Extreme precipitation over Southeastern Brazil in the austral summer and relations with the Southern Hemisphere annular mode

ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 11: 21 26 (2010) Published online 12 January 2010 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/asl.247 Extreme precipitation over Southeastern Brazil in the austral summer and relations with the Southern Hemisphere annular mode Fernanda Cerqueira Vasconcellos* and Iracema F. A. Cavalcanti Center for Weather Forecasting and Climate Studies, National Institute for Space Research, Avenida dos Astronautas 1758, 12227-010 São José dos Campos, São Paulo, Brazil *Correspondence to: Fernanda Cerqueira Vasconcellos, Center for Weather Forecasting and Climate Studies, National Institute for Space Research, Avenida dos Astronautas 1758, 12227-010 São José dos Campos, São Paulo, Brazil. E-mail: fernanda.cerqueira@cptec.inpe.br Abstract Southeastern Brazil is a highly industrialized and populated region which largely contributes to the country s economy. Extreme precipitation over this region can cause floods and landslides occurrences or lack of precipitation that can affect many sectors of economy. The objective of this work is to identify atmospheric characteristics associated with austral summer extreme precipitation over part of Southeastern Brazil. The atmospheric characteristics obtained from wet and dry composites suggest that extreme precipitation was associated with anomalous circulation over the region, forced by a PSA-like wavetrain intensified by the Southern Annular Mode. Copyright 2010 Royal Meteorological Society Received: 20 August 2009 Revised: 14 October 2009 Accepted: 26 October 2009 Keywords: Extreme precipitation; PSA pattern; S. H. annular mode; Southeastern Brazil 1. Introduction The Southeastern Brazil region is affected by synoptic systems such as frontal systems and the South Atlantic Convergence Zone (SACZ) during summer (December, January, February), which can produce intense precipitation with floods and landslides occurrences near the coastal regions. On the other hand, periods of drought also occur in the region and affect mainly the agriculture and energy sectors. One of these cases was reported by Cavalcanti and Kousky (2001) on the severe drought over Southeastern and Central Brazil, where the lack of rains produced a serious energy crisis in the country, since the generation of power relies on the hydroelectricity management. Anomalous convection and precipitation associated with the SACZ has been discussed by Carvalho et al. (2004) and Cunningham and Cavalcanti (2006) on an intraseasonal scale, Mo (2000) and Mo and N-Paegle (2001) on a monthly scale and Liebmann et al. (1999) on a submonthly scale. In these studies, the Pacific-South America (PSA) teleconnection pattern was associated with the SACZ anomalies. The PSA pattern, one of the dominant modes of climate variability in the Southern Hemisphere (SH), was first identified by Mo and Ghil (1987) in EOF analysis as a wavetrain from the central Pacific to South America. Two PSA modes in quadrature with each other, in the intraseasonal band during winter, were analyzed by Mo and Higgins (1998). Mo and N-Paegle (2001) showed a dipole of rainfall deficits over Northern South America and enhanced precipitation over Eastern Brazil associated with PSA 1 during summer. Another precipitation dipole between the SACZ region and Uruguay/Argentina was associated with PSA 2. Teleconnection patterns, including PSA wavetrain influence on South America, were discussed by Cavalcanti (2000). Cunningham and Cavalcanti (2006) also discussed the influence of tropical-extratropical intraseasonal variability through the PSA pattern and the tropical intraseasonal variability through the MJO on SACZ. Another main mode of climate variability in SH is the Southern Annular Mode (SAM) or Antarctic Oscillation (AAO). This oscillation between the middle and high southern latitudes was described by Gong and Wang (1999) and Thompson and Wallace (2000). The prominent feature in the AAO pattern is the strong negative relationship between pressures at 40 and 65 S, as discussed by Gong and Wang (1999). They defined an objective index for the AAO, based on the difference between normalized monthly zonal mean Sea Level Pressure (SLP) at these two latitudes. The positive (negative) index for this oscillation indicates less (higher) than normal SLP at high latitudes. This oscillation is also identified in geopotential data by Thompson and Wallace (2000). Silvestri and Vera (2003) showed negative correlations of AAO phase with precipitation over Paraguay and adjacent regions in November/December and positive correlations in July/August in the same region. