Quantifying Key Mechanisms That Contribute to the Deviation of the Tropical Warming Profile From a Moist Adiabat

Climate models project tropical warming is amplified aloft relative to the surface in response to increased CO2. Here we show moist adiabatic adjustment overpredicts the multimodel mean 300 hPa temperature response by 16.6-25.3% across the CMIP5 model hierarchy. We show three mechanisms influence overprediction: climatological large-scale circulation, direct effect of increased CO2, and convective entrainment. Accounting for the presence of a climatological large-scale circulation and the direct effect of CO2 reduces the CMIP5 multimodel mean overprediction by 0.7-7.2% and 2.8-3.9%, respectively, but does not eliminate it. To quantify the influence of entrainment, we vary the Tokioka parameter in aquaplanet simulations. When entrainment is decreased by decreasing the Tokioka parameter from 0.1 to 0, overprediction decreases by 9.6% and 10.4% with and without a large-scale circulation, respectively. The sensitivity of overprediction to climatological entrainment rate in the aquaplanet mostly follows the predictions of zero-buoyancy bulk-plume and spectral-plume models.