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

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Abstract

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 cold-event 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 (90°W) 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 ENSO 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 (33.5°S) 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 (41.5°S).

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