Accepted by Journal of Atmospheric Sciences
The feedback concept has been used by several authors in the climatology field to describe model behavior and to assess the importance of different intervening mechanisms. Here, a simple global model of climate has been built to analyze the water vapor feedback, making use of elementary laws and parameterizations as determined by GCM results of CO2 doubling experiments. Beyond a static quantification of the water feedback, a more general formal definition of feedback gain based on the Tangent Linear System is introduced. This definition recovers the dynamical aspect of the system response to perturbation of Bode’s original concept.
Two conclusions are drawn from the model : (i) The water vapor effect is found to have a feedback gain of 36 % (1.6 factor), comparable with results from GCM analyzes, but with a very long characteristic time of over four years. (ii) The water vapor feedback is found negative for time scales below four years and positive for longer time scales. As a consequence, the water vapor feedback is fully active only in response to perturbations that last ten years at least. This suggests that the water vapor feedback could reduce the natural variability due to tropospheric temperature perturbations over short time scales while enhancing it over longer time scales.