Search for: All records

Abstract Recent satellite and in-situ measurements show that forests can influence regional and global cloud cover through biophysical processes. However, forest’s possible local and non-local impacts on clouds remain unclear. By analyzing the model simulations from the Coupled Model Intercomparison Project Phase 6, here we show that deforestation-induced cloud cover changes have a strong latitudinal dependence, with decreased cloudiness in the tropics but increased cloudiness in the temperate and boreal regions. We further disentangle the local and non-local effects in influencing the cloudiness changes in model simulations. Results show that deforestation leads to a local cloud reduction in the tropics and a non-local cloud enhancement in the temperate and boreal regions. We demonstrate that the relationship between changes in cloud cover and deforestation would be misinterpreted without considering the non-local signals. Furthermore, our modeling results are inconsistent with recent observational studies, with enhanced clouds in model simulations but reduced clouds in observations in the temperate and boreal regions. Further efforts to explore the non-local effect and to reduce the model uncertainty could help advance our understanding of the biophysical effects of deforestation. 

Abstract Recent summer surface air temperature (SAT) variations over Central East Asia (CEA) have been influenced by greenhouse gas and aerosol forcing since 1960. But how CEA SAT responds to contrasting changes in Asian, and European and North American aerosol sources remains unclear. By analyzing observations and model simulations, here we show that aerosol‐forced summer SAT changes over CEA since 1960 come mostly from the effects of aerosols outside Asia, with relatively small influences from Asian aerosols. Unlike Europe, where direct and indirect aerosol effects on surface solar radiation drive the SAT long‐term trend and decadal variations, over CEA atmospheric circulation response to aerosols outside Asia plays an important role. Aerosol‐forced anomalous low‐level low pressure in mid‐latitude Eurasia may influence the SAT anomalies downstream over mid‐latitude Asia, including a warm anomaly around CEA. The results suggest that caution is needed in attributing SAT changes around CEA to anthropogenic aerosols from Asia. 

Abstract The recent summer surface air temperature (SAT) changes over densely populated Eurasia exhibit a non‐uniform pattern with amplified warming over Europe and East Asia (EA) but weak warming over Central Asia (CA), forming a wave train‐like structure. However, the key factors that determine this non‐uniform warming pattern remain unclear. By analyzing observations and model simulations, here, we show that more than half of the SAT multidecadal variations from 1950 to 2014 over Europe‐west Asia and EA may have resulted from external forcing, rather than from internal variability in the Atlantic as previously thought. In contrast, the recent SAT over CA is influenced mainly by internal variations in the Atlantic and Pacific oceans. Large ensemble model simulations suggest that the forced SAT multidecadal variations over Eurasia are mainly caused by changes in greenhouse gases and aerosols. Our findings provide strong evidence for major impacts of external forcing on multidecadal climate variations over Eurasia. 

Abstract By analyzing observations and model simulations, here we show that there exists a significant anticorrelation on interannual to multidecadal time scales between the Sahel and southeast Amazon rainfall during July‐August‐September. This rainfall seesaw, which is strongest on decadal to multidecadal scales, is due to an anomalous meridional gradient of sea surface temperatures across the tropical Atlantic that pushes the Intertropical Convergence Zone and its associated rain belt toward the anomalously warm hemisphere. Large ensemble model simulations suggest that the seesaw pattern is likely caused by decadal changes in anthropogenic and volcanic aerosols, rather than internal climate variability. Our results suggest that the recent decadal to multidecadal climate variations in and around the North Atlantic basin are largely externally forced and that projected large North Atlantic warming could lead to a wetter Sahel but drier Amazon in the future.