CAPE VARIATIONS IN THE CURRENT CLIMATE AND IN A CLIMATE-CHANGE

Observed variations of convective available potential energy (CAPE) in the current climate provide one useful rest of the performance of cum ulus parameterizations used in general circulation models (GCMs). It i s found that frequency distributions of tropical Pacific CAFE, as well as the dependence of CAFE on surface wet-bulb potential temperature ( Theta(w)) simulated by the Goddard Institute for Space Studies's GCM, agree well with that observed during the Australian Monsoon Experiment period, CAFE variability in the current climate greatly overestimates climatic changes in basinwide CAFE in the tropical Pacific in respons e to a 2 degrees C increase in sea surface temperature (SST) in the GC M because of the different physics involved. In the current climate, C AFE variations in space and time are dominated by regional changes in boundary layer temperature and moisture, which in turn are controlled by SST patterns and large-scale motions. Geographical thermodynamic st ructure variations in the middle and upper troposphere are smaller bec ause of the canceling effects of adiabatic cooling and subsidence warm ing in the rising and sinking branches of the Walker and Hadley circul ations. In a forced equilibrium global climate change, temperature cha nge is fairly well constrained by the change in the moist adiabatic la pse rate and thus the upper troposphere warms to a greater extent than the surface. For this reason, climate change in CAFE is better predic ted by assuming that relative humidity remains constant and that the t emperature changes according to the moist adiabatic lapse rate change of a parcel with 80% relative humidity lifted from the surface. The mo ist adiabatic assumption is not symmetrically applicable to a warmer a nd colder climate: In a warmer regime moist convection determines the tropical temperature structure, but when the climate becomes colder th e effect of moist convection diminishes and the large-scale dynamics a nd radiative processes become relatively important. Although a predict ion based on the change in moist adiabat marches the GCM simulation of climate change averaged over the tropical Pacific basin, it does not match the simulation regionally because small changes in the general c irculation change the local boundary layer relative humidity by 1%-2%. Thus, the prediction of regional climate change in CAFE is also depen dent on subtle changes in the dynamics.