Variations in zonal surface temperature gradients and zonally asymmetric
tropical overturning circulations (Walker circulations) are examined over a
wide range of climates simulated with an idealized atmospheric general
circulation model (GCM). The asymmetry in the tropical climate is generated by
an imposed ocean energy flux, which does not vary with climate. The range of
climates is simulated by modifying the optical thickness of an idealized
longwave absorber (representing greenhouse gases).
The zonal surface temperature gradient in low latitudes generally decreases
as the climate warms in the idealized GCM simulations. A scaling relationship
based on a two-term balance in the surface energy budget accounts for the
changes in the zonally asymmetric component of the GCM-simulated surface
temperature gradients.
The Walker circulation weakens as the climate warms in the idealized
simulations, as it does in comprehensive simulations of climate change. The
wide range of climates allows a systematic test of energetic arguments that
have been proposed to account for these changes in the tropical circulation.
The analysis shows that a scaling estimate based on changes in the hydrological
cycle (precipitation rate and saturation specific humidity) accounts for the
simulated changes in the Walker circulation. However, it must be evaluated
locally, with local precipitation rates. If global-mean quantities are used,
the scaling estimate does not generally account for changes in the Walker
circulation, and the extent to which it does is the result of compensating
errors in changes in precipitation and saturation specific humidity that enter
the scaling estimate
