Water vapor and cloud feedback mechanisms: inferences from satellite observations and numerical modeling

Satellite observations show large sensitivity of water vapor and cloud forcings to sea surface temperature changes during major climate events such as the El Nino Southern Oscillation (ENSO). This leads to two very important questions: What are the mechanisms of the observed large sensitivity? And can we use ENSO as a surrogate to understand mechanisms for climate change? This paper addresses the above questions. The author has studied the sensitivity of water vapor and cloud forcings to ocean temperature changes and their relationship with large scale circulation regimes using AVHRR and ERBE satellite outgoing longwave radiation, sea surface temperature and large scale circulation data. Results show that on interannual time scales, changes in the large scale circulation regimes contribute substantially to anomalies in the water vapor and cloud forcing observed during major climate events such as ENSO. Experiments show that over 75% of the observed apparent sensitivity of water vapor and cloud feedback sensitivity are due to changes in the large scale circulation with direct radiative feedback accounting for less than 20% of the observed changes. On interannual time scales, when adjustments are made for changes in the large scale circulation, the so-called supergreenhouse effect, i.e., run away greenhouse warming, does not exist. The results point to possible pitfalls in interpreting results for cloud and radiation feedback processes and that ENSO or other natural climate variations such as the seasonal cycle cannot simply be used as a surrogates for climate change without taking into account the effect of large scale dynamics.