No abstract

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.

Summertime (December-February) precipitation is virtually the only water resource over the South American Altiplano, a semiarid, high-level plateau entrenched in the central Andes. On the interannual timescale, Altiplano rainfall exhibits pronounced fluctuations between drought and very wet conditions, with subsequent impacts on agriculture and hydrology. In this work, the large-scale patterns of convective cloudiness and circulation associated with interannual variability of the summer rainfall over this region are investigated using a regression analysis between relevant atmospheric fields (NCEP-NCAR reanalysis, outgoing longwave radiation) and an index of convection over the Altiplano. It is found that the seasonal-mean, large-scale zonal flow over the central Andes is directly related with the number of days with easterly flow within the season, that, in turn, favor the occurrence of summertime deep convection on the Altiplano by transporting moist air from the interior of the continent. Consequently, interannual variability of the seasonal-mean zonal wind explains nearly half of the variance of summertime convection over the Altiplano through an easterly/wet-westerly/dry pattern. The circulation anomalies are in geostrophic balance with changes in the meridional baroclinicity at the southern border of the tropical belt. Thus, a previously documented relationship between El Niño-Southern Oscillation (ENSO) phenomenon and interannual rainfall variability over the Altiplano is explained by the generalized warming (cooling) of the tropical troposphere during the negative (positive) phase of ENSO and the associated strengthening (weakening) of the westerlies over the central Andes.

No abstract

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.