HARINA PRECOCIDA DE CHACHAFRUTO

Lamb’s record has been applied far beyond the simple description of seasonal and annual frequencies, with research carried out into the relationships between Lamb weather types (LWTs) and temperature and precipitation (O’Hare and Sweeney, 1993). For the purpose of this project, relationships with precipitation patterns hold the key for linking synoptic climatology to the Scottish flood record.

Perry (1968) found that periods of progressiveness (displayed by a high P-index) are strongly correlated with above average precipitation. The tracks of depressions across the

country were also found to be influential: a depression tracking across the British Isles will give little precipitation in northern regions whilst one which moves mainly between Scotland and the Faroes is more likely to result in precipitation in the north west. Perry (1976) later noted that the general decrease in Westerlies experienced from the late 1950s onwards (Lamb, 1972) was likely to be associated with a specific decrease in January (and possibly the winter season as a whole) Westerly frequencies.

In order to explain the west-east precipitation gradient in Scotland, Smithson (1969) used an alternative to Lamb’s classification to relate these rainfall patterns to their synoptic origin. Using three categories to classify the synoptic origin of precipitation (frontal, airstream and cyclonic), Smithson concluded that precipitation on the west coast was predominantly frontal in origin, whilst warm sector polar maritime air precipitation was dominant in mountainous regions. Precipitation in the east was found to be more variable but could often be linked to stagnating non-frontal depressions. This study is particularly relevant since it deals specifically with synoptic- precipitation relationships in Scotland. The over-riding problem is, however, the absence of a long-term record of precipitation origin which could be incorporated into this project.

Further studies on the incidence of LWTs indicate that during the period 1967-82, summers were dominated by blocked, anticyclonic conditions. Such conditions are often associated with easterly airflows over the British Isles, and under such circumstances a reversal of the usual west-east rainfall gradient could be expected. However, during this period the distribution of precipitation across Scotland was, in fact, typical of a dominance of Westerly airflows (Hughes, 1984). These findings cast some doubt on the accuracy of LWTs in representing the synoptic situation and consequent weather conditions to all locations in the British Isles, and Scotland in particular. This is a point noted by other authors (Sweeney and O’Hare, 1992).

Jones and Kelly (1982) used Principal Component Analysis (PCA) to establish what compensatory relationships exist between LWTs. The PSCM index (Murray and Lewis, 1966) assumes a rather simple dependent inverse relationship between weather

types. A highly Cyclonic year, for example, is determined by looking at Cyclonic and Anticyclonic types only, and this paper questions the simplicity of that approach. PCA, a statistical technique which identifies groups of inter-correlated variables (Johnston, 1978), was applied to the database of annual frequencies of the 27 LWTs over the period 1861-1980. Four principal components (PC1-PC4) accounted for 71% of the variance displayed by the annual database:

• PCI links years with a high (low) frequency of Anticyclones with a low (high) incidence of Westerlies;

• PC2 indicates a year with a high (low) frequency of Anticyclones and/or Westerlies, and a low (high) frequency of Cyclonic systems;

• PC3 represents years with a dominance (absence) of synoptic systems to an absence (dominance) of North West directional flows;

• PC4 indicates meridional flow: low (high) Southerly frequencies and high (low) frequencies of Northerly and/or North westerly flows.

Although this paper has emphasised the complex counteracting processes which exist between weather types, it provides useful information on which scenarios are likely to cause inter-annual variations in weather type frequencies.

Further analysis using these principal components (Briffa el al, 1990) suggest the decline in Westerlies since the 1950s is a feature mirrored in all seasons (but to a lesser extent in Autumn). Although this decrease can be associated with increases in the frequency of synoptic systems (Anticyclonic and Cyclonic), the precise relationship does vary at seasonal and annual scales. Sweeney and O'Hare (1992) found a continued decrease in Westerlies and Cyclonic-Westerlies during the late 1980s which again coincided with increases in both the Anticyclonic and Cyclonic weather types.

Although the variability displayed by the dominant weather types (Westerly, Cyclonic and Anticyclonic) is important in determining the effect upon the climate of the British Isles, the changing incidence of the less frequent weather types should not be overlooked. The last decade (1981-90) has seen an apparent increase in the frequency of Southerly airflows. Murray (1993) confirmed that the 1980s had the greatest (and

the only positive) S-value (southerly index; Murray and Lewis, 1966) of any decade since 1861, a feature evident in all seasons, but most pronounced in autumn and spring. Whether this increase is simply a peculiarity within the climate or a possible indicator of climate change is not yet known. However, this paper does imply some recent distortions in the general circulation. Sweeney and O'Hare (1992) similarly observed a twofold increase in the frequency of the South-Westerly type over the period 1960-90 coupled with a sharp decrease in North Westerlies during the 1980s.

Sweeney and O'Hare (1992) provide a useful summary of the associations between LWTs and the resultant spatial precipitation patterns (Table 2.2). This information suggests that the Westerly, Cyclonic, Southerly and South-Westerly LWTs are associated with the highest precipitation totals in the British Isles. Westerlies, Southerlies and South-Westerlies are responsible for the west-east gradient apparent in the distribution of Scottish rainfall.