The study of infrasound is experiencing a renaissance since it was chosen as a verification technique for the Comprehensive Nuclear-Test-Ban Treaty. The success of the verification technique strongly depends on knowledge of upper atmospheric processes. The ability of infrasound to probe the upper atmosphere starts to be exploited, taking the field beyond its monitoring application. Processes in the stratosphere couple to the troposphere and influence our daily weather and climate. Infrasound delivers actual observations on the state of the stratosphere with a high spatial and temporal resolution. Here we show the infrasonic signature, passively obtained, of a drastic change in the stratosphere due to the major Sudden Stratospheric Warming (SSW) of January 2009. With this study, we infer the enormous capacity of infrasound in acoustic remote sensing of stratospheric processes on a global scale with surface based instruments.

The back azimuth of infrasound between 0.1 and 1.0 Hz as function of time, i.e., Jan 01 up to Feb 15 2009. Analyzed are infrasound array data from IS18 on Greenland (left) and IS53 in Alaska (right). A back azimuth of zero degrees means a source to the north of the array and 90 deg to east. Color coded are the number of detections per hour where five or more detections are colored red. The source in the 0.1-1.0 Hz frequency band are microbaroms resulting from the interacting of oceanic waves in the Atlantic and Pacific Ocean. The location of these sources with respect to the arrays are estimated by the solid (Atlantic) and dashed line (Pacific) from ECMWF oceanic wave analysis (significant wave height and frequency). Coincidence between the resolved back azimuths and the source location should be noted. For example in January at IS53 for the Pacific maximum. A sudden change in the detections of microbaroms appears between Jan 15 and Jan 30. Pacific microbaroms are hardly detected while the Atlantic ones become more pronounced. As the source activity remains unchanged, this drastic change is due to changes in the atmosphere and caused by a major Sudden Stratospheric Warming. The temperature change around 20 km altitude appeared to be 50 deg C (between 65 and 95N) and occurred only within a few days, at the same time the polar vortex was reversed from eastward to westward. This reversal partly explains the observations, the lack of a westward component in the polar vortex makes the detection of Pacific microbaroms unlikely. The wind direction at 50 km altitude near the arrays are given by the dark green lines and come from ECMWF analysis. Part of the observations is also explained by the wind direction (see for example the coincidence at IS53 between late Jan and early Feb). Here, the source location remains unchanged while the changes in observed back azimuths are guided by the wind direction at 50 km altitude.

Reference:
Evers, L.G. and P. Siegmund, The infrasonic signature of the 2009 major Sudden Stratospheric Warming Geophys. Res. Lett., 2009, 36, L23808, doi:10.1029/2009GL041323.