Interannual variability and trends of stratospheric gravity waves from nearly two decades of Atmospheric Infrared Sounder (AIRS) and Microwave Limb Sounder (MLS) observations

Stratospheric gravity waves (SGWs) originate in the troposphere and form as a result of disturbances in stably stratified boundary layers. These waves propagate upwards and break in the middle or upper atmosphere, acting as a vehicle for energy and momentum transport. Therefore, studying SGWs and the way they evolve is important in understanding lower-middle-upper atmosphere coupling and how this coupling changes with time and other forcings. Using nearly 17 years (Sep. 2002-Jun. 2019) of Atmospheric Infrared Sounder (AIRS) and 15 years (Aug. 2004-Apr. 2019) of Microwave Limb Sounder (MLS) high-quality Level 1 radiance data, we observe a consistent positive trend of SGW variances in the middle stratosphere (30-40 km), where both instruments exhibit stable noise levels throughout the observation period. The magnitude of this positive trend is in phase of a poleward shift of the Northern-Hemisphere (NH) polar night jet and a strengthening of the Southern-Hemisphere (SH) polar night jet during the past 17 years, as indicated in the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) dataset. Additionally, because of their different viewing geometries, AIRS and MLS observations of SGWs show contrasting responses to the solar cycle, the quasi-biennial oscillation (QBO), and the El Niño-Southern Oscillation (ENSO). Specifically, AIRS is more sensitive to vertically propagating gravity waves (GWs), while MLS is more sensitive to slantwise-propagating GWs. Because of these differing sensitivities to contrasting parts of the GW spectrum, AIRS-observed GWs exhibit inter-hemispheric symmetric response patterns against all three major interannual forcings, with the largest responses centered on the equatorial stratosphere, while MLS-observed GWs exhibit a strong positive ENSO (solar) response at SH (NH) high latitudes. Such inter-hemispheric asymmetry implies a new pathway of teleconnection between the tropical troposphere and high-latitude stratosphere.