Thermodynamic and dynamic mechanisms for large-scale changes in the hydrological cycle in response to global warming

The mechanisms of changes in the large scale hydrological cycle projected by 15 models participating in the Coupled Model Intercomparison Project 3 and used for the Intergovernmental Panel on Climate Change Assessment Report Four are analyzed by computing differences between 2046-65 and 1961-2000. The contributions to changes in precipitation minus evaporation, P − E, caused thermodynamically by changes in specific humidity, dynamically by changes in circulation and by changes in moisture transports by transient eddies are evaluated. The thermodynamic and dynamic contributions are further separated into advective and divergent components. The non-thermodynamic contributions are then related to changes in the mean and transient circulation. The projected change in P −E involves an intensification of the existing pattern of P −E with wet areas (the ITCZ and mid to high latitudes) getting wetter and arid and semi-arid regions of the subtropics getting drier. In addition the subtropical dry zones expand poleward. The accentuation of the 20 Century pattern of P−E is in part explained by increases in specific humidity via both advection and divergence terms. Weakening of the tropical divergent circulation partially opposes the thermodynamic contribution by creating a tendency to less P −E in the ITCZ and to increased P −E in the descending branches of the Walker and Hadley Cells. The changing mean circulation also causes decreased P − E on the poleward flanks of the subtropics as the descending branch of the Hadley Cell expands and the mid-latitude meridional circulation cell shifts poleward. Subtropical drying and poleward moistening are also contributed to by an increase in poleward moisture transport by transient eddies. The thermodynamic contribution to changing P − E, arising from increased specific humidity, is almost entirely accounted for by atmospheric warming under fixed relative humidity.