Planetary wave activity in the Arctic and Antarctic lower stratospheres during 2007 and 2008

Abstract

Temperature data from the COSMIC GPS-RO satellite constellation are used to study planetary wave activity in both polar stratospheres from September 2006 until November 2008. One major and several minor sudden stratospheric warmings (SSWs) were recorded during the boreal winters of 2006/2007 and 2007/2008. Planetary wave morphology is studied using space-time spectral analysis while individual waves are extracted using a linear least squares fitting technique. Results show the planetary wave frequency and zonal wavenumber distribution varying between hemisphere and altitude. Most of the large Northern Hemisphere wave activity is associated with the winter SSWs, while the largest amplitude waves in the Southern Hemisphere occur during spring. Planetary wave activity during the 2006/2007 and 2007/2008 Arctic SSWs is due largely to travelling waves with zonal wavenumbers |s|=1 and |s|=2 having periods of 12, 16 and 23 days and stationary waves with |s|=1 and |s|=2. The latitudinal variation of wave amplification during the two Northern Hemisphere winters is studied. Most planetary waves show different structure and behaviour during each winter. Abrupt changes in the latitude of maximum amplitude of some planetary waves is observed co-incident in time with some of the SSWs.

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ACPD
9, 14601–14643, 2009
Planetary waves in
the polar
stratospheres
S. P. Alexander and
M. G. Shepherd
Title Page
AbstractIntroduction
Conclusions References
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Atmos. Chem. Phys. Discuss., 9, 14601–14643, 2009
www.atmos-chem-phys-discuss.net/9/14601/2009/
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
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and Physics
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This discussion paper is/has been under review for the journal Atmospheric Chemistry
and Physics (ACP). Please refer to the corresponding final paper in ACP if available.
Planetary wave activity in the Arctic and
Antarctic lower stratospheres during 2007
and 2008
S. P. Alexander1and M. G. Shepherd2
1Australian Antarctic Division, Kingston, Tasmania, Australia
2Centre for Research in Earth and Space Science, York University, Toronto, Canada
Received: 28 May 2009 – Accepted: 25 June 2009 – Published: 7 July 2009
Correspondence to: S. P. Alexander (simon.alexander@aad.gov.au)
Published by Copernicus Publications on behalf of the European Geosciences Union.
14601

