Principal component analysis of the Lamb catalogue of daily weather types: Part 2, Seasonal frequencies and an update to 1987

Principal component analysis is applied to seasonal weather-type frequencies from the Lamb Catalogue of Daily Weather Types for the British Isles. Four combinations of the major weather types are identified, which represent around 70 per cent of the variance of the data set. Long-term trends in the seasonal principal component time series are described and relationships with temperature and precipitation are identified. It is shown that a pronounced change in the character of the synoptic circulation over the British Isles has occurred in recent decades. A decline in the frequency of westerly weather types has been compensated for by an increase in the frequency of pure cyclonic and anticyclonic types and, dependent on season, directional types. This development suggests greater variability in the atmospheric circulation and related climatic parameters. The westerly decline has not been as marked in autumn. In this season, though, there has been a notable shift towards greater southerliness at the expense of northerly weather types.