| Summary: | Observational data sets in the polar middle atmosphere are extremely valuable for understanding the polar dynamics and coupling between lower and middle atmosphere. Using the long-term data sets observed with an OH all-sky imager, a Fabry-Perot Interferometer at Resolute Bay Observatory, Canada (74.7 degrees N, 94.9 degrees W), Microwave Limb Sounder and reanalysis data, we study the characteristics of small-scale gravity waves (GWs) with the horizontal wavelength less than 20 km in the Arctic winter mesosphere during 2014-2016. Most GWs propagate nearly against the mesospheric and stratospheric winds, consistent with the wind filtering theory. A small amount (7 of 36 cases) of small-scale GWs in large regions (area >1/2 of the whole image) propagate nearly westward with slower phase speeds, larger horizontal wavelengths, and longer periods than those in limited regions. We also find that multiple GWs with different propagating directions can sometimes occur simultaneously. This is likely due to different wave sources and/or the ducted region in the atmosphere. The observed small-scale GWs may also likely be excited in the mesopause region, such as secondary wave generated by primary wave breaking or a result of baroclinic instability processed in the stratosphere and the interaction of planetary waves with the background winds. In addition, almost all of the small-scale GWs occurred during the strong El Nino and anomalous quasi-biennial oscillation events in 2015/2016 winter. AGS-1339918
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