| The Northern hemisphere winter circulation is probed for deviations from
Gaussianity. A projection pursuit approach is applied that search for
directions in phase space that maximize an index of interest. Different
indices gauging different aspects of non-Gaussianity such as flatness,
bimodality, and multimodality are considered. The projection pursuit
approach allows high-dimensional data spaces to be investigated and it
therefore complements previous studies that usually have been confined
to spaces of a few dimensions. Both the stratospheric and tropospheric
circulations are studied at daily, monthly, and annual timescales.
The statistical significance of the results is considered by a Monte
Carlo method where results from the atmospheric data are compared with
results from Gaussian distributed surrogate data. The surrogate data
are generated so that they have the same temporal structure as the
original data. Careful considerations of the statistical significance
are of particular importance in studies with exploratory methods such
as projection pursuit.
In the stratosphere the moderate evidence for bimodality in the
inter-annual variability of extended winter means is confirmed. On monthly
and daily timescales strong evidence for non-Gaussianity but no evidence
for bi- or multimodality is found. In the troposphere only evidence for
non-Gaussianity is found and mainly on the shortest timescales. |