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Previous studies have reported that the tropospheric climate shifted around the 2000s, but the recent trend in the stratospheric wave intensity (SWI) is still unclear. The results in this study show that the SWI in December during 2001-2015 weakens, which is opposite to that during 1979-2000, implying a shift around the 2000s. The weakened SWI is dominated by its wave number-1 component, which is related to the wave propagation impeded by the decreased refractive index at high latitudes and the weakened wave activity in the troposphere. Changes in refractive index are mainly contributed by changes in the meridional potential vorticity gradient via the barotropic term. This shift in the wave number-1 waves leads to a shift in the stratospheric Arctic temperature, that is, warming (cooling) during 1979-2000 (2001-2015), implying that similar shifting phenomena may appear regardless of the continued greenhouse gas emissions.

Plain Language Summary Recent studies have reported that some tropospheric climate (e.g., global mean surface temperature, the Pacific trade wind, and the Walker circulation) and stratospheric circulations (e.g., Arctic vortex and Brewer-Dobson circulation) have shifted around the 2000s. The planetary waves, which play a key role in the stratosphere-troposphere dynamical coupling, can not only impact the stratospheric temperature and circulation via wave-mean interactions but also modify the tropospheric weather and climate via downward wave coupling. Therefore, it is important to examine whether the planetary waves have shifted around the 2000s. In this study, we investigated the trends in stratospheric wave intensity in December during 1979-2000 and 2001-2015 and found that the stratospheric wave intensity during 2001-2015 weakens, which is opposite to that during 1979-2000, implying a shift around the 2000s. This shift in planetary waves suggests that temperature, circulation, and tracers, associated with planetary waves may have also shifted around the 2000s, even though global warming continues.