Evidence of mesospheric gravity‐waves generated by orographic forcing in the troposphere

We report on the first imaging observations of stationary mesospheric gravity‐waves in the three nightglow emissions of OH (695–1050 nm), Na (589.3 nm) and O( 1 S) (557.7 nm). The waves were observed in all‐sky images at the El Leoncito Observatory (ELO), Argentina (31.8°S, 69.3°W), during the period of 1–8 July 2008. Over the course of each night, the band‐like wave features exhibited a zero or near‐zero observed horizontal phase speed and had lifetimes of several hours. These factors, coupled with the winds measured locally, and the orientation of the waves to the Andes mountain range located nearby, suggested that the waves were mountain waves generated by ∼70 ms −1 eastward winds flowing over the Andes.