Synoptic aspects of the central chile rainfall variability associated with the southern oscillation

Autor: José A. Rutllant, Humberto Fuenzalida
Rok vydání: 2007
Předmět:
Zdroj: International Journal of Climatology. 11:63-76
ISSN: 1097-0088
0899-8418
DOI: 10.1002/joc.3370110105
Popis: Central Chile winter (June, July, August (JJA)) rainfall shows positive anomalies during the developing stage of warm events of the Southern Oscillation. Conversely, cold events correspond quite closely to dry conditions. A synoptic characterization of major storms during the most recent warm events is presented. Dry months during coldevent years are described in terms of average 500-hPa contour anomaly fields. Significant departures from this general behaviour are also discussed. It is found that major winter storms associated with warm events are related to blocking highs frequently located around the Bellingshausen Sea (9OOW) within hemispheric circulation anomaly patterns where zonal wavenumbers 4 and 3 dominate. This phenomenon seems consistent with observed teleconnection wavetrains stemming from the anomalous atmospheric heat source above the equatorial Pacific during ENS0 events. Cold years, often immediately preceding or following a warm event, bring dry conditions in the study area owing to a well-developed south-east subtropical anticyclone with enhanced zonal westerly flow at middle latitudes. Frequency distributions of 500-hPa daily blocking indices (BI) at 90°W, derived from 1980 to 1987 European Centre for Medium Range Weather Forecasts hemispheric analyses, show a significant departure towards positive BI values for the available warm-event winters; the opposite being also true. However, the JJA rainfall variability at Santiago (33.53) also seems to be related to the regional strength of the south-east Pacific anticyclone, as represented by seasonal 500-hPa geopotential anomalies at Puerto Montt, Chile (413"s). The apparent relationship between the phases of the Southern Oscillation (SO) and rainfall anomalies in central Chile (30-35"s) has been reported by several authors. Rubin (1955), while taking into consideration pressure anomalies in the Southern Hemisphere, found that the precipitation in central Chile stays below normal during the positive phase of the SO, namely when the south-east Pacific subtropical anticyclone is stronger than average. The strength and position of the aforementioned anticyclone has also been related by Pittock (1980) to the interannual rainfall variability in the central part of Chile. Quinn and Neal (1983), in an attempt to relate long series of annual precipitation in Santiago (33*5"S, 70.7"W) and in Valparaiso (33.0°S, 71.6"W) with an El Niiio index, obtained a good correspondence of the interannual rainfall variability with area averaged sea-surface temperatures in the south-eastern tropical Pacific. More recently, Aceituno (1987) has correlated the pressure, temperature, wind, and precipitation fields with a Southern Oscillation Index (SOI), concluding that the tendency to positive rainfall anomalies in central Chile during the negative phase of the SO is associated with a weak and northerly displaced southeast Pacific subtropical anticyclone; together with an overall increase in baroclinicity at the subtropical latitudes produced by tropospheric cooling in the southern part of South America and a corresponding warming in tropical latitudes. At present, the Chilean rainfall anomaly seems to have gained a place in the world-wide sequence of major climatic anomalies related 0899-84 1 8/9 1/0 10063-1 4$07.OO 0 1991 by the Royal Meteorological Society
Databáze: OpenAIRE
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