The response of the extratropical hydrological cycle to global warming

The change in the hydrological cycle in the extratropics under global warming is studied using the climate models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. The changes in hydrological quantities are analyzed with respect to the increases expected from the Clausius-Clapeyron (C-C) equation, which describes the rate of increase of a hydrological quantity per temperature increase. The column-integrated water vapor increases at a rate close to the C-C rate, which is expected if relative humidity remains nearly constant. The poleward moisture transport and the precipitation increase with temperature at a rate less than the C C rate, with the precipitation increasing the least. In addition, the intermodel variance of poleward moisture transport and precipitation is explained significantly better when the zonal-mean zonal wind change as well as the temperature change is taken into account. The percent increase in precipitation per temperature increase is smallest during the warm season when energy constraints on the hydrological cycle are more important. In contrast to other hydrological quantities, the changes in evaporation in the extratropics are not explained well by the temperature or zonal wind change. Instead, a significant portion of the intermodel spread of evaporation change is linked to the spread in the poleward ocean heat transport change.