WATER-VAPOR AND TEMPERATURE LAPSE-RATE FEEDBACKS IN THE MIDLATITUDE SEASONAL CYCLE

The ongoing scientific debate regarding the role of water vapor feedba ck in climate change centers on the effects of water vapor vertical di stribution and variation with climate change. The temperature lapse ra te is another vertically dependent variable; its combination and inter action with water vapor feedback have an even greater potential for in fluencing the climate. Here we examine the vertical dependence of the feedbacks, as presently observed in the midlatitude seasonal cycle, by means of a radiative-convective model with climatological lapse rates . It is found that both the water vapor and lapse rate feedbacks are p ositive, amplifying the seasonal surface temperature cycle by approxim ately 23% and 15%, respectively, and have considerable thermal couplin g. The feedback contributions from the various vertical levels are ess entially additive. These vertical contributions are characterized by a bimodal water vapor feedback, with upper and lower troposphere maxima , and by a predominant lapse rate feedback from the lower troposphere. Although a small negative water vapor feedback contribution is found at 150 mb, its significance is overshadowed by the strong positive fee dbacks at all other levels, especially in the lower troposphere. The i mplications for climate change are discussed.