Numerical Modeling of the Generation of Tertiary Gravity Waves in the Mesosphere and Thermosphere During Strong Mountain Wave Events Over the Southern Andes

Abstract

AbstractWe investigate the effects on the mesosphere and thermosphere from a strong mountain wave (MW) event over the wintertime Southern Andes using a gravity wave (GW)‐resolving global circulation model. During this event, MWs break and attenuate at z∼50–80 km, thereby creating local body forces that generate large‐scale secondary GWs having concentric ring structure with horizontal wavelengths λH=500–2,000 km, horizontal phase speeds cH=70–100 m/s, and periods τr∼3–10 hr. These secondary GWs dissipate in the upper mesosphere and thermosphere, thereby creating local body forces. These forces have horizontal sizes of 180–800 km, depending on the constructive/destructive interference between wave packets and the overall sizes of the wave packets. The largest body force is at z=80–130 km, has an amplitude of ∼2,400 m/s/day, and is located ∼1,000 km east of the Southern Andes. This force excites medium‐ and large‐scale “tertiary GWs” having concentric ring structure, with λH increasing with radius from the centers of the rings. Near the Southern Andes, these tertiary GWs have cH=120–160 m/s, λH=350–2,000 km, and τr∼4–9 hr. Some of the larger‐λH tertiary GWs propagate to the west coast of Africa with very large phase speeds of cH≃420 m/s. The GW potential energy density increases exponentially at z∼95–115 km, decreases at z∼115–125 km where most of the secondary GWs dissipate, and increases again at z>125 km from the tertiary GWs. Thus, strong MW events result in the generation of medium‐ to large‐scale fast tertiary GWs in the mesosphere and thermosphere via this multistep vertical coupling mechanism.