Variabilidade da precipitação em Portugal e relações com o sitema oceano-atmosfera no Atlântico Norte

Abstract

RAINFALL VARIABILITY IN PORTUGAL AND SOME RELATIONS WITH THE OCEAN-ATMOSPHERE SYSTEM IN THE NORTH ATLANTIC - The scope of this study is to analyse rainfall variability in the Portuguese rainy season (Nov-Mar) during the 20th century, and examine some relations with the sea surface temperatures (SST) and geopotencial heights at 850 hPa (Z850) in the North Atlantic region. To build the Portuguese rainfall index (PRI), a principal component analysis (PCA) was performed on a matrix containing seven Portuguese stations with secular records (variables) and the years 1900 to 1993 (observations). This index is the first principal component (PC), which explains always over 60% of the total variance and all the seven stations used in the PCA have positive correlationswith the first component. Therefore we can assume that the first PC describes the rainfall variability in Portugal and the PCA has detectedonly one coherent area in Portuguese territory. The analysis of cycles reveals a variability that might be linked with the North Atlantic ocean atmosphere system. Very significant cycles of 3/4 or 7/8 years were found either in the PRI or in the first PC of SST time-series. In order to search the causes of the rainfall variability in Portugal, several correlations and composites analysis were made. Correlations between the PRI and the North Atlantic Oscillation (NAO) index are always negative for all rainy season months (Nov-Mar). The correlation between the PRI and the SST in North Atlantic shows a central area with significant negative coefficients (west of Portuguese coastline). Positive correlations above 50ºN and below 30ºN. The correlation between the PRI and Z850 field shows a North Atlantic Oscillation (NAO) pattern: positive correlations in the north and negative correlations in the south of North Atlantic region. In additon, a compositive analysis was made for wet and dry years to test the reaction of the SST field. A distinct pattern was found in the SST field for wet and dry years