The quasi-biennial oscillation (QBO) is the strongest mode of variability in the tropical stratosphere and plays an important role in stratospheric dynamics and chemistry. The QBO is notably deficient in many climate models, including the Energy Exascale Earth System Model (E3SM) developed by the US Department of Energy. In this work, we utilize vertical grid refinement to reduce the lower stratospheric grid spacing from roughly 1 km to 500 m to facilitate more realistic synoptic wave activity in lower stratosphere in E3SM version 2. The refinement results in a simulated QBO with a reasonable amplitude and easterly-westerly transition, but still has a longer period than observed, slower easterly downward propagation speed, and shallower vertical depth. Similar refinement in the multi-scale modeling framework (MMF) configuration of E3SM yields similar improvements. By analyzing the forcing contributions from different wave types, we find that most of the QBO forcing still comes from parameterized gravity wave drag from convection. The improved QBO forcing contributions from resolved waves, especially equatorial Kelvin waves and resolved small scale gravity waves can be attributed to the grid refinement.

The simulated quasi-biennial oscillation (QBO) in the Energy Exascale Earth System Model version 2 (E3SMv2) is characterized by a shorter period and stronger amplitude compared with its predecessor E3SMv1. This is primarily attributed to updates to the deep convection parameterization which makes convection more intense but less frequent while leaving the time-mean convective heating tendency almost unchanged. Amplitudes of momentum fluxes associated with parameterized gravity waves, which are determined by the square of convective heating tendency, are consequently intensified. In addition, stronger planetary waves, another contributor to the QBO driving, are simulated in E3SMv2 in part due to tropospheric processes. Furthermore, there is evidence that planetary wave generation is present in the stratosphere due to dissipation of amplified parameterized gravity waves. These factors are found to be responsible for the stronger and faster QBO simulated in E3SMv2.

This work documents version two of the Department of Energy’s Energy Exascale Earth System Model (E3SM). E3SM version 2 (E3SMv2) is a significant evolution from its predecessor E3SMv1, resulting in a model that is nearly twice as fast and with a simulated climate that is improved in many metrics. We describe the physical climate model in its lower horizontal resolution configuration consisting of 110 km atmosphere, 165 km land, 0.5° river routing model, and an ocean and sea ice with mesh spacing varying between 60 km in the mid-latitudes and 30 km at the equator and poles. The model performance is evaluated by means of a standard set of Coupled Model Intercomparison Project Phase 6 (CMIP6) Diagnosis, Evaluation, and Characterization of Klima (DECK) simulations augmented with historical simulations as well as simulations to evaluate impacts of different forcing agents. The simulated climate is generally realistic, with notable improvements in clouds and precipitation compared to E3SMv1. E3SMv1 suffered from an excessively high equilibrium climate sensitivity (ECS) of 5.3 K. In E3SMv2, ECS is reduced to 4.0 K which is now within the plausible range based on a recent World Climate Research Programme (WCRP) assessment. However, E3SMv2 significantly underestimates the global mean surface temperature in the second half of the historical record. An analysis of single-forcing simulations indicates that correcting the historical temperature bias would require a substantial reduction in the magnitude of the aerosol-related forcing.