Arquivo do Trabalho - IAG - USP

1APRIL 2009 M U Z A E T A L . 1695FIG. 12. The 850-hPa total wind (m s 21 ) during (a), (c) wet and (b), (d) dry extreme intraseasonal events over(a), (b) SEBr; (c), (d) WAtl. Solid lines indicate regions where the average magnitude of the meridional wind is abovethe 55th percentile of the climatology (lines are plotted every 10% interval). Dashed lines indicate regions where themagnitude of the meridional wind is below the 25th percentile. This analysis is performed only where the intraseasonalanomalies of the meridional wind are statistically significant at 5% significance level (gray shades). Dottedcontours show the terrain elevation beginning at 750 m with 500-m intervals.where the intensity is above the 75th percentile. Thesefeatures are observed along with a decrease in magnitudeof V over Northeastern Brazil extending toward theAtlantic Ocean and have not been documented before.Wet extreme events over WAtl (Fig. 12c) are associatedwith the intensification of the northerly windsover eastern South America and over the AtlanticOcean, north of WAtl. Unlike SEBr wet events (Fig.12a), these winds have intensity above the upper quartileof the climatology only over the Atlantic and thesoutheastern coast of SA (Fig. 12c). Interesting enough,dry extreme events over WAtl (Fig. 12d) are not associatedwith an enhancement of V east of the Andes butover southeastern Brazil.6. Relationships between intraseasonal andlow-frequency extreme eventsThe occurrence of extreme events on low-frequencytime scales certainly results from the interplay of phenomenaoccurring on several time scales. For example, alarge number of storms or intense mesoscale systemsduring one season must increase the total seasonalprecipitation. Analogously, more frequent cold fronts orlong-lasting blocking conditions can also play a role inchanging the seasonal climatology. The objective of thepresent section is to examine the importance of intraseasonalanomalies in contributing to extreme seasonalprecipitation.