Surface circulation associated with frost in the wet Pampas

The aim of this paper is to make a synoptic-climatic classification of atmospheric circulation in order to obtain synoptic frost-related patterns in the wet Pampas. Both partial (recorded in 25 to 75% of the meteorological stations) and widespread (recorded in more than 75%) frost events that occurred during the winters (June-July-August) of 1972-83 are included in this study. Frost days are grouped into Neutral (G1), El Nino (G2), and La Nina (G3) years. In addition, the complete dataset, called the total group, is analysed for reference purposes. Each group is analysed using the unrotated and Varimax rotated principal component analysis using the T-mode approach. Six synoptic situations accounted for 94% of the variance associated with frosts in the area studied. In general, the principal component score patterns given by the unrotated and rotated components are similar. The biggest difference between unrotated and rotated solutions was in variance redistribution. After rotation, only one of the two possible situations (direct/inverse) of each pattern represented a real synoptic type associated with frost in the wet Pampas. Persistence and location cause temperature drops in the area studied. The most frequent patterns in rotated results are those termed A, B, C* (in G2) and D (in G1 and G3). They are connected with cold anticyclones, which cause advective and/or radiative frosts. The remaining patterns accounted for about 5% of the variance and represent real, though rare, situations which are important because of their effect on the wet Pampas. The results obtained for the different groups show that inter-winter variability of the equatorial Pacific signal produces changes in the frequency of frost-connected patterns, rather than different patterns for cold events. Copyright (C) 2003 Royal Meteorological Society.