1672 / 2019-08-26 12:21:14

Dynamics of Climate Response to Anthropogenic Aerosol Forcing: From Global to Regional Scale

9、大气科学与全球变化 > 专题6：区域气候变化动力学

Anthropogenic aerosols are a major driver of the twentieth century climate change. Highly variable in space and time, anthropogenic aerosols induce changes in atmospheric circulation and regional climate, including rainfall change in Asian Monsoons and the African Sahel. In climate models, the aerosol forcing, larger in the Northern than the Southern Hemisphere, induces an interhemispheric Hadley circulation. In support of the model result, we detected a robust change in the zonal mean cross-equatorial wind over the second half of the twentieth century from ship observations and reanalyses, accompanied by physically consistent changes in atmospheric pressure and marine cloud cover. Single-forcing experiments indicate that the observed change in zonal mean cross-equatorial wind is a fingerprint of global climate response to anthropogenic aerosol forcing.
In Asian Monsoon regions, anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST) mediated change, we diagnose the physical mechanisms of anthropogenic aerosol induced changes in the East Asian Summer Monsoon (EASM) and South Asian Summer Monsoon (SASM). Using coupled and atmospheric general circulation models, we show that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20°N and 40°N, and weakens the EASM north of 20°N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to the direct atmospheric response to radiative forcing. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, the SST feedback causes a substantial weakening of the SASM through Bjerknes feedback, and reorganizes the regional meridional atmospheric overturning circulation. With large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the inter-model spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean-atmosphere coupling.
As societies awaken to reduce aerosol emissions, a phase reversal of the aerosol induced climate response modes are expected in the 21st century, including the northward shift of the Inter-Tropical Convergence Zone and the recovery of Asian Summer Monsoon.