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Abstract The boreal‐winter stratospheric polar vortex is more disturbed when the quasi‐biennial oscillation (QBO) in the lower stratosphere is in its easterly phase (eQBO), and more stable during the westerly phase (wQBO). This so‐called “Holton–Tan effect” (HTE) is known to involve Rossby waves (RWs) but the details remain obscure. This tropical–extratropical connection is re‐examined in an attempt to explain its intraseasonal variation and its relation to Rossby wave breaking (RWB). Reanalyses in isentropic coordinates from the National Centers for Environmental Prediction Climate Forecast System for the 1979–2017 period are used to evaluate the relevant features of RWB in the context of waveguide, wave–mean‐flow interaction, and the QBO‐induced meridional circulation. During eQBO, the net extratropical wave forcing is enhanced in early winter with ∼25% increase in upward propagating planetary‐scale Rossby waves (PRWs) of zonal wave‐number 1 (wave‐1). RWB is also enhanced in the lower stratosphere, characterized by convergent anomalies in the subtropics and at high latitudes and strengthened waveguide in between at 20°N–40°N, 350–650 K. In late winter, RWB leads to finite amplitude growth, which hinders upward propagating PRWs. The effect is most significant for zonal wave‐numbers 2 and 3 (wave‐2‐3). During wQBO, RWB in association with wave‐2‐3 is enhanced in the upper stratosphere. Wave absorption/mixing in the surf zone reinforces a stable polar vortex in early to middle winter. A poleward confinement of the extratropical waveguide in the upper stratosphere forces RWB to extend downward around January. A strengthening of upward propagating wave‐2‐3 follows and the polar‐vortex response switches from reinforcement to disturbance around February, thus a sign reversal of the HTE in late winter.
Key Findings • Rossby wave breaking (RWB) is enhanced in the height regions where the zero‐wind line is shifted into the winter hemisphere and where the QBO‐induced meridional circulation is directed toward the winter pole
• Polar vortex responses differ in terms of the height location of RWB, zonal wave‐number‐dependent disturbances and seasonal development
• Significant increase in wave‐1 occurs when the QBO is in its easterly phase
• A cumulative effect of RWB results in enhanced wave forcing of zonal wave‐numbers 2 and 3 during westerly QBO, which manifests in a sign reversal of the Holton–Tan effect in late winter.
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Abstract We compare the response of the Quasi‐Biennial Oscillation (QBO) to a warming climate in eleven atmosphere general circulation models that performed time‐slice simulations for present‐day, doubled, and quadrupled CO2climates. No consistency was found among the models for the QBO period response, with the period decreasing by 8 months in some models and lengthening by up to 13 months in others in the doubled CO2simulations. In the quadrupled CO2simulations, 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 it could still be identified in the deseasonalized zonal mean zonal wind timeseries. In contrast, all the models projected a decrease in the QBO amplitude in a warmer climate with the largest relative decrease near 60 hPa. In simulations with doubled and quadrupled CO2, the multi‐model mean QBO amplitudes decreased by 36 and 51%, respectively. Across the models the differences in the QBO period response were most strongly related to how the gravity wave momentum flux entering the stratosphere and tropical vertical residual velocity responded to the increases in CO2amounts. Likewise it was found that the robust decrease in QBO amplitudes was correlated across the models to changes in vertical residual velocity, parametrized gravity wave momentum fluxes, and to some degree the resolved upward wave flux. We argue that uncertainty in the representation of the parameterized gravity waves is the most likely cause of the spread among the eleven models in the QBO's response to climate change.
