WINTER ANTICYCLONE CHANGES ON THE CENTRAL GULF-COAST OF THE USA

A modified cyclone-model-based weather classification system is used t o identify long-term trends in anticyclonic weather type frequencies o ver the central Gulf Coast of the USA. Results suggest that days domin ated by mid-latitude cold-season anticyclones of continental origin (C H days) have decreased in frequency from 1961, to 1989. At the same ti me, anticyclones of Pacific origin (PH days) have become more common o ver the same region. In both cases, most of the trends are characteriz ed by abrupt 'step changes' rather than gradual changes. Principal com ponents analysis in combination with a clustering procedure is used to analyse long-term trends in the surface air mass properties at New Or leans during times of influence of these weather types. The intensitie s of these weather types as a whole do not display any significant tre nds. However, several individual weather variables do show significant change, especially on CH days. Although some trends may represent cha nges in trajectory and intensity of the air masses, many may be indica tive of more general atmospheric tendencies not limited to CH and PH d ays, such as the urban heat island and reduced visibility associated w ith anthropogenic activity. Future research should be conducted to imp rove our understanding of how changes in the presence and properties o f these anticyclones relate to changes in the larger scale steering fl ow patterns governing their trajectories and intensities. (C) 1997 by the Royal Meteorological Society.