HF Radar Observations of Inter-annual variations in Mid-Latitude
Mesospheric Winds
Garima Malhotra
The equatorial Quasi Biennial Oscillation (QBO) is known to be an important source of
inter-annual variability at mid and high latitudes in both hemispheres. Coupling between
QBO and the polar vortex has been extensively studied over the past few decades, however,
less is known about QBO influences in the mid-latitude mesosphere. One reason for
this is the relative lack of instrumentation available to study mesospheric dynamics at midlatitudes.
In this study, we have used the mid-latitude SuperDARN HF radar at Saskatoon
(52.16◦N, -106.53◦E ) to study inter-annual variation in mesospheric winds. The specific
aim was to determine whether or not a Quasi Biennial signature could be identified in the
Saskatoon mesosphere, and if so, to understand its relationship with the equatorial stratospheric
QBO. To achieve this goal, a technique has been developed which extracts meteor
echoes from SuperDARN near-range gates and then applies least-squares fitting across all
radar beam directions to calculate hourly averages of the zonal and meridional components
of the mesospheric neutral wind. Subsequent analysis of 13 years (2002-2014) of zonal wind
data produced using this technique indicates that there is indeed a significant QBO signature
present in Saskatoon mesospheric winds during late winter (Jan-Feb). This mesospheric
QBO signature is in opposite phase with the equatorial stratospheric QBO, such that when
QBO (at 50 hPa) is in its easterly (westerly) phase, the late winter winds in Saskatoon mesosphere
become more (less) westerly. To further examine the source of the signature, we also
analyzed winds in the Saskatoon stratosphere between 5 hPa and 70 hPa using the ECMWF
ERA-Interim reanalysis data set, and found that the late winter winds stratospheric winds
become less (more) westerly when QBO is easterly (westerly). This QBO signature in the
mid-latitude stratospheric winds is essentially the same as that observed for the polar vortex
in previous studies but it is opposite in phase to the mid-latitude mesospheric QBO.
We therefore conclude that filtering of gravity waves through QBO-modulated stratospheric
winds plays a major role in generating the mesospheric QBO signature we have identified
in the Saskatoon HF radar data. When the Saskatoon stratospheric winds are anomalously
westward during easterly QBO, the gravity waves having westward momentum might be
filtered out, depositing a net eastward momentum in the mesosphere as they propagate
upwards. This would result in increased westerly mesospheric winds at Saskatoon. The
opposite would happen when the equatorial QBO is westerly