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ACPD
9, 14601–14643, 2009
Planetary waves in
the polar
stratospheres
S. P. Alexander and
M. G. Shepherd
Title Page
AbstractIntroduction
Conclusions References
TablesFigures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Abstract
Temperature data from the COSMIC GPS-RO satellite constellation are used to study
planetary wave activity in both polar stratospheres from September 2006 until Novem-
ber 2008. One major and several minor sudden stratospheric warmings (SSWs) were
recorded during the boreal winters of 2006/2007 and 2007/2008. Planetary wave mor-
phology is studied using space-time spectral analysis while individual waves are ex-
tracted using a linear least squares fitting technique. Results show the planetary wave
frequency and zonal wavenumber distribution varying between hemisphere and alti-
tude. Most of the large Northern Hemisphere wave activity is associated with the win-
ter SSWs, while the largest amplitude waves in the Southern Hemisphere occur during
spring. Planetary wave activity during the 2006/2007 and 2007/2008 Arctic SSWs is
due largely to travelling waves with zonal wavenumbers |s|=1 and |s|=2 having periods
of 12, 16 and 23 days and stationary waves with s=1 and s=2. The latitudinal variation
of wave amplification during the two Northern Hemisphere winters is studied. Most
planetary waves show different structure and behaviour during each winter. Abrupt
changes in the latitude of maximum amplitude of some planetary waves is observed
co-incident in time with some of the SSWs.
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1Introduction
Large amplitude planetary waves dominate the winter middle atmosphere and their in-
teraction with the zonal mean flow is a major driver of winter stratosphere dynamics
(Andrews et al., 1987). Planetary wave amplitudes are larger in the Northern Hemi-
sphere than in the Southern Hemisphere due to larger themal and orographic forcing
(Andrews et al., 1987). Stratospheric waves generally propagate eastward relative to
the ground in the Southern Hemisphere (Hartmann et al., 1984; Shiotani et al., 1990),
while quasi-stationary waves dominate the Northern Hemisphere (Chshyolkova et al.,
2005). However, planetary waves generally propagate westward relative to the zonal
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ACPD
9, 14601–14643, 2009
Planetary waves in
the polar
stratospheres
S. P. Alexander and
M. G. Shepherd
Title Page
Abstract Introduction
ConclusionsReferences
TablesFigures
??
??
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Interactive Discussion
mean flow (Holton, 2004).
Planetary waves propagate upward from tropospheric sources (Hartmann et al.,
1984; Randel, 1987; Kr¨ uger et al., 2005). Several studies have shown the connection
between tropospheric and stratospheric planetary wave activity. Leovy and Webster
(1976) and Mechoso and Hartmann (1982) discussed the strong vertical coherence
of travelling planetary waves. Randel (1987) used geopotential height below 1 hPa to
find a decrease in propagation time from the troposphere to stratosphere for higher
zonal wavenumber waves. Westward propagating waves with zonal wavenumber s=1
and s=2 were identified in the stratosphere using satellite data and shown to agree
with Rossby modes for an isothermal atmosphere (Hirota and Hirooka, 1984). Recent
satellite datasets have enabled wave characteristics and propagation to be followed up
to the mesosphere and lower thermosphere (Hirooka, 2000; Palo et al., 2005).
Sudden stratospheric warmings (SSWs) are much more prevalent in the Arctic than
in the Antarctic due to the larger Northern Hemisphere planetary wave forcing (Manney
et al., 2005; Pancheva et al., 2008a). Indeed, only one major SSW has been recorded
in the Antarctic, during spring 2002 (Newman and Nash, 2005). An increase in middle
atmospheric planetary wave activity is noticed prior to the onset of an SSW which pre-
conditions the atmosphere (e.g. Palo et al., 2005; Chshyolkova et al., 2006; Hoffmann
et al., 2007), leading to an upward and poleward motion of heat flux (Andrews et al.,
1987). Planetary waves interact dramatically with the background mean flow during
the SSW, leading to a reversal of the meridional temperature gradient at 10 hPa for
minor warmings and additionally for major warmings, a reversal of the eastward flow
(Labitzke and Naujokat, 2000). Matsuno (1971) showed that SSWs can be explained
by the interaction of upward propagating transient planetary waves on the background
zonal mean flow. A downward circulation causing adiabatic heating in the stratosphere
results from the deceleration of the eastward flow by planetary waves (Liu and Roble,
2002).
The stratosphere-mesosphere system is coupled via planetary wave propagation
during SSWs (Liu and Roble, 2005; Hoffmann et al., 2007; Pancheva et al., 2009a),
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ACPD
9, 14601–14643, 2009
Planetary waves in
the polar
stratospheres
S. P. Alexander and
M. G. Shepherd
Title Page
AbstractIntroduction
ConclusionsReferences
TablesFigures
??
??
BackClose
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Interactive Discussion
which act to change the stratopause structure (Manney et al., 2008) and the nature
of waves in the mesosphere (Chshyolkova et al., 2006; Shepherd et al., 2007; Mur-
phy et al., 2007; Pancheva et al., 2008b). During the austral winter of 2002, multiple
planetary waves reached the mesosphere, weakening and eventually reversing the po-
lar night jet, thus changing wave transmission and breaking conditions (Kr¨ uger et al.,
2005; Liu and Roble, 2005). SSWs alter the background stratospheric circulation and
thereby influence directly the gravity wave fluxes reaching the mesosphere. The west-
ward winds observed during major SSWs allow eastward propagating gravity waves
to reach the MLT region and deposit their momentum there (Dunkerton and Butchart,
1984). Gravity wave activity in the stratosphere is generally enhanced during SSWs
(Duck et al., 1998; Ratnam et al., 2004; Alexander et al., 2009), although in the meso-
sphere, gravity wave activity depends upon the warming itself and the location of the
observations (Dowdy et al., 2007; Hoffmann et al., 2007).
The recent advent of GPS Radio Occultation (RO) satellite missions has resulted in
the collection of highly accurate (sub-Kelvin accuracy) and increasingly dense temper-
ature profiles from near the surface to 40 km altitude (Kursinski et al., 1997; Tsuda
et al., 2000). The launch of the Constellation Observing System for Meteorology, Iono-
sphere and Climate Global Positioning System Radio Occultation (COSMIC GPS-RO)
satellites in April 2006 has resulted in towards 2000 profiles per day distributed about
the globe (Anthes et al., 2008). These profiles are distributed fully in longitude and local
time, making them ideal for global scale wave studies (Alexander et al., 2008b). COS-
MIC data were used to show large planetary wave activity in the 2006 Antarctic early
summer, more consistent with winter-time activity (Shepherd and Tsuda, 2008). COS-
MIC data are also dense enough to quantify changes in gravity wave activity over short
time scales (on the order of several days) and in particular have been used to study
gravity wave activity associated with recent Arctic SSWs (Alexander et al., 2009).
In this paper, we study planetary wave activity using COSMIC temperature data
between 15 km and 35 km altitude from September 2006 to November 2008. We use
two methods to study planetary waves in the Arctic (60◦N–70◦N) and Antarctic (60◦S–
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ACPD
9, 14601–14643, 2009
Planetary waves in
the polar
stratospheres
S. P. Alexander and
M. G. Shepherd
Title Page
AbstractIntroduction
ConclusionsReferences
TablesFigures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
70◦S). Firstly, space-time spectral analysis provides a morphology of planetary waves
during this period and allows quantification and analysis of travelling and stationary
wave components. Wave periods, wave numbers and phase speeds are examined.
Then we extract individual waves from the temperature perturbations using a least
squares fitting method. From this we look at the height structure and periods of these
waves through time. Lastly, we identify individual waves and study their latitudinal
distribution during the major and minor sudden stratospheric warmings of the Arctic
winters of 2006/2007 and 2007/2008.
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2Data analysis
2.1COSMIC data
10
The COSMIC version 2.0 dry temperature data product is used, which is derived from
the measured refractivity profile by neglecting humidity. Sufficient data for this analysis
are available from September 2006 onwards. The original GPS-RO data are available
at 0.1 km vertical resolution but they have an effective vertical resolution on the or-
der of 1 km in the lower stratosphere (Kursinski et al., 1997). Therefore the data are
interpolated to the approximate real resolution of 1 km. The precision of the COS-
MIC refractivity is 0.7% at 30km (Schreiner et al., 2007). The accuracy of the derived
temperature is better than 0.5 K (Kursinski et al., 1997). Only a small bias of 1–2%
between radiosondes and COSMIC at 25 km altitude was observed by Hayashi et al.
(2009), thus gravity wave and planetary wave activity observed with COSMIC agrees
well with model results (Alexander et al., 2008a,b; Kawatani et al., 2009). COSMIC data
are available from near the surface to 40 km, although we consider altitudes 15 km–
35 km here to neglect humidity in the lower regions and to maintain accuracy at the
upper altitudes.
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