Seasonality of the ENSO-Related Rainfall Variability in Central Chile and Associated Circulation Anomalies

Abstract

The seasonality of the ENSO–rainfall relationship in central Chile (308–418S) and associated circulation anomalies are studied using correlation and compositing techniques. During El Nin˜o episodes there is a tendency for the occurrence of above-average precipitation between 308 and 358S in winter [June–July–August (JJA)] and from 358 to 388S in late spring [October–November (ON)], while rainfall deficit is typically observed from around 388 to 418S during the following summer [January–February–March (JFM)], when El Nin˜o reaches its maximum development. Opposite rainfall anomalies are characteristic during La Nin˜a events. This study confirms results from previous investigations indicating that enhanced blocking activity over the Amundsen–Bellingshausen Seas area in the southeastern (SE) Pacific during El Nin˜o is a key feature explaining the wet conditions in winter. It is also shown that the same circulation anomaly explains the relatively wet conditions in late spring in the 358–388S region during El Nin˜o episodes. Furthermore, the southward displacement from winter to late spring of the area with significant ENSO-related rainfall anomalies seems associated with the seasonal migration of the boundary separating the region under the influence of the subtropical domain from the extratropical domain, where the westerly regime and associated disturbances prevail. Blocking episodes in the SE Pacific during El Nin˜o seem to be part of a wave structure, particularly intense during spring, characterized by a sequence of positive and negative quasi-barotropic height anomalies stretching southeastward from the equator toward the SE Pacific and back to the southwestern Atlantic. On the other hand, anomalously dry conditions in winter and late spring during La Nin˜a are favored by long-lasting and intense ridges at subtropical latitudes over the SE Pacific and South America resulting in a southward migration of the midlatitude storm tracks. In summer, a higher frequency of ridges in the southern tip of the South America during El Nin˜o episodes presumably contributes to reinforcement of the southern edge of the subtropical anticyclone in the SE Pacific, which at this time of the year reaches its southernmost position, resulting in the annual rainfall minimum. On the other hand, an increased frequency of cyclonic circulation anomalies crossing the southern tip of the continent is associated with relatively wet conditions in southern-central Chile, particularly during La Nin˜a events.