Causes of Reduced Climate Sensitivity in E3SM From Version 1 to Version 2

Abstract

The effective climate sensitivity in the Department of Energy's Energy Exascale Earth System Model (E3SM) has decreased from 5.3 K in version 1 to 4.0 K in version 2. This reduction is mainly due to a weaker positive cloud feedback that leads to a stronger negative radiative feedback. Present‐day atmosphere‐only experiments with uniform 4 K sea surface temperature warming are used to separate the contributions of individual model modifications to the reduced cloud feedback. We find that the reduced cloud feedback is mostly driven by changes over the tropical marine low cloud regime, mainly related to a new trigger function for the deep convection scheme and modifications in the cloud microphysics scheme. The new trigger function helps weaken the low cloud reduction by increasing the cloud water detrainment at low levels from deep convection under warming. Changes to the formula of autoconversion rate from liquid to rain and an introduced minimum cloud droplet number concentration threshold in cloud microphysical calculations help sustain clouds against dissipation by suppressing precipitation generation with warming. In the midlatitudes, the increased Wegener‐Bergeron‐Findeisen (WBF) efficiency strongly reduces present‐day liquid water and leads to a stronger negative cloud optical depth feedback. The reduced trade cumulus cloud feedback in v2 is closer to estimates from recent observational and large‐eddy modeling studies but might not be due to the right physical reasons. The reduced mid‐latitude cloud feedback may be more plausible because more realistic present‐day mixed‐phase clouds are produced through the change in the WBF efficiency.

Slab Ocean Component of the Energy Exascale Earth System Model (E3SM): Development, Evaluation, and Application to Understanding Earth System Sensitivity

“…The large sensitivity to ocean heat convergence and overcompensation between atmosphere and ocean heat transports in the Southern Hemisphere indicates that mid-latitude marine clouds are too sensitive to SSTs in E3SMv2. A recent assessment of the cloud feedbacks in the CMIP5/CMIP6 models suggests a relatively large positive net cloud feedback in E3SM (Golaz et al, 2019;Qin et al, 2024;Zelinka et al, 2022). These studies also show greater significance for tropical marine low-level clouds, whereas the results here point to a large midlatitude marine low-level cloud sensitivity to SSTs.…”

“…The v1.HR.Control and the SOM.v1.HR-OHC experiments use different versions of the same atmosphere model (EAMv1 and EAMv2), with significant differences in their parameter tunings (Golaz et al, 2022;Qin et al, 2024). Our testing of the SOM configuration in E3SMv1 (not shown) indicates that the SOM experiments with the EAMv1 atmosphere were generally cooler than those of EAMv2.…”

“…As a result, these tunings can overcompensate for biases in the ocean or other model components. Here, we apply the SOM to test the sensitivity of the E3SMv2 baseline climate to a more realistic ocean heat transport since the model can reproduce observed global SSTs and reasonable ECS despite its biases in ocean transports and sea ice extent (Golaz et al, 2022;Qin et al, 2024). This sensitivity study will provide insight into how atmospheric parameters might be overcompensating for the biases in ocean transports.…”