Geophysical Research Letters
Rossby Wave Propagation into the Northern Hemisphere
Stratosphere: The Role of Zonal Phase Speed
Daniela I. V. Domeisen
, Olivia Martius
, and Bernat Jiménez-Esteve
Institute for Atmospheric and Climate Science, ETH Zürich, Zürich, Switzerland,
Institute of Geography, Oeschger Centre
for Climate Change Research, University of Bern, Bern, Switzerland
Sudden stratospheric warming (SSW) events are to a dominant part induced by upward
propagating planetary waves. While theory predicts that the zonal phase speed of a tropospheric wave
forcing aﬀects wave propagation into the stratosphere, its relevance for SSW events has so far not
been considered. This study shows in a linear wave diagnostic and in reanalysis data that phase speeds tend
eastward as waves propagate upward, indicating that the stratosphere preselects eastward phase speeds
for propagation, especially for zonal wave number 2. This also aﬀects SSW events: Split SSW events tend
to be preceded by anomalously eastward zonal phase speeds. Zonal phase speed may indeed explain part
of the increased wave ﬂux observed during the preconditioning of SSW events, as, for example, for the
record 2009 SSW event.
Upward propagating planetary-scale Rossby waves are the dominant cause of day-to-day variability in the
extratropical winter stratosphere. Anomalous wave forcing can lead to strong disruptions of the stratospheric
ﬂow, so-called sudden stratospheric warming (SSW) events, which in turn aﬀect tropospheric variability
(e.g., Baldwin & Dunkerton, 2001) and predictability (e.g., Domeisen et al., 2015; Karpechko et al., 2017).
The magnitude and duration of the wave forcing can be diagnosed from the vertical component of the
Eliassen-Palm (EP) ﬂux (e.g., Vallis, 2006), which is proportional to the meridional heat ﬂux and often consid-
erably enhanced for a sustained period before SSW events (Polvani & Waugh, 2004; Sjoberg & Birner, 2012).
The anomalous wave forcing has been linked to tropospheric anomalies such as blocking (Martius et al., 2009;
Nishii et al., 2011; Quiroz, 1986; Woollings et al., 2010). However, both tropospheric forcing (Davies, 1981) and
internal stratospheric variability (de la Cámara et al., 2017) have to be considered as factors playing a role in
preconditioning SSW events.
While both the magnitude and duration of a wave forcing can inﬂuence the preconditioning of SSW events,
it is known from theoretical considerations (Charney & Drazin, 1961) that the vertical propagation of waves
into the stratosphere is also aﬀected by their zonal phase speed. Waves with nonzero zonal phase speed,
that is, traveling waves, are ubiquitous in the stratosphere, especially in the Southern Hemisphere (Labitzke,
1981). In the absence of longitudinally asymmetric surface forcing such as topography, Domeisen and Plumb
(2012) show that the generation of traveling planetary-scale waves is dominated by nonlinear interaction
among synoptic-scale baroclinic eddies in the troposphere, as suggested by Scinocca and Haynes (1998). In
the Northern Hemisphere, the majority of the wave spectrum consists of stationary waves (e.g., Watt-Meyer
& Kushner, 2015a). Traveling waves are, however, also observed, for example before the 1979 SSW event
(Madden & Labitzke, 1981). Traveling waves have in addition been linked to resonant behavior before SSW
events (e.g., Geisler, 1974; Plumb, 1981).
The aim of this study is to elucidate the role of traveling Rossby waves for the Northern Hemisphere winter
stratosphere. The paper is structured as follows: Section 2 introduces the role of phase speed in upward wave
propagation, section 3.1 expands the analysis using a two-dimensional wave propagation diagnostic, and
section 3.2 compares these results to reanalysis. Section 3.3 examines the role of zonal phase speed ahead of
SSW events, and section 3.4 discusses the results in the framework of the 2009 SSW event. Section 4 provides
a summary and discussion of the results. The supporting information provides further details on the methods
used in this study.
• Waves with eastward phase speeds
are preferred for upward propagation
into the stratosphere
• Split SSW events are preceded by an
eastward tendency in zonal phase
speed of wave 1 and wave 2
• Extreme eastward phase speeds likely
favored the record upward EP ﬂux
before the 2009 SSW
• Supporting Information S1
D. I. V. Domeisen,
Domeisen, D. I. V., Martius, O.,
& Jiménez-Esteve, B. (2018).
Rossby wave propagation
into the Northern Hemisphere
stratosphere: The role of zonal
phase speed. Geophysical
Research Letters, 45, 2064–2071.
Received 4 AUG 2017
Accepted 1 FEB 2018
Accepted article online 8 FEB 2018
Published online 22 FEB 2018
©2018. American Geophysical Union.
All Rights Reserved.
DOMEISEN ET AL.