Improved Simulation of the QBO in E3SMv1

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AbstractThe quasi‐biennial oscillation (QBO) of the zonal mean wind is a prominent feature of the tropical lower stratosphere and has been shown to influence stratospheric and tropospheric variability. Although observed for many decades, the QBO is difficult to be accurately captured in Earth system models. The newly developed U.S. Department of Energy Energy Exascale Earth System Model, Version 1 (E3SMv1) is capable of resolving stratospheric dynamics as it has a top at 0.1 hPa and 72 levels. The tropical oscillation of the zonal mean zonal wind in the default version of the model is too fast and too strong compared to the observed QBO. Here we evaluate the current simulation of the QBO and present simulations with modified convective gravity wave parameterization parameters that make the oscillation more realistic, especially in terms of the period. Although improvements in the QBO are obtained through modifications to the parameterized gravity waves, we find that the forcing of the QBO from Kelvin and mixed‐Rossby gravity waves is small due to weak sources in the troposphere. The changes to the parameterized convective gravity waves (and QBO) are primarily limited to the tropical stratosphere. Small changes to mean wind and temperature are also found in Northern Hemisphere winter in the uppermost troposphere between 10°N and 60°N. Modifications to parameterized convectively generated gravity waves result in changes in interannual variability, primarily in the Southern Hemisphere, and modest changes in sea level pressure variability over the Pacific are also noted.