Coupled Ocean-Atmosphere Response to Solar Forcing in the Early 20th Century Compared to Greenhouse Gas Forcing in the Late 20th Century

Authors: Gerald A. Meehl, Warren M. Washington, T.M.L. Wigley, Julie M. Arblaster, and Aiguo Dai
Affiliation: National Center for Atmospheric Research

Ensemble experiments with a global coupled climate model for the 20th century with time evolving observed solar, greenhouse gas (GHG), sulfate aerosol (direct effect), and ozone (tropospheric and stratospheric) forcing are analyzed to study the differences in response of the climate system to solar and GHG forcing. Observed global warming in the 20th century occurred in two distinct periods, one in the early 20th century from about 1900 until the mid-1940s, and one later in the century from the 1960s to 2000. The climate system response to the combination of solar forcing with greenhouse gases, sulfate aerosols, and ozone approximates the early 20th century warming, while the radiative forcing from increasing greenhouse gases is dominant for the response in the late 20th century. Of interest here is the transient climate system response in these two periods when the nature of the forcing was fundamentally different. This difference is manifested by the fact that solar forcing is more spatially heterogeneous (i.e. acting most strongly in areas where sunlight reaches the surface) while greenhouse gas forcing is more spatially uniform. Consequently, solar forcing is subject to feedbacks involving temperature gradient-driven circulation regimes that can alter clouds. Over relatively cloud-free oceanic regions in the subtropics, the enhanced solar forcing produces greater evaporation. More moisture then converges into the precipitation convergence zones, intensifying the regional monsoon, Hadley, and Walker circulations, causing less clouds over the subtropical ocean regions, and even more solar input. Since the greenhouse gases are more spatially uniform, such regional circulation feedbacks are not as strong. An additional response to solar forcing in northern summer is an enhancement of the meridional temperature gradients due to greater solar forcing over land regions that contribute to stronger west African and south Asian monsoons.