Environmental & Climate Sciences Department Seminar

"Effects of Solar Geoengineering on Clouds, Energy Transport and the ITCZ"

Presented by Rick Russotto, University of Washington

Monday, July 31, 2017, 11:00 am — Conference Room Bldg 815E

The polar amplification of warming and the ability of the inter-tropical convergence zone (ITCZ) to shift to the north or south are two very important problems in climate science. Examining these behaviors in global climate models (GCMs) running solar geoengineering experiments is helpful not only for predicting the effects of solar geoengineering, but also for understanding how these processes work under increased CO2. Both polar amplification and ITCZ shifts are closely related to the meridional transport of moist static energy (MSE) by the atmosphere. This study examines changes in MSE transport in 10 fully coupled GCMs in Experiment G1 of the Geoengineering Model Intercomparison Project, in which the solar constant is reduced to compensate for abruptly quadrupled CO2 concentrations. In this experiment, poleward MSE transport decreases relative to preindustrial conditions in all models, in contrast to the CMIP5 abrupt4xCO2 experiment, in which poleward MSE transport increases. Since poleward energy transport decreases rather than increasing, and local feedbacks cannot reverse the sign of an initial temperature change, the residual polar amplification in the G1 experiment must be due to the different spatial patterns of the simultaneously imposed solar and CO2 forcings. However, the reduction in poleward energy transport likely plays a role in limiting the polar warming in G1. The seasonal migration of the ITCZ is dampened in G1 relative to abrupt4xCO2 due to preferential cooling of the summer hemisphere by the solar reduction. The ITCZ shifts northward in G1 by 0.14 degrees in the annual, multi-model mean, with an inter-model range of -0.33 to 0.89 degrees. These shifts are anticorrelated with changes in cross-equatorial MSE transport. An attribution study with a moist energy balance model shows that cloud feedbacks are the largest source of uncertainty regarding changes in cross-equatorial energy transport under solar geoengineering. Analysis of cloud changes in

Hosted by: Mike Jensen

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