Quantifying the deviation of the tropical upper tropospheric temperature
response to surface warming from a moist adiabat
Abstract
Climate models project that tropical warming is amplified aloft in
response to increased CO$_2$. Amplification aloft is expected
following moist adiabatic adjustment and the Clausius-Clapeyron
relation. Here, we show that moist adiabatic adjustment overpredicts the
multi-model mean temperature response at 300 hPa by
12.9–25.3\% across the model hierarchy. We show that
overprediction is influenced by at least three mechanisms: large-scale
circulation, direct effect of CO$_2$, and convective entrainment.
Accounting for the large-scale circulation and the direct effect of
CO$_2$ reduces overprediction by 5.7\% and
3.8\% respectively, but does not eliminate it. To test
the influence of entrainment, we vary the Tokioka parameter in
aquaplanet simulations with and without a large-scale circulation. When
varying the climatological entrainment rate in the aquaplanet,
overprediction varies from 6.7–17.9\%. The sensitivity
of overprediction to climatological entrainment rate in the aquaplanet
configured in radiative-convective equilibrium agrees well with the
predictions of zero-buoyancy bulk-plume models.