| Abstract: We examine 800-year time series of
internally generated variability in both a coupled ocean-atmosphere
model where water vapor anomalies are not allowed to interact with
longwave radiation and one where they are. The ENSO-like phenomenon in
the experiment without water vapor feedback is drastically suppressed
both in amplitude and geographic extent relative to the experiment with
water vapor feedback. Surprisingly, the reduced amplitude of ENSO-related
sea surface temperature anomalies in the model without water vapor
feedback cannot be attributed to greater longwave damping of sea surface
temperature. (Differences between the two experiments in radiative
feedback due to clouds counterbalance almost perfectly the differences
in radiative feedback due to water vapor.) Rather, the interaction
between water vapor anomalies and longwave radiation affects the ENSO-like
phenomenon through its influence on the vertical structure of radiative
heating: Because of the changes in water vapor associated with it, a
given warm equatorial Pacific sea surface temperature anomaly is
associated with a radiative heating profile that is much more
gravitationally unstable when water vapor feedback is present. The warm
sea surface temperature anomaly therefore results in more convection in
the experiment with water vapor feedback. The increased convection, 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
manner, the interaction between water vapor anomalies and longwave
radiation magnifies the air-sea interactions at the heart of the ENSO
phenomenon; without this interaction, the coupling between sea surface
temperature and wind stress is effectively reduced, resulting in smaller
amplitude ENSO episodes with a more limited geographical extent. |