A. Butler, S. Hardiman (presenting), N. Butchart, and D. Seidel
Representation of Stratospheric Sudden Warmings in Reanalyses and
Comparisons with Stratospheric Sounding Unit Temperature Observations
Sudden warmings are among the most dramatic features of the wintertime
Northern Hemispheric polar stratosphere and are thought to be among
the most important linkages between stratospheric and tropospheric
variability on intraseasonal to seasonal time scales.
Reanalyses have been a main data source for recent studies of
stratospheric sudden warmings (SSWs) because they offer spatially
and temporally complete and internally consistent dynamical
and thermodynamical fields. But the validity of conclusions about
SSW occurrences and the likelihood of associated tropospheric
climate variations depends on two assumptions, neither of which
has yet been carefully tested. The first assumption is that
findings are not sensitive to the particular reanalysis used,
i.e., that different reanalyses have consistent representations
of the three-dimensional structure and the temporal evolution
of these events, and of subsequent tropospheric signals.
The second assumption is that the traditional World Meteorological
Organization definition of SSWs allows robust identification
of SSWs in reanalyses. Using this threshold definition (zonal mean
zonal wind reversal at 60N and 10hPa), the list of "observed" SSWs
differs slightly between different reanalysis datasets.
We will explore the validity of these two assumptions, as a
contribution to the emerging SPARC Reanalysis/analysis
Intercomparison Project. We will present a preliminary
intercomparison of SSW occurrences during the period 1979-2009
in 4 different reanalyses. To evaluate the depiction of
temperature variations throughout the polar stratosphere,
we will also examine the spatial and temporal structure of SSWs
from the Stratospheric Sounding Units flown on NOAA polar orbiting
satellites during 1979-2005, and compare reanalyses to this unique
observational dataset.
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