Abstract
Gravity waves (GWs) generated by orographic forcing, also known as
mountain waves (MWs) have been studied for decades. First measured in
the troposphere, then in the stratosphere, they were only imaged at
mesospheric altitude in 2008. Their characteristics have been
investigated during several recent observation campaigns, but many
questions remain concerning their impacts on the upper atmosphere, and
the effects of the background environment on their deep propagation.
An Advanced Mesospheric Temperature Mapper (AMTM) and the Southern
Argentina Agile MEteor Radar (SAAMER) have been operated simultaneously
during the Austral winter 2018 from Rio Grande, Argentina (53.8°S). This
site is located near the tip of South America, in the lee of the Andes
Mountains, a region considered the largest MW hotspot on Earth.
New AMTM image data obtained during a 6-month period show almost 100
occurrences of MW signatures penetrating into the upper mesosphere. They
are visible ~30% of time at the height of the winter
season (mid-May to mid-July). Their intermittency is highly correlated
with the zonal wind controlled by the semi-diurnal tide, revealing the
direct effect of the atmospheric background on MW penetration into the
Mesosphere Lower Thermosphere (MLT, altitude 80-100 km). Measurements of
their momentum fluxes (MF) were determined to reach very large values
(average ~250 m/s), providing strong evidence of the
importance and impacts of small-scale gravity waves at mesospheric
altitudes.