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The mesosphere/lower thermosphere (MLT, 80–100 km) region is an important boundary between Earth's atmosphere below and space above and may act as a sensitive indicator for anthropogenic climate change. Existing observational and modeling studies have shown the middle atmosphere and the MLT is cooling and contracting because of increasing greenhouse gas emissions. However, trend analyses are highly sensitive to the time periods covered, their length, and the measurement type and methodology used. We present for the first time the linear and 11‐year solar cycle responses in the meteor ablation altitude distributions observed by 12 meteor radars at different locations. Decreasing altitudes were seen at all latitudes (linear trends varying from −10.97 to −817.95 m dec−1), and a positive correlation with solar activity was seen for most locations. The divergence of responses at high latitudes indicates an important and complex interplay between atmospheric changes and dynamics at varying time scales. Plain Language Summary: High up in our atmosphere lies the mesosphere/lower thermosphere region (80–100 km); an important transition zone between the atmosphere below and space above. Existing studies indicate that this region is changing (cooling and contracting) in response to increasing greenhouse gas emissions, quite unlike the net warming we see near the surface. However these trend studies are often highly sensitive to choice and length of time period covered, and the methodology and type of measurements used. Here we present for the first time a self‐consistent methodology applied to 12 different meteor radar station datasets located at a diverse range of latitudes. We looked at changes in the mean peak altitude of individual meteoroid detections, and found decreasing peak altitudes at all locations examined (linear trends varying from −10.97 to −817.95 m decade−1) consistent with a global cooling and contracting of the upper atmosphere. We also examined the response to the 11‐year solar cycle and found a positive correlation with solar activity (i.e., increased meteoroid peak altitudes during solar maximum, and vice versa) for low and mid‐latitude locations. However we found an anti‐correlation at high latitudes suggestive of an important and complex interplay between atmospheric changes and dynamics at varying time scales. Key Points: Use of geographically diverse meteor radar peak detection altitudes to assess long‐term and 11‐year solar cycle (SC) trends in mesopause regionThe altitude of observed peak meteor height has decreased over time at all locations, regardless of latitude and data setPositive correlation at low‐ and mid‐latitude locations with the 11‐year SC, but more complex response at high‐latitudes [ABSTRACT FROM AUTHOR]

Atmospheric gravity waves (GWs) are key drivers of the atmospheric circulation, but their representation in general circulation models (GCMs) is challenging, leading to significant biases in middle atmospheric circulations. Unresolved GW momentum transport in GCMs must be parameterized, but global directional GW observations are needed to constrain this. Here we present an 18‐year climatology of directional stratospheric GW momentum flux (GWMF) from global AIRS/Aqua 3‐D satellite observations during 2002 to 2019. Striking hemispheric asymmetries are found at high latitudes, including dramatic reductions and reversals of GWMF during sudden stratospheric warmings. During Southern Hemisphere winter, a lateral convergence of GWMF toward 60°S is found that has no Northern Hemisphere counterpart. In the tropics, we find that zonal GWMF in AIRS measurements is strongly modulated by the semiannual oscillation (SAO) but not the quasi‐biennial oscillation (QBO). Our results provide guidance for future GW parameterizations needed to resolve long‐standing biases in GCMs. Plain Language Summary: Gravity waves (GWs) are traveling waves that can occur in geophysical fluid environments subject to the gravitational force. In the Earth's atmosphere, GWs transport momentum that helps to drive the atmospheric circulation, especially in the middle atmosphere. But for numerical weather and climate models, accurately simulating GWs has proven very challenging because of a lack of GW observations, leading to significant model biases. Here we present an 18‐year climatology of GW observations near 40‐km altitude for 2002 to 2019. For the first time, we present multidecadal global measurements of directional GW momentum flux derived from 3‐D satellite observations from National Aeronautics and Space Administration's (NASA's) AIRS instrument. We find that during Southern Hemisphere winter, GWs travel laterally toward 60°S each year. This is significant because most models underestimate GW momentum near 60°S and do not typically include lateral propagation in their parameterizations. In the tropical stratosphere, we find no modulation of GWs in AIRS observations by the quasi‐biennial oscillation (QBO). This is consistent with the hypothesis that GW‐QBO interactions occur at shorter vertical wavelengths, which are invisible to AIRS. Our results will help to guide future GW parameterizations, leading to more accurate atmospheric simulations and ultimately better forecasts of weather and climate. Key Points: Directional stratospheric gravity wave momentum flux (GWMF) is derived from global 3‐D AIRS/Aqua satellite observations from 2002 to 2019Dramatic reductions and reversals of high‐latitude GWMF are seen during SSWs; in the tropics the SAO modulates the AIRS GWMF but not the QBOA lateral convergence of wintertime GWMF toward 60°S occurs each year that has no counterpart in the Northern Hemisphere [ABSTRACT FROM AUTHOR]

Satellite observations show that there is a wintertime hotspot of gravity wave activity, located mainly over the ocean, around 60°S in the stratosphere. However, the sources of the gravity waves making up this hotspot are varied and complex and remain unclear. Here we use radiosonde observations from 11 Antarctic stations and selected small islands close to 60°S to examine the horizontal directional pseudo‐momentum flux and energy density distributions of upward propagating gravity waves in the lower stratosphere. This paper shows, for the first time, that short vertical wavelength gravity waves in the lower stratosphere clearly propagate meridionally toward 60°S during the winter months. This result supports previous studies that show that this belt of gravity wave activity over the ocean is contributed to by wave sources outside 60°S. Plain Language Summary: Atmospheric gravity waves are buoyancy waves, where the restoring force is gravity, which can have horizontal wavelengths from a few tens to hundreds of kilometers. They can be generated by a variety of sources including wind flow over mountains and storms. Over the Drake Passage and out to the east around 60°S, a large hotspot of gravity waves has been observed in the wintertime stratosphere using satellite observations. The sources for this gravity wave hotspot are unclear as it mainly located over the open ocean. In this paper radiosonde observations are used to examine, for the first time, the directional variation in short vertical wavelength gravity waves close to 60°S in the lower stratosphere. It is found that during the winter months there is a meridional convergence of gravity waves toward 60°S, implying that the gravity wave sources of the hotspot are in fact be outside this region. Key Points: This paper provides evidence of a winter time convergence of gravity waves and their momentum flux toward 60°SThis is the first time this has been observed for short vertical wavelength waves in the lower stratosphere [ABSTRACT FROM AUTHOR]