Apr 29, 2010

Thermodynamic and Dynamic Mechanisms for Large-Scale Changes in the Hydrological Cycle in Response to Global Warming

Richard Seager, Naomi Naik, Gabriel A. Vecchi
American Meteorological Society
  • Analyzes the mechanisms of changes in the large-scale hydrological cycle projected by 15 models participating in the Coupled Model Intercomparison Project phase 3 and used for the Intergovernmental Panel on Climate Change’s Fourth Assessment Report
  • Evaluates 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
  • Holds that the projected change in P − E involves an intensification of the existing pattern of P − E with wet areas [the intertropical convergence zone (ITCZ) and mid- to high latitudes] getting wetter and arid and semiarid regions of the subtropics getting drier
  • Finds the accentuation of the twentieth-century pattern of P − E is in part explained by increases in specific humidity via both advection and divergence terms
  • Finds that weakening of the tropical divergent circulation partially opposes the thermodynamic contribution by creating a tendency to decreased P − E in the ITCZ and to increased P − E in the descending branches of the Walker and Hadley cells
  • Finds the changing mean circulation also causes decreased − E on the poleward flanks of the subtropics because the descending branch of the Hadley Cell expands and the midlatitude meridional circulation cell shifts poleward
  • Finds that an increase in poleward moisture transport by transient eddies also contributes to subtropical drying and poleward moistening
  • Results suggest the thermodynamic contribution to changing P − E, arising from increased specific humidity, is almost entirely accounted for by atmospheric warming under fixed relative humidity