Sammendrag
Weather Regimes in the Euro-Atlantic sector - Do they exist?
Hovedfagsoppgave i Meteorologi, 2002
Kandidat: Ivar Ambjørn Seierstad, Veileder: Nils Gunnar Kvamstø
Geofysisk Institutt, UiB

Weather Regimes in the Euro-Atlantic sector - Do they exist?

In the recent debate on climate change and global warming a central problem is to separate anthropogenic and natural climate variability. Corti et.al (1999) have in this context proposed a theoretically appealing result. They claim that the large scale atmospheric circulation is characterised by preferred states or regimes, and that any external forcing (i.e anthropogenic) will be manifested in terms of changes to the frequency of occurrence of such natural atmospheric circulation regimes. However, some recent studies have doubted the existence of hemispheric regimes (Stephenson 2002; Hsu and Zwiers 2001). In this thesis it is therefore investigated whether regimes can be identified in the Euro-Atlantic sector.
It is argued that testing the simple hypothesis of multivariate normality can be used for this purpose. It is also shown that there exist alternative paradigms, beside the regime view, for understanding the large scale circulation.

The 500 hPa geopotential height for the extended winter (NDJFMA) for the years 1948-2001 is chosen for analysis. The multivariate normal hypothesis is tested in the subspace spanned by the seven leading principal components. It is found that this hypothesis can be rejected for the principal components calculated from both daily and monthly mean values. The departure from normality is due to significant skewness. By using a mixture model clustering technique it is shown that this skewness can be interpreted to represent two marginal regimes: the Euro-Atlantic "blocked" and "zonal" regime. However, the value of such marginal regimes is questionable.