large-scale circulation 10,25,[40][41][42][43] , with a specific focus on ACI. By identifying the importance of considering the impacts of ACI on a broader spatial-temporal scale, we hope to inspire further research and collaboration to advance our understanding of these crucial processes.…”

“…4a and ref. 25). The increased water vapour convergence into the deep regime results in enhanced precipitation, stronger latent heat release and an increased export of dry static energy from the deep regime (Fig.…”