Rosa H. Compagnucci

Surface Pressure Patterns During The Year Over Southern South America

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.

Variability in subtropical Andean Argentinean Atuel river; a wavelet approach

A wavelet filter was employed for removing the strong annual wave in the Atuel river runoff data to analyze for other wavelength phenomena of interest and to examine the influence of the ENSO events. After this removal, the influence of ENSO signal in different frequency bands and indications of climatic changes in bands larger than 10 years could be observed. Other significant phenomena were also observed. As the Andean Argentinean rivers in the Cuyo region have significant similarities, the conclusions about the Atuel river could be extended to other rivers in this region. Copyright

INTERANNUAL AND INTERDECADAL VARIABILITY IN STREAM FLOW FROM THE ARGENTINE ANDES

The associations between mean annual stream flow records from 15 rivers draining the Argentine Andes, ranging from the Altiplano to Patagonia, and the Southern Oscillation Index (SOI), as established through simple lag cross-correlation, are examined. The results indicate that rivers in the Cuyo region (30° to 40°S) are negatively associated with the SOI but that the response is lagged by about one year through the regional winter maximum of precipitation and subsequent snowmelt. Stream flow in the area also has a strong positive association with winter precipitation totals in Central Chile. A statistical analysis of recent synoptic variability indicates that the difference between regional zonal and meridional flow patterns is responsible for observed fluctuations of winter precipitation. This is in keeping with the general observation of greater meridional activity globally during warm phases of El Niño-Southern Oscillation (ENSO), and a dominance of zonal flows during cold phases. Both regional stream flow and a record of historic El Niño events suggest that some lower frequency control exists in both of these series. Three periods—1908 to 1935, 1936 to 1975, and 1976 to present—are identified in the 90-year gauged record, and the distributions of annual flows and ENSO frequency are shown to vary between these, perhaps in concordance with periods of increasing global temperatures. Indications of similar associations, both with ENSO and at lower frequencies, over a longer time period are found in historic and proxy precipitation series of Central Chile. Wetter 5-year periods generally correspond to times of increasing temperatures and extended dry periods to declining temperatures, suggesting stronger El Niño events during periods of increasing temperatures. The implications for human activity of such extended wet and dry periods in regions known to currently be effected by ENSO are tentatively drawn and paralleled by archaeological evidence in northern Peru, the Bolivian Altiplano, and Easter Island.