VERTICAL STRUCTURE OF WEATHER REGIMES AND THEIR ROLE IN CLIMATE CHANGE

When examined in terms of diagnostics of the atmospheric circulation, recent climate change may be interpreted in terms of increases or decreases in the frequency of occurrence of pre-existing preferred modes of atmospheric behaviour rather than a simple linear shift in the mean climate with superimposed noise. This contrasts with the characteristics of the observed changes in temperature-related diagnostics, which appear much closer to the simple linear picture. While not necessarily inconsistent, the implications of these two views of the nature of climate change need to be explored. In this framework the main purpose of this paper is to use a 50-yr NCEP reanalysis dataset in order to explore the vertical structure of circulation regimes and (at the same time) illustrate the potential role of weather regimes and non-linearity in the emerging anthropogenic signal. The variables considered in this study are Northern Hemisphere extended-winter (November to April) monthly means of: i) geopotential height; ii) temperature; iii) mean sea level pressure. An empirical orthogonal function (EOF) analysis is first applied in order to define, for each variable, a reduced phase space based on the leading modes of variability. Then a simultaneous analysis of the four fields is carried out performing a further (vectorial) EOF analysis in the subspace spanned by the 10 leading EOFs of each field. This produced a multi-variable EOF picture of the large-scale vertical (and thermal) structure of the atmosphere. With the three-dimensional structure available, it was possible to assess whether the observed temperature changes are consistent with an increase in certain regime frequencies. Moreover the hypothesis that regime frequency change more than regime structure was checked by performing a regime analysis separately on the first and last 25-yr periods.