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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