Response of the quasi-biennial oscillation to a warming climate in global climate models
<p>We compare the response of the quasi-biennial oscillation (QBO) to a warming&#160;climate in eleven atmosphere general circulation models that performed time-slice&#160;simulations for present-day, doubled,&#160; and&#160; quadrupled CO<sub>2</sub> climates.&#160; No consistency was found among the models for the QBO period response, with the period&#160;decreasing by eight months in some models and lengthening by up to thirteen months in others in the doubled CO<sub>2</sub>&#160; simulations.&#160; In the&#160;quadruped CO<sub>2</sub> simulations&#160; a reduction in QBO period of 14 months was found in some models, whereas in several others the tropical oscillation no longer resembled the present day QBO, although&#160;could still be identified in the deseasonalized zonal mean zonal wind timeseries.&#160; In contrast, all the models projected a decrease in the&#160; QBO&#160;amplitude in a warmer climate with the largest relative decrease&#160; near 60 hPa. In simulations with doubled and quadrupled CO<sub>2</sub> the multi-model mean QBO amplitudes decreased by 36\% and 51\%, respectively. Across the&#160; models the differences in the QBO period response were most strongly&#160;related to how the gravity wave momentum flux entering the stratosphere and&#160;tropical vertical residual velocity responded to the increases&#160;in CO<sub>2</sub> amounts. Likewise it was found that the robust decrease in QBO amplitudes was&#160;correlated across the models to changes in vertical residual velocity,&#160;parameterized gravity wave momentum fluxes, and to some degree the resolved upward wave flux.&#160; We argue that uncertainty in the representation of the parameterized gravity waves is the most likely cause&#160;of the spread among the eleven models in the QBO's response to climate change.</p>