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1APRIL 2009 M U Z A E T A L . 1687FIG. 4. Box-plot diagram illustrating the median, upper, lowerquartiles and the interquartile range of the distributions of persistenceof extreme dry and wet events on (left) intraseasonal and(right) low-frequency time scales for all regions. The limits of theboxes represent the interquartile range. Outliers are data values $ 2times the interquartile range.clearly related to phases of ENSO, as both positive andnegative extremes are associated with negative anomaliesin the equatorial Pacific.Low-frequency extremes over the western subtropicalSouth Atlantic (WAtl; Figs. 5c,d) are characterized byanomalies of smaller magnitude and scale than thoseover the continent previously discussed. No coherentsignal of precipitation anomalies is observed over continentalSouth America during extreme dry or wet eventsin that region. In addition, the patterns of precipitationover the Atlantic and Pacific Oceans differ from thoseobserved for wet or dry events over SEBr (cf. Figs. 5a,b)and do not resemble the canonical patterns of precipitationrelated to ENSO. Only 32% (38%) of the total pentadsassociated with extreme wet (dry) anomalies overWAtl occurred simultaneously with wet (dry) eventsover SEBr. These results suggest that distinct mechanismsmodulate the variability of extreme precipitationover the oceanic portion of the SACZ and the continentalSAZC, in agreement with previous observations(Carvalho et al. 2002a, 2004).a. Sea surface temperatureThe characteristics of SST low-frequency anomalies(SST LF ) associated with patterns of extreme dry and wetevents previously discussed are examined here. Consistentwith the pattern of precipitation anomalies, extremewet and dry events over SEBr are both relatedto negative SST LF anomalies over the eastern Pacific(Figs. 6a,b). Dry events over SEBr do not exhibit a reversalin the sign of the SST LF anomalies over easternPacific (cf. Fig. 6a). Nonetheless, relatively large magnitudesand westward extension of negative SST LFanomalies are observed during dry events (Fig. 6b).Three out of six dry events occurred during cold phases,and two during neutral ENSO phases. ENSO phasesand intensity are based on the NCEP/Climate PredictionCenter classification.Another interesting feature of the SST LF is observedover the South Atlantic Ocean, where extremes areassociated with opposite phases of the South AtlanticSST dipole (Robertson and Mechoso 2000). Wet (dry)extreme events over SEBr occur when the SST LFanomalies gradient over the South Atlantic is southward(northward). Extreme wet (Fig. 6c) and dry (Fig. 6d)events over WAtl seem also related to distinct SST LFgradients over the Atlantic Ocean. However, there areno systematic relationships with low-frequency extremeprecipitation anomalies over SEBr.b. Upper-level circulationKodama (1992, 1993) suggested that the SACZ, similarto other subtropical convergence zones, appearswhen two necessary conditions in midlatitude circulationare satisfied: first, subtropical jet flows in the subtropicallatitudes (308–358S) and second, low-level poleward flowsprevail along the western peripheries of the subtropicalhighs. It has been postulated that if these conditions arenot satisfied, the convergence zone is weak. It is importantto point out that Kodama focused mainly on the oceanicportion of the SACZ. The objective of the present analysisis to investigate the importance of low-frequencyanomalies in low- and high-level circulations along withvariations in the Pacific–South American (PSA) pattern(Mo and Nogués-Paegle 2001) for the occurrence of extremewet/dry events over the regions considered here.For this purpose zonal wind low-frequency anomalies at200 hPa (U200 LF ) and wind low-frequency anomalies at850 hPa (UV850 LF ) during extreme wet and dry events onlow-frequency time scales are examined.Figure 7 shows that extreme wet (dry) events in allregions are observed in association with a local enhancementof U200 LF westerly (easterly) anomalies. ThePSA pattern characterized by the alternating U200 LFanomalies over western South America is evident duringwet events over SEBr (Fig. 7a). Dry episodes over SEBr,on the other hand, are characterized by a large extent ofeasterly anomalies over large portions of the tropics ofthe Southern Hemisphere (Fig. 7b). Westerly anomaliesare observed over the equator between 1808 and 1208Wand also over the extratropics of the Southern Hemisphere,indicating an intensification of the zonal symmetryin high-level circulation.Composites of U200 LF show clear evidence that theoccurrence of extreme wet (Fig. 7c) and dry (Fig. 7d)