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

Abstract

Quasi-biennial oscillations (QBOs) in thirteen atmospheric general circulation models forced with both observed and annually repeating sea surface temperatures (SSTs) are evaluated. In most models the QBO period is close to, but shorter than, the observed period of 28 months. Amplitudes are within ±20% of the observed QBO amplitude at 10 hPa, but typically about half of that observed at lower altitudes (50 and 70 hPa). 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 of the descending westward shear. Westward phases are generally too weak, and most models have an eastward time mean wind bias throughout the depth of the QBO. The 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 non-orographic gravity wave (NOGW) sources are coupled to tropospheric parameters and not prescribed with a fixed value. Overall, however, modelled QBOs are very similar whether or not the prescribed SSTs vary interannually. A portrait of the overall ensemble performance is provided by a normalized 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 70 hPa in most models. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Progress in Simulating the Quasi‐Biennial Oscillation in CMIP Models

“…For reference, we also show the amplitude of the QBO from Experiment 1 of QBOi (Bushell et al, ; Butchart et al, ). Thirteen models carried out this experiment (60LCAM5, AGCM3‐CMAM, CESM1 (WACCM5‐110L), ECHAM5sh, EMAC, HadGEM2‐A, HadGEM2‐AC, LMDz6, MIROC‐AGCM‐LL, MIROC‐ESM, MRI‐ESM2, UMGA7, and UMGA7gws), which consisted of a simulation with observed prescribed sea surface temperatures (SSTs) and sea ice as well as external forcings for the period between January 1979 and February 2009.…”

“…Thirteen models carried out this experiment (60LCAM5, AGCM3‐CMAM, CESM1 (WACCM5‐110L), ECHAM5sh, EMAC, HadGEM2‐A, HadGEM2‐AC, LMDz6, MIROC‐AGCM‐LL, MIROC‐ESM, MRI‐ESM2, UMGA7, and UMGA7gws), which consisted of a simulation with observed prescribed sea surface temperatures (SSTs) and sea ice as well as external forcings for the period between January 1979 and February 2009. Details of the QBOi models and experiments can be found in Butchart et al () and Bushell et al ().…”

“…The QBO is a prominent feature of the equatorial lower stratosphere and upper troposphere, with alternating easterly and westerly zonal mean wind shear zones descending from ∼10 to 100 hPa. The period of the observed QBO ranges from about 20 to 32 months with an average of ∼28 months (Baldwin et al, ; Bushell et al, ). The QBO impacts the variability of many parts of the Earth system.…”

“…In CMIP5, only five models were able to simulate a QBO, which in large part motivated the Stratosphere Processes and their Role in Climate (SPARC) QBO initiative (QBOi) to improve the simulation of tropical stratospheric variability in general circulation models (GCMs) and ESMs (Butchart et al, ). As part of QBOi, simulations of the QBO were examined in 13 atmosphere‐only models run under present‐day climate (Bushell et al, ), and 11 models' QBO response to doubling and quadrupling CO 2 was examined by Richter et al (). In addition, the proximal forcing of the QBO was examined by Holt et al ().…”

“…In most models, a parameterization of nonorographic gravity waves (GWs) is required in order to simulate the QBO. Among QBOi models, parameterized GWs provide 40% to 80% of the momentum forcing required to drive the QBO, and these parameterizations are highly tuned in order to obtain the correct QBO characteristics (Bushell et al, ; Butchart et al, ).…”