In this thesis, the dynamics and coupling in the middle atmosphere over the Southern
Hemisphere are investigated using SuperDARN high frequency (HF) radar wind data,
satellites, light detection and ranging (LIDAR), the South African National Antarctic
Expedition (SANAE) imaging riometer and models. In particular, the study focuses on
the unprecedented 2002 major stratospheric warming and its role in coupling the middle
atmosphere. The dynamics of the middle atmosphere is investigated in terms of mean
wind, temperature, gravity waves and planetary wave activity.
Studying the middle atmospheric thermal structure over Southern Africa is an important
activity to improve the understanding of atmospheric dynamics of this region. Observation
of a middle atmosphere thermal structure over Durban (29.9 S, 31.0 E, South Africa)
using LIDAR data collected from April 1999 to July 2004 (277 nights), including closest
overpasses of the Sounding of the Atmosphere using Broadband Emission Radiometry
(SABER) and Halogen Occultation Experiments (HALOE) satellites, and the COSPAR
International Reference Atmosphere (CIRA-86) are presented in this thesis. The observations
from the LIDAR instrument, satellites and CIRA-86 exhibit the presence of annual
oscillation in the stratosphere, whereas in the mesosphere the semi-annual oscillation seems
to dominate the annual oscillation at some levels. The stratopause is observed in the height
range of 40-55 km for all the instruments, with the stratopause temperatures being 260-
270 K for the LIDAR, 250-260 K for the SABER, and 250-270 K for the HALOE. Data
from the LIDAR, satellites and CIRA-86 model indicate almost the same thermal structure
of the middle atmosphere over Durban. This indicates a good agreement between
LIDAR, satellites and the CIRA-86 model.
Mean wind and planetary waves are investigated on a climatological scale in this study.
Mean wind observations from the SANAE SuperDARN HF radar are compared with
observations from Halley SuperDARN HF radar. There is a good agreement between the
observations from the two stations both in the zonal and meridional wind components.
Zonal wind is observed to be consistently larger than the meridional wind. The zonal
wind is also consistently more eastward at both stations with maxima occurring during
the solstice months. High latitude summer zonal mean
ow at 94 km is observed to
be weaker and more variable compared to the eastward winter mean circulation owing to
tropospherically forced planetary waves propagating through the middle atmosphere. The
zonal mean wind shows greater seasonal variability than does the meridional mean wind.
This seasonal behaviour is reasonably well understood in terms of the upward propagating
planetary waves and gravity waves interacting with the mean
ow. The Coriolis force also
plays an important role in the case of meridional wind component.
The climatology of planetary waves both in the zonal and meridional wind components indicates
an ampli cation of planetary waves of shorter wavenumbers (s = 3) in the winter
months. During summer, long period oscillations (e.g. >10 days) which are dominant in
winter disappear, and oscillations with shorter period (e.g. <10 days) become dominant.
vi
There is a strong planetary wave coupling between the stratosphere and mesosphere-lower
thermospheric (MLT) during the year 2002 winter season, whilst the coupling is observed
to be relatively weak during the other years. The strong planetary wave coupling in 2002
is understandable because during this year the middle atmosphere winter months were
characterised by strong planetary wave activity which led to the rst ever detection of the
SSW in the Southern Hemisphere.
In the year 2002 winter period the mean circulation in the stratosphere is characterized
by a series of planetary wave events that weakened the polar vortex and triggered the
sudden stratospheric warming in late September. In particular, in the stratosphere there
is a presence of a quasi 10-day eastward propagating planetary wave of wavenumber s=1,
while in the MLT a quasi 14-day eastward propagating planetary wave of wavenumber
s=1 is observed to be dominant. The Eliassen Palm
ux (E-P)
ux shows that strong
planetary wave activity observed in the middle atmosphere originates from the troposphere.
Zonal winds at the MLT show reversal approximately 7 days before the reversal at
stratosphere, indicating a downwards propagation of circulation disturbance in the middle
atmosphere. Eastward zonal winds dominate the winter MLT, but during the 2002 winter
there are many periods of westward winds observed compared to the other years. The
SABER vertical temperature pro les indicate cooling of the MLT region during the SSW
occurrence. Gravity wave horizontal phase velocities and horizontal wavelengths as seen
by the SANAE imaging riometer are observed to reduce dramatically over SANAE during
the occurrence of the stratospheric warming. The disturbance of the middle atmosphere
during the Southern Hemisphere stratospheric warming in year 2002 winter preconditioned
the region for gravity waves to propagate upward to the MLT. The potential energy of
these gravity waves is observed to increase with height up until they reach the lower thermosphere.
At the MLT they lose their energy, thus depositing their momentum, leading
to the MLT cooling and mean wind reversal.
Keywords: SSW, Planetary waves, Gravity waves, Stratosphere, MLT, SuperDARN radar,
Mean wind, Temperature, Middle atmosphere, SANAE. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2012.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/9731 |
| Date | January 2012 |
| Creators | Mbatha, Nkanyiso Bongumusa. |
| Contributors | Venkataraman, Sivakumar., Malinga, Sandile., Pillay, Sadhasivan. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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