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The effect of the Madden-Julian Oscillation (MJO) on the Northern Hemisphere wintertime stratospheric polar vortex in the period preceding stratospheric sudden warmings is evaluated in operational subseasonal forecasting models. Reforecasts which simulate stronger MJO-related convection in the Tropical West Pacific also simulate enhanced heat flux in the lowermost stratosphere and a more realistic vortex evolution. The time scale on which vortex predictability is enhanced lies between 2 and 4 weeks for nearly all cases. Those stratospheric sudden warmings that were preceded by a strong MJO event are more predictable at similar to 20 day leads than stratospheric sudden warmings not preceded by a MJO event. Hence, knowledge of the MJO can contribute to enhanced predictability, at least in a probabilistic sense, of the Northern Hemisphere polar stratosphere.

Plain Language Summary Atmospheric variability in the polar stratosphere, the atmospheric layer between approximately 10 km and 50 km above the surface, strongly impacts surface climate over the eastern United States and Eurasia. It is thought that variability in this region can be predicted up to 15 days in advance. Here we show that knowledge of the state of convection in the tropical Pacific and Indian Ocean can lead to enhanced predictability up to 4 weeks in advance.