We investigated the variability of short-period (<1 hour) atmospheric gravity waves (AGWs) in the high-latitude mesosphere-lower-thermosphere/ionosphere (MLTI) region using OH (3,1) band emission data from the Advanced Mesospheric Temperature Mapper (AMTM) at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) in Norway. These OH intensity maps from January 2014, 2015, and 2016 were analysed to characterise AGW activity at these altitudes. We derived phase velocity spectra of AGWs by applying the Matsuda-transform to the OH intensity maps and calculated spectral power across different phase speed ranges and propagation directions to study the day-to-day and intraday variability of AGWs. Our results reveal significant differences in AGW activity between these years, with 2015 exhibiting lower spectral power and less variability compared to 2014 and 2016. The vertical propagation efficiency of AGWs was estimated using principles of critical-level filtering, incorporating winds from the ERA5 dataset for altitudes between 0–50 km. The relatively lower AGW activity and spectral power observed in the MLTI region in 2015 were associated with higher Arctic Oscillation (AO) index values, suggesting gravity wave filtering by eastward winds in the upper stratosphere. In contrast, lower AO index values in 2014 and 2016 indicated minimal filtering, leading to more diverse spectra of AGWs observed in the OH images. These findings highlight the strong influence of stratospheric wind structures on AGW variability in the high-latitude MLTI region during winter.

A document by Emily J Lear. Click on the document to view its contents.

\cite{Narayanan_2024}Gravity waves (GWs) play an important role in the dynamics and energetics of the mesosphere. Geomagnetic activity is a known source of GWs in the upper atmosphere. However, how deep the effects of geomagnetic activity induced GWs penetrate into the mesosphere remains an open question. We use temperature measurements from the SABER/TIMED instrument between 2002 - 2018 to study the variations of mesospheric GW activity following intense geomagnetic disturbances identified by AE and Dst indices. By considering several case studies, we show for the first time that the GWs forced by geomagnetic activity can propagate down to about 80 km in the high latitude mesosphere. Only regions above 55° latitudes show a clear response. The fraction of cases in which there is an unambiguous enhancement in GW activity following the onset of geomagnetic disturbance is smaller during summer than other seasons. Only about half of the events show an unambiguous increase in GW activity during non-summer periods and about one quarter of the events in summer show an enhancement in GWs. In addition, we also find that the high latitude mesopause is seen to descend in altitude following onset of geomagnetic activity in the non-summer high latitude region.

During winter, the latitude belt at 60S is one of the most intense hotspots of stratospheric gravity wave (GW) activity. However, producing accurate representations of GW dynamics in this region in numerical models has proved exceptionally challenging. One reason for this is that questions remain regarding the relative contributions of different orographic and non-orographic sources of GWs here. We use 3-D satellite GW observations from the Atmospheric InfraRed Sounder (AIRS) from winter 2012 in combination with the Gravity-wave Regional Or Global Ray Tracer (GROGRAT) to backwards ray trace GWs to their sources. We trace over 14.2 million rays, which allows us to investigate GW propagation and to produce systematic estimates of the relative contribution of orographic and non-orographic sources to the total observed stratospheric GW momentum flux in this region. We find that in winter 56% of momentum flux (MF) traces back to the ocean and 44% to land, despite land representing less than a quarter of the region’s area. This demonstrates that, while orographic sources contribute much more momentum flux per unit area, the large spatial extent of non-orographic sources leads to a higher overall contribution. The small islands of Kerguelen and South Georgia specifically contribute up to 1.6% and 0.7% of average monthly stratospheric MF, and the intermittency of these sources suggests that their short-timescale contribution is even higher. These results provide the important insights needed to significantly advance our knowledge of the atmospheric momentum budget in the Southern polar region.

Gravity waves (GWs) are key drivers of atmospheric dynamics, with major impacts on climate and weather processes. However, they are challenging to measure in observational data, and as a result no large-area multi-decadal GW time series yet exist. This has prevented us from quantifying the interactions between GWs and long-timescale climate processes. Here, we exploit temperatures measured by commercial aircraft since 1994 as part of the IAGOS atmospheric chemistry research programme to produce a novel 26-year time series of upper troposphere/lower stratosphere (UTLS) GW measurements across most of the northern hemisphere. We analyse 90\,342 flight-hours (76.2 million flight-kilometres) of data, typically at a temporal resolution of seconds and with high temperature precision. We show that GW activity in the northern-hemisphere UTLS is consistently strongest north of and above the upper tropospheric jet. We also show that GW sources not typically observed in stratospheric data but assumed in model schemes, such as the Rocky Mountains, are visible at these altitudes, suggesting that wave momentum from these sources is deposited specifically between $\sim$200–50\,hPa. Our data shows no significant impact of the Quasi Biennial Oscillation, the Northern Annular Mode, or climate change. However, we do see strong evidence of links with the El Ni\ no-Southern Oscillation, which modulates the measured GW signal by $\sim$25\%, and weak evidence of links with the 11-year solar cycle. These results have important implications for atmospheric process modelling and for understanding large-scale climate teleconnections.

Aeolus is the first Doppler wind lidar in space. It provides unique high-resolution measurements of horizontal wind in the sparsely-observed upper-troposphere/lower-stratosphere (UTLS), with global coverage. In this study, Aeolus’ ability to resolve atmospheric gravity waves (GWs) is demonstrated. The accurate representation of these small-scale waves is vital to properly simulate dynamics in global weather and climate models. In a case study over the Andes, Aeolus GW measurements show coherent phase structure from the surface to the lower stratosphere, with wind perturbations >10 m/s, a vertical wavelength ~8 km and an along-track horizontal wavelength ~900 km. Good agreement is found between Aeolus and colocated satellite, ground-based lidar and reanalysis data sets for this example. Our results show that data from satellites of this type can provide unique information on GW sources and propagation in the UTLS, filling a key knowledge gap that underlies known major deficiencies in weather and climate modelling.