The response of extratropical precipitation and moisture transport to increased CO2.

An ensemble of climate simulations performed for the fourth assessment report of the Intergovernmental Panel on Climate Change are used to study the response of extratropical precipitation and moisture transport to human induced climate change. The poleward moisture transport across the mid-latitudes increases strongly under climate change and the inter-model spread in temperature sensitivity largely explains the inter-model spread of moisture transport. Moreover, a plot of the fractional change in poleward moisture transport versus global temperature change follows the slope suggested by the Clausius-Clapeyron relationship under the assumptions of no change in either relative humidity or circulation. A simple atmospheric GCM with passive water-like tracers reproduces the response of the fully-coupled models and is used to provide additional insight into the precipitation response, which is somewhat smaller than the moisture transport response. The results suggest that the large-scale changes in poleward moisture transport across the mid-latitudes can be explained by the thermodynamics embodied in the Clausius-Clapeyron relationship without invoking changes in atmospheric circulation.