Top: 4-day running mean amplitude of the 12-hour variability in the meridional wind observed with the SKiYMET meteor radar at Trondheim, Norway. Bottom: The three components of the semidiurnal tide resolved from the longitudinal chain of SuperDARN radars. Data span the 2013 Stratospheric Sudden Warming over a five-week interval starting one week before the onset date of 5 January 2013, indicated by a vertical black line. Credit: Hibbins et al. [2019], Figure 6
Source: Journal of Geophysical Research: Atmospheres

A variety of weather processes in the troposphere, solar radiation absorption in stratospheric ozone and nonlinear wave-mean flow interactions excite a longitude-dependent spectrum of atmospheric tides – global scale wave motions – with periods defined by Earth’s rotation rate. Growing exponentially in magnitude when propagating upward, tides are a key ingredient in coupling the lower with the upper atmosphere and cause regular semidiurnal wind oscillations on the order of +-50 meters per second in the upper mesosphere.

During Stratospheric Sudden Warmings, a deformation or split of the polar vortex, the semidiurnal tide in the upper mesosphere changes by as much as its average amplitude. Understanding the reasons for this dramatic impact requires to resolve the global wavenumber spectrum of the semidiurnal tidal oscillations on a day-to-day basis. This has always been a challenge because satellites do not have the necessary time resolution and single ground-based observations lack the spatial information.

Hibbins et al. [2019] present a new approach using meteor wind data from the Super Dual Auroral Radar Network (SuperDARN) in the northern hemisphere to resolve the semidiurnal wave spectrum at 95-kilometer altitude around 60oN latitude with high time resolution. By separating migrating (Sun-synchronous) and nonmigrating (non Sun-synchronous) tides, the authors are able to conduct a statistical analysis of short-term tidal variability in response to Stratospheric Sudden Warming using 20 years of available data.

They demonstrate that the migrating semidiurnal tide is absent during the period immediately following the Stratospheric Sudden Warming onset but anomalously large during the recovery phase due to propagating conditions and stratospheric ozone driving. Nonmigrating tides play a lesser role. This study may also help to better understand how stratospheric sudden warmings impact the ionosphere through dynamo processes because of the global nature of the tides and to guide further model developments.

Citation: Hibbins, R. E., Espy, P. J., Orsolini, Y. J., Limpasuvan, V., & Barnes, R. J. [2019]. SuperDARN observations of semidiurnal tidal variability in the MLT and the response to sudden stratospheric warming events. Journal of Geophysical Research: Atmospheres, 124.

—Jens Oberheide, Associate Editor, JGR: Atmospheres

Text © 2019. The authors. CC BY-NC-ND 3.0
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