Suppression of ENSO in a coupled model without water vapor feedback

We examine 800-year time series of internally generated variability in both a coupled ocean-atmosphere model where water vapor anomalies are not allow ed to interact with longwave radiation and one where they are. The ENSO-lik e phenomenon in the experiment without water vapor feedback is drastically suppressed both in amplitude and geographic extent relative to the experime nt with water vapor feedback. Surprisingly, the reduced amplitude of ENSO-r elated sea surface temperature anomalies in the model without water vapor f eedback cannot be attributed to greater longwave damping of sea surface tem perature, (Differences between the two experiments in radiative feedback du e to clouds counterbalance almost perfectly the differences in radiative fe edback due to water vapor.) Rather, the interaction between water vapor ano malies and longwave radiation affects the ENSO-like phenomenon through its influence on the vertical structure of radiative heating: Because of the ch anges in water vapor associated with it, a given warm equatorial Pacific se a surface temperature anomaly is associated with a radiative heating profil e that is much more gravitationally unstable when water vapor feedback is p resent. The warm sea surface temperature anomaly therefore results in more convection in the experiment with water vapor feedback. The increased conve ction, in turn, is related to a larger westerly wind-stress anomaly, which creates a larger decrease in upwelling of cold water, thereby enhancing the magnitude of the original warm sea surface temperature anomaly. In this ma nner, the interaction between water vapor anomalies and longwave radiation magnifies the air-sea interactions at the heart of the ENSO phenomenon; wit hout this interaction, the coupling between sea surface temperature and win d stress is effectively reduced, resulting in smaller amplitude ENSO episod es with a more limited geographical extent.