Surface Pressure Patterns During The Year Over Southern South America

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Principal component analysis was carried out on daily sea-level pressure maps for the period 1972–1983 in southern South America to determine the main synoptic types. Each month in the whole period was studied in order to analyse the variability through the annual cycle. The results show that the first six principal components (PC) explain more than 90 per cent of the total variance and they give approximately the same patterns for all the months. For each pattern the most important difference observed in the features through the annual cycle was the shift towards high latitudes in the synoptic systems during the summer months. It means that there are no remarkable seasonal contrasts apart from the latitudinal position and differences in the variances accounted for by the patterns. Half or more than half of the whole variance was explained by the first PC, the spatial pattern of which is quite similar to the feature of the monthly mean pressure field. This synoptic situation represents the major contribution to the zonal flow for surface circulation at mid-latitudes of the area studied. The remaining PC patterns have been recognized as features corresponding to the most frequent baroclinic perturbations to the basic flow. Furthermore, these synoptic situations account for the principal meridional flow component. In brief, the present results are found to be in general agreement with previous knowledge of southern South America. © 1997 by the Royal Meteorological Society.