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.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.