Can gravity waves significantly impact PSC occurrence in the Antarctic? Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
Received: 11 Dec 2008 – Published in Atmos. Chem. Phys. Discuss.: 03 Feb 2009 – Published: 23 Nov 2009Abstract. A combination of POAM III aerosol extinction and CHAMP RO temperature
measurements are used to examine the role of atmospheric gravity waves
in the formation of Antarctic Polar Stratospheric Clouds (PSCs). POAM III
aerosol extinction observations and quality flag information are used to
identify Polar Stratospheric Clouds using an unsupervised clustering algorithm.
A PSC proxy, derived by thresholding Met Office temperature analyses with the
PSC Type Ia formation temperature (TNAT), shows general agreement with
the results of the POAM III analysis. However, in June the POAM III observations
of PSC are more abundant than expected from temperature threshold crossings in
five out of the eight years examined. In addition, September and October PSC
identified using temperature thresholding is often significantly higher than
that derived from POAM III; this observation probably being due to dehydration
and denitrification. Comparison of the Met Office temperature analyses with
corresponding CHAMP observations also suggests a small warm bias in the Met
Office data in June. However, this bias cannot fully explain the differences observed.
Analysis of CHAMP data indicates that temperature perturbations associated
with gravity waves may partially explain the enhanced PSC incidence observed
in June (relative to the Met Office analyses). For this month, approximately
40% of the temperature threshold crossings observed using CHAMP RO data are
associated with small-scale perturbations. Examination of the distribution of
temperatures relative to TNAT shows a large proportion of June data
to be close to this threshold, potentially enhancing the importance of gravity
wave induced temperature perturbations.
Inspection of the longitudinal structure of PSC occurrence in June 2005 also
shows that regions of enhancement are geographically associated with the
Antarctic Peninsula; a known mountain wave "hotspot". The latitudinal variation
of POAM III observations means that we only observe this region in June–July,
and thus the true pattern of enhanced PSC production may continue operating into later months.
The analysis has shown that early in the Antarctic winter stratospheric
background temperatures are close to the TNAT threshold (and PSC
formation), and are thus sensitive to temperature perturbations associated with
mountain wave activity near the Antarctic peninsula (40% of PSC formation).
Later in the season, and at latitudes away from the peninsula, temperature
perturbations associated with gravity waves contribute to about 15% of the
observed PSC (a value which corresponds well to several previous studies).
This lower value is likely to be due to colder background temperatures already
achieving the TNAT threshold unaided. Additionally, there is a reduction
in the magnitude of gravity waves perturbations observed as POAM III samples
poleward of the peninsula.
Citation: McDonald, A. J., George, S. E., and Woollands, R. M.: Can gravity waves significantly impact PSC occurrence in the Antarctic?, Atmos. Chem. Phys., 9, 8825-8840, doi:10.5194/acp-9-8825-2009, 2009.