Atmosphere Ocean Science Colloquium

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Clouds, Water Vapor and (Extra-) Tropical Circulation Changes Under Global Warming

Speaker: Aiko Voigt, Columbia University

Location: Warren Weaver Hall 1302

Date: Wednesday, March 4, 2015, 3:30 p.m.

Synopsis:

Global warming will not only change surface temperatures but will also affect the large-scale atmospheric circulation, and thereby lead to regional patterns of global change. Previous work indicates substantial circulation changes under global warming, ranging from a restructuring of the inter-tropical convergence zone (ITCZ) TO poleward shifts of the extratropical jet. In this talk I will present idealized fixed SST-aquaplanet simulations to show that radiative changes of clouds and water vapor are essential to both tropical and extra-tropical circulation changes and cause much, if not most, of the model spread. First, I will show that the model-dependent response of the Hadley circulation and the narrow/widening of the ITCZ critically depends on changes in deep-convective in deep-convective ice clouds and a radiative-dynamic feedback between these ice clouds and ascending motion. Second, I will show that clouds and water vapor, by creating meridional and vertical temperature gradients in the upper troposphere and lower stratosphere (UTLS), affect the location of the subtropical Hadley cell edge and the extratropical jet. Third, I will argue that model spread in the jet shift seen in the CMIP5 aquaplanet archive is linked to model spread in the UTLS temperature change, suggesting that uncertainties in the magnitude of jet shifts is, at least partly, caused by uncertainties in UTLS clouds and water vapor. Overall these results show that radiative changes of clouds and water vapor are not only integral to the magnitude of future global-mean warming but also determine patterns of regional climate change.