CO2-INDUCED AND AEROSOL-INDUCED CHANGES IN VERTICALLY INTEGRATED ZONAL MOMENTUM BUDGET IN A GCM EXPERIMENT

The German Climate Computing Center recently conducted a model experim ent in which separate runs simulate the climatic response to increasin g CO2 alone and to increasing CO2 together with direct radiative forci ng by sulfate aerosols. One of the variables that shows interesting di fferences between the different runs is the nearsurface zonal-mean zon al wind. As compared with the control run, the midlatitude surface wes terlies intensify and shift poleward in the CO2-only run in both hemis pheres in both the northern winter (DJF) and summer (JJA). However, th e aerosol forcing moderates these changes in general and, in particula r, reverses the pattern of change in the Northern Hemisphere in JJA. C onsistent differences between the various runs occur in the meridional distribution of sea level pressure. The origin of these simulated cha nges is studied by using the vertically integrated zonal-mean zonal mo mentum budget, utilizing the intimate linkage between the low-level wi nd and the surface stress and the close time-mean balance between the surface stress and the other terms in the budget. Regardless of the fo rcing used, momentum convergence in transient eddies is found to be th e dominant agent of change in the extratropical Southern Hemisphere an d in the Northern Hemisphere midlatitudes in JJA, In particular, the c hanges in the contribution of high-pass transients are relatively larg e and they seem to be qualitatively traceable to the changes in the tr opospheric meridional temperature gradient. In the northern extratropi cs in DJE stationary eddies make an even larger contribution than the transients.