Evaluation of the Quasi-Biennial Oscillation in global climate models for the SPARC QBO-initiative

The Stratosphere-troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) seeks to improve confidence in general circulation and earth system model (GCM and ESM) simulations of the QBO, a prominent feature of middle atmosphere tropical variability first identified nearly sixty years ago. Although only five out of 47 models contributing to the Coupled Model Intercomparison Project Phase 5 (CMIP5) had spontaneous QBOs, simulated QBOs are anticipated to be more common among CMIP6 models as more atmospheric GCMs are able to reproduce the phenomenon, both by ensuring adequate vertical resolution in the stratosphere and by parametrizing accelerations due to subgrid nonorographic gravity waves (NOGWs). The complexity of CMIP6 models and their forcing scenarios, however, is an obstacle to using the CMIP6 multimodel ensemble for analysis of modelling uncertainties that are specific to the QBO and its impacts. The QBOi multimodel ensemble represents an alternative approach in which modelling uncertainties related to the QBO are assessed by performing coordinated experiments with atmospheric GCMs that have simplified external forcings and boundary conditions, designed to characterize QBO representation and its response to idealised future climate scenarios. 

Results are presented from an analysis of QBOs in thirteen atmospheric GCMs forced with both observed and annually repeating sea surface temperatures (SSTs). Mean QBO periods in most of these models are close to, though shorter than, the period of 28 months observed in ERA-Interim. Amplitudes are within ±20% of the observed QBO amplitude at 10hPa, but typically about half of that observed at lower altitudes (50 and 70hPa). For almost all models the oscillation's amplitude profile shows an overall upward shift compared to reanalysis and its meridional extent is too narrow. Asymmetry in the duration of eastward and westward phases is reasonably well captured though not all models replicate the observed slowing as the westward shear descends. Westward phases are generally too weak, and most models have an eastward time mean wind bias throughout the depth of the QBO. Intercycle period variability is realistic and in some models is enhanced in the experiment with observed SSTs compared to the experiment with repeated annual cycle SSTs. Mean periods are also sensitive to this difference between SSTs but only when parametrized NOGW sources are coupled to tropospheric parameters and not prescribed with a fixed value. But, overall, modelled QBOs are very similar whether or not the prescribed SSTs vary interannually. A portrait of the overall ensemble performance is provided by a normalised grading of QBO metrics. To simulate a QBO all but one model used parametrized NOGWs, which provided the majority of the total wave forcing at altitudes above 70hPa in most models. Thus the representation of NOGWs either explicitly or through parametrization is still a major uncertainty underlying QBO simulation in these present-day experiments.