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The nature of the seasonality of precipitation in eastern and southern coastal Spain (including both the Mediterranean and the far southwestern Atlantic provinces) is examined using monthly precipitation values available for 410 sites for the period 1964-1993. Important contrasts are illustrated between eastern (Mediterranean) and southern (Atlantic) areas. In the east, seasonality is more subdued due to incursions of fronts from the north at all times of the year. In the far south, the months of July and August are almost completely dry, but the period October-February is wet due to incursions of active Atlantic frontal systems. In central areas (Almería, Murcia, Alacant, València and the Illes Balears) the significance of extensive severe thunderstorm development during September and October produces an autumn peak in precipitation amount. The spatial variation of precipitation seasonality is further examined using the seasonality index derived by Walsh and Lawler (Walsh PD, Lawler DM. 1981. Rainfall seasonality: description, spatial patterns and change through time. Weather 36: 201-208). Trends of this index through the 30-year period are identified for some areas using linear regression on 5-year running means of the index, and indicate that seasonality is increasing in the south (Andalucía), around the uplands of Catalunya (near Lleida) in the east, and in a few smaller areas in between. In the south, the increase in seasonality is due to a greater concentration of precipitation during the cooler part of the year, so that the period January-March is becoming drier, but October-December, wetter. The overall cool season precipitation remains much as for the present, though some evidence for drying is indicated for Málaga, Jaén and Granada. Precipitation in the normally dry month of July is also increasing. In upland Catalunya, there are indications of a greater concentration of cool season precipitation, with, notably, higher amounts in October and some evidence of warm season drying, notably in June.

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Daily precipitation data from 121 sites for five autumn-winter seasons (September to January, 1982January, -1983January, to 1986 have been analysed to determine discrete precipitation areas within Wales, using S-mode principal components (PCA) and cluster analyses. Using unrotated PCA, much of the variation in daily precipitation is explained by the first five PCs, representing an accumulated variance of 75.7 per cent. The first four PCs appear to relate respectively to: altitude (608 per cent of the total variance explained), an exposure effect relative to north or north-east winds (6.0 per cent), a 'westerly exposure' effect (4.2 per cent) and an 'easterly exposure' effect (2.9 per cent). The fifth PC, with high scores only over Ynys M6n and Gwent, could not be attributed to any particular influence. Oblique rotation produced a better simple structure, enhanced the apparent importance of exposure to wind from the main compass directions, but lessened that of altitude. Three methods of clustering PCs were subsequently tried on the Oblimin solution: centroid, median and Ward. The precipitation areas produced by each were broadly similar, and the boundaries between them often coincided with clear topographic units, influenced by exposure and shelter effects. The Ward technique yielded the areas best defined in terms of topography and exposure, and best satisfied criteria determined by the nature of the data themselves.

The long-term variability of rainfall conditions in Nigeria in terms of the onset, retreat, and length of the rainy season has been analysed, using pentad data for the period 1919-1985. Data were grouped into four areas, arranged in a south-north transect; the Coastal, Guinea-Savanna, Midland and Sahelian Zones. The series for retreat of rainfall showed evidence for quasi-triennial and quasi-6-year oscillations, while that for rainy season length displayed quasi-biennial and quasi-triennial oscillations. No consistent spectral peaks emerged for changes in the date of onset of the rainy season. There is spatial coherence in variation in the date of the retreat of rainfall over the whole country, whilst for the date of onset of the season spatial coherence is limited to southern Nigeria (Coastal and Guinea-Savanna Zones). Northern Nigeria (Midland and Sahelian Zones) and southern Nigeria (Coastal and Guinea-Savanna) emerge as distinct areas in terms of spatial coherence in the variation of the length of the rainy season. There is also evidence for a secular change in the date of the retreat of rainfall for the whole country during the period 1939-1985, and in the date of onset of rainfall for southern Nigeria for 1968-1985.

Daily rainfall affinity areas are derived for the island of Mallorca using four years of data, and a 9 6 x 9 6 intersite correlation matrix. An initial visual impression of the nature of rainfall organization is afforded by the mapping of correlation links between sites. The Serra de Tramontana form a climatological as well as topographic barrier, producing a marked alignment in spatial rainfall occurrence in that part of the island. Elsewhere 'core' sites are identified, about which rainfall correlation is high for distances of up to 15 km. These centre on higher elevation locations in the Serra de Llevant and in the central area, and also the Palma and Campos coastal embayments. Use of rotated principal components and cluster analyses applied to the correlation matrices permits the division of the island into seven major rainfall areas. The uplands of the north-west and east of the island provide clear units in terms of both exposure and altitude, and their general exposure to prevailing rain-bearing winds appears important, whilst an urban effect due to Palma and clear sea-breeze convergence effects near the centre of the island are also implied.

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This study is a preliminary attempt to associate the distribution of significant rainfalls over the island of Mallorca with recognized dominant surface circulation types. Based on the extraction of five prominent PCs from a principal components analysis using a T-mode (time) correlation matrix, six dominant daily rainfall spatial patterns have been isolated using cluster analysis for significant rainfall days (area mean 2 5.0 mm) for the island of Mallorca in the western Mediterranean. These six 'pattern groups', and a number of important subgroups, are linked closely to the prevailing near-surface circulation. Seven dominant circulation types have been identified, which are closely, although not perfectly, associated with rainfall distribution, so that a conceptual spatial rainfall model for the island seems possible. Such a model would be most useful if used in conjunction with NWP forecasts of probable rainfall amount issued for the area, and would provide valuable local information on likely heavy rainfall occurrence. AIMS OF THE STUDYThe main aim of this paper is to test the strength of association between the regional surface circulation and synopsis, and resultant rainfall patterns on days with significant falls for a part of the western Mediterranean: the island of Mallorca in the Spanish Balearic Islands. A Cyear run of daily rainfall data was available (19861989), so that derived associations must be regarded as 'pointers' to indicate the directions that future in-depth studies may take. Results from the study thus aim to provide a first approximation to rainfall patterns over the island of Mallorca linked to the more common synoptic situations and resultant near-surface airflows. Given that NWP forecasts yield mean rainfall predictions for areas larger than is generally considered the locul scale, knowledge of the relutiue distribution of rainfall within these NWP forecast areas is all that is required to provide the potential for more detailed local area forecasting, superimposing a predicted rainfall pattern on the NWP forecast area mean.An additional important aspect of this study is that surface circulation should be based on pre-existing classification schemes for the purposes of comarability and clarity. Further, the full range of synoptic patterns within a given classification may yield comparatively few distinctive rainfall distributions. Closely similar rainfall patterns may result from a number of different surface circulations, and, indeed, many surface CCC 0899-841 81951060673-24 0 1995 by the Royal Meteorological Society