We propose that the first two empirical orthogonal function (EOF) modes of tropical Pacific sea surface temperature (SST) anomalies do not describe different phenomena (i.e., El Niño‐Southern Oscillation (ENSO) and “El Niño Modoki”) but rather the nonlinear evolution of ENSO. We introduce two new uncorrelated indices (E and C), based on the leading EOFs, that respectively account for extreme warm events in the eastern and cold/moderate warm events in the central equatorial Pacific, corresponding to regimes with different evolution. Recent trends in ENSO can be described as an increase in the central Pacific (C) variability that is associated with stronger cold events, as well as a reduction in the eastern Pacific (E) variability within the cold/moderate warm regime, consistent with model projections. However, little can be said observationally with respect to the extreme warm regime.

The seasonality of the ENSO-rainfall relationship in central Chile (30Њ-41ЊS) and associated circulation anomalies are studied using correlation and compositing techniques. During El Niño episodes there is a tendency for the occurrence of above-average precipitation between 30Њ and 35ЊS in winter [June-July-August (JJA)] and from 35Њ to 38ЊS in late spring [October-November (ON)], while rainfall deficit is typically observed from around 38Њ to 41ЊS during the following summer [January-February-March (JFM)], when El Niño reaches its maximum development. Opposite rainfall anomalies are characteristic during La Niña 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 Niño 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 35Њ-38ЊS region during El Niño 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 Niño 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 Niña 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 Niño 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 Niña events.

Central Chile, home to more than 10 million inhabitants, has experienced an uninterrupted sequence of dry years since 2010 with mean rainfall deficits of 20–40%. The so‐called Mega Drought (MD) is the longest event on record and with few analogues in the last millennia. It encompasses a broad area, with detrimental effects on water availability, vegetation and forest fires that have scaled into social and economical impacts. Observations and reanalysis data reveal that the exceptional length of the MD results from the prevalence of a circulation dipole‐hindering the passage of extratropical storms over central Chile—characterized by deep tropospheric anticyclonic anomalies over the subtropical Pacific and cyclonic anomalies over the Amundsen–Bellingshausen Sea. El Niño Southern Oscillation (ENSO) is a major modulator of such dipole, but the MD has occurred mostly under ENSO‐neutral conditions, except for the winters of 2010 (La Niña) and 2015 (strong El Niño). Climate model simulations driven both with historical forcing (natural and anthropogenic) and observed global SST replicate the south Pacific dipole and capture part of the rainfall anomalies. Idealized numerical experiments suggest that most of the atmospheric anomalies emanate from the subtropical southwest Pacific, a region that has experienced a marked surface warming over the last decade. Such warming may excite atmospheric Rossby waves whose propagation intensifies the circulation pattern leading to dry conditions in central Chile. On the other hand, anthropogenic forcing (greenhouse gases concentration increase and stratospheric ozone depletion) and the associated positive trend of the Southern Annular Mode also contribute to the strength of the south Pacific dipole and hence to the intensity and longevity of the MD. Given the concomitance of the seemingly natural (ocean sourced) and anthropogenic forcing, we anticipate only a partial recovery of central Chile precipitation in the decades to come.

The seasonality of the simultaneous relationship between tropical Pacific SST and rainfall, as well as rainfall predictability one season in advance in subtropical South America (25Њ-40ЊS), is studied using different multivariate techniques. This study shows that ENSO-related rainfall anomalies in subtropical South America are restricted mostly to regions on the eastern and western sides of the continent and mainly during the second half of the year. The relationship is almost exclusively of the warm-wet/cold-dry type, but a more widespread impact is found when anomalously warm conditions prevail in the equatorial Pacific. A spatially coherent region with a significant warm-wet/cold-dry signal is detected in southeastern South America during austral spring (October-November), including southern Brazil, southern Paraguay, Uruguay, and eastern Argentina. This signal moves inland toward the west from spring to early summer. During late winter (July-August), a similar SST-rainfall relationship is found in subtropical Chile and southern Brazil. In Chile, a southward propagation of the signal is observed from winter to spring. Most significant ENSO-related rainfall anomalies seem to occur after the maximum in the precipitation annual cycle. The combined analysis of seasonal diagnostics and predictability of rainfall show that the seasonal rainfall predictability in subtropical South America based on tropical Pacific SST to a greater extent is restricted to a specific time of the year and regions that broadly coincide with those where the simultaneous SST-rainfall relationship is significant. This fact suggests that persistence of tropical Pacific SST anomaly is the major source of seasonal rainfall predictability in this region, when SST is used as a predictor.

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