Clouds, water vapor and the response of the extratropical jets to global warming

Abstract:

Climate models suggest that global warming will cause substantial changes of the mid-latitude circulation, including meridional shifts of the extratropical jets and storm tracks. The magnitude, and in some circumstances even the sign, of these shifts remains subject to large model uncertainties, however. In this talk I will report on recent work that demonstrates the importance of longwave radiative effects of clouds and water vapor for the jet position and its response to warming. To this end, I will apply a hierarchy of climate models ranging from CMIP5 models in realisitic setups to dry idealized general circulation models. I will show that cloud changes, in particular those of the tropics and mid-latitudes, and high-latitude water vapor changes push the jet towards the pole under global warming, whereas equatorial water vapor changes pull the jet towards the equator. These radiative impacts of clouds and water vapor on the jet are found to be consistent with our understanding of the response of the dry circulation to diabatic heating. I will also discuss the extent to which mid-latitude clouds are controlled by the jet. Finally, I will show that CMIP5 model spread in warming-induced jet shifts is correlated with model spread in regional changes of clouds and water vapor. These results provide evidence that part of the climate model uncertainty in projections of future jet shifts might result from uncertainty in how clouds and water vapor respond to global warming, and how they modify the longwave radiation inside the atmosphere.