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The impact of parametrized nonorographic gravity wave drag (NOGWD) on the stratosphere-troposphere dynamical coupling in atmospheric models is relatively unexplored. Using the European Centre for Medium-Range Weather Forecasts Integrated Forecast System, we find that changes in NOGWD strength have a substantial impact on the tropospheric eddy-driven jet (EDJ) in both hemispheres, but the sense of the impact is opposite in the two hemispheres. In the Northern Hemisphere the impact occurs via changes in the amplitude and persistence of stratospheric anomalies. In the Southern Hemisphere it occurs instead via differences in the sensitivity of the EDJ to a given stratospheric anomaly, arising from changes in the seasonal cycle leading up to the polar vortex breakdown. Increasing NOGWD eliminates the springtime phase of the Southern Hemisphere tropospheric semiannual oscillation, resulting in a more equatorward annual-mean EDJ and showing that the semiannual oscillation cannot be explained entirely from tropospheric mechanisms.

Plain Language Summary The parametrization of the drag exerted by unresolved nonorographic gravity waves (NOGWD) in atmospheric models is highly uncertain but is known to be of first-order importance in the representation of stratospheric and mesospheric circulation. Because stratospheric variability affects tropospheric circulation, NOGWD might be expected to affect the troposphere as well. This question is explored using an atmospheric model. We find that increasing the strength of NOGWD weakens the impact of stratospheric variability on the tropospheric midlatitude jet in the Northern Hemisphere but strengthens the impact in the Southern Hemisphere. Beyond being relevant to quantifying the importance of NOGWD and the relationships between model biases, the experiments shed light on the different nature of stratosphere-troposphere coupling in the two hemispheres, an important topic in atmospheric dynamics. They also show that the tropospheric Southern Hemisphere semiannual oscillation in surface pressure cannot be explained entirely from tropospheric mechanisms, as is usually assumed.