Recent explorations of the state-dependence of Earth’s equilibrium climate sensitivity (ECS) have revealed a pronounced peak in ECS at a surface temperature of approximately 310 K. This ECS peak has been observed in models spanning the model hierarchy, suggesting a robust physical source. Here we propose an explanation for this ECS peak using a novel spectrally-resolved decomposition of clear-sky longwave feedbacks. We show that the interplay between spectral feedbacks in H2O- and CO2-dominated portions of the longwave spectrum, along with moist-adiabatic amplification of upper-tropospheric warming, conspire to produce a minimum in the feedback parameter, and a corresponding peak in ECS, at a surface temperature of 310 K. Mechanism denial tests highlight three key ingredients for the ECS peak: 1) H2O continuum absorption to quickly close spectral windows at high surface temperature; 2) moist-adiabatic tropospheric temperatures to enhance upper-tropospheric warming; and 3) energetically-consistent increases of CO2 with surface temperature.

Meridional gradients in CO2 forcing are known to drive increases in poleward heat transport (Huang and Zhang 2014, Huang et al. 2017). However, the climate factors which control these meridional forcing gradients are not well understood. Building upon the work of Wilson (2012), we build a first-principles, analytical model for CO2 forcing which requires as input only the temperatures at the surface and roughly 30 hPa, as well as column relative humidity. This model quantitatively captures global variations in clear-sky CO2 forcing, and shows that the meridional forcing gradient is directly attributable to the meridional surface temperature gradient.

The overturning tropical Pacific circulation known as the Walker circulation embodies complex interactions between large-scale circulations, deep and shallow convection, stratocumulus clouds, and microphysical cloud processes. The large and multi-scale nature of the Walker circulation has made high resolution modeling costly, while disentangling the relevant circulations and processes in a global model with more parameterizations is often challenging. This work uses the framework of the Walker Circulation as a unifying experiment for both high-resolution and global models with the goal of identifying how deep tropical convective heating and low-level clouds interact with and are influenced by the circulations in which they are embedded. A high resolution model with explicit convection (1km and 2km grid-spacing) is used to examine the system free of the complications inherent in convective parameterizations. The same model is also used at GCM-like resolutions with parameterized convection (25km and 100km grid-spacing) to gain insight into how the clouds and circulations interact in a GCM configuration. We define the idealized Walker circulation with a prescribed sea surface temperature dipole pattern, no rotation, uniform insolation, fully interactive radiation, and a channel domain (100km x 4000km). All simulations use the the same nonhydrostatic dynamical core (FV3) with the physics based on those in the AM4 GFDL atmospheric model. We find large differences in the total condensate between the high-resolution model and the GCM with the high-resolution model tending to have less low-level condensate but more condensate in the deep convective regions. This is reflected in the relative humidity fields as well. The parameterized entrainment of deep convection and the feedbacks of low-level tropical clouds are both leading factors contributing to the large spread of the climate sensitivity. With this in mind experiments are performed with the GCM in which the lateral mixing rate of deep convective plumes is varied. In addition, the detailed representation of cloud fraction between the two models is investigated. Our goal is to determine to what extent deep tropical convection can influence remote low-level clouds in regions with a subsiding free troposphere.