|Title||Comparison of climate response to anthropogenic aerosol versus greenhouse gas forcing: Distinct patterns|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Wang H, Xie SP, Liu QY|
|Journal||Journal of Climate|
|Type of Article||Article|
|Keywords||20th-century; asian summer monsoon; CMIP5 models; driven; drought; late; ocean; pacific; precipitation; rainfall; tropical circulation|
Spatial patterns of climate response to changes in anthropogenic aerosols and well-mixed greenhouse gases ( GHGs) are investigated using climate model simulations for the twentieth century. The climate response shows both similarities and differences in spatial pattern between aerosol and GHG runs. Common climate response between aerosol and GHG runs tends to be symmetric about the equator. This work focuses on the distinctive patterns that are unique to the anthropogenic aerosol forcing. The tropospheric cooling induced by anthropogenic aerosols is locally enhanced in the midlatitude Northern Hemisphere with a deep vertical structure around 40 degrees N, anchoring a westerly acceleration in thermal wind balance. The aerosol-induced negative radiative forcing in the Northern Hemisphere requires a cross-equatorial Hadley circulation to compensate interhemispheric energy imbalance in the atmosphere. Associated with a southward shift of the intertropical convergence zone, this interhemispheric asymmetric mode is unique to aerosol forcing and absent in GHG runs. Comparison of key climate response pattern indices indicates that the aerosol forcing dominates the interhemispheric asymmetric climate response in historical all-forcing simulations, as well as regional precipitation change such as the drying trend over the East Asian monsoon region. While GHG forcing dominates global mean surface temperature change, its effect is on par with and often opposes the aerosol effect on precipitation, making it difficult to detect anthropogenic change in rainfall from historical observations.
"We have developed a method to highlight the distinct patterns of climate response to anthropogenic aerosols versus the GHG changes. In the tropics, the aerosol-induced negative radiative forcing in the NH midlatitudes requires an anomalous Hadley circulation across the equator according to the cross-equatorial energy transport theory. This is also an important mechanism to propagate the aerosol effect to the extratropical SH. Eddy–mean flow interactions are important in the adjustment (Ceppi et al. 2013; Xu and Xie 2015). In the NH midlatitudes, patterns unique to anthropogenic aerosols include the North Pacific cooling, the drying trend over eastern Asia, and the southward shift of the NH westerly jet. Comparisons of climate response pattern indices quantify the distinct regional climate change between anthropogenic aerosol and GHG runs. The interhemispheric asymmetric mode, represented by the anomalous cross-equatorial gradient of SST, SLP, and precipitation, shows a much larger response to aerosol than to GHG forcing."