Observation of Mountain Waves and Secondary Gravity Waves in the Mesosphere Lower Thermosphere Above Patagonia

Abstract

We find observational evidence for mountain waves (MWs) and secondary gravity waves (2GWs) in the OH*(3‐1) layer above Tierra del Fuego, Argentina. On the night of 21–22 May 2018, the Advanced Mesospheric Temperature Mapper (AMTM) obtained temperatures at ∼84 km. During the westerly phase of the semi‐diurnal tide, arc‐shaped quasi‐stationary structures in temperature maps indicated MWs with horizontal wavelengths of 20–40km. A co‐located temperature measurement from the COmpact Rayleigh Autonomous Lidar (CORAL) confirmed quasi‐stationary structures down to 15 km, also indicating MWs. Temperature profiles revealed a convectively unstable region within a MW phase front at 58–66 km—a sign of MW breaking, momentum deposition, local body forces, and generation of 2GWs. After a wind reversal in the mesosphere/lower thermosphere (MLT) observed by the Southern Argentina Agile Meteor Radar (SAAMER), a gravity wave propagated southeastward with an intrinsic phase speed >90m/s. Using 1‐D cross‐wavelet analysis, we derived spectral properties of this fast wave and performed ray‐tracing, using the Japanese Whole Atmosphere Reanalysis as background. The fast wave likely originated near 63 km above the Torres del Paine region, known for large‐amplitude MWs. We conclude that tropospheric forcing excites MWs over Tierra del Fuego (observed) and over the Torres del Paine massif (not observed) ∼500km northwest. The latter MWs likely generate 2GWs upon breaking in the lower mesosphere, observed over Tierra del Fuego after a turning‐level disappearance. A novel momentum‐flux (MF) retrieval, based on co‐located AMTM, CORAL, and SAAMER measurements, automatically identifies dominant wave features, incorporating measurement and OH*(3‐1) layer variability to infer realistic MF uncertainties.