Long-term changes in gravity wave activity in the middle atmosphere from satellite and ground-based observations

Atmospheric gravity waves (GWs) are one of the most important drivers of the circulation of the middle and upper atmosphere. Usually generated in the lower atmosphere and propagating upwards through the atmospheric layers, the aggregated forcing of these waves drives circulations in the middle atmosphere that are far from that expected under radiative equilibrium. Circulations in the mesosphere and lower thermosphere (MLT) and above, especially in polar regions, have shown extreme sensitivity to GW parameterisations in recent high-top modelling simulations and can exhibit significant and limiting biases compared to observations. This uncertainty in the role of GW dynamics between models has made predictions of how these high-altitude circulations are expected to respond to a changing climate very challenging. This is confounded by a relative scarcity of global observations of GW activity in the middle and upper atmosphere with which to understand these connections over climate timescales. Since the early 2000s, satellite and ground-based instrumentation has provided an unprecedented observational view of middle atmospheric dynamics and composition, especially for the study of GWs. However, due to different instrument capabilities and limited hardware lifetimes, examining long term trends of GW properties observationally has been challenging due to the need to re-establish baselines. Here we examine results from some of the longest known single-instrument records of GW activity in the middle and upper atmosphere spanning more than two decades. We explore changes in GW amplitudes, wavelengths and directional momentum flux in the stratosphere from a 22-year climatology derived from global 3-D satellite observations from the AIRS/Aqua, the longest single-instrument climatology of this type. We also explore changes in wind, temperature and large-scale GW activity in the polar MLT from nearly 20 years of single-station meteor wind radar observations in the Arctic and Antarctic. We compare these trends to equivalent analysis of other long-term satellite GW datasets and resolved GW activity in ERA5 stratospheric reanalysis. Finally, we discuss limitations and best practise for considering observed trends in GW observations, such as how changes in circulation can affect GW propagation and their apparent sensitivity to satellite remote sensing techniques.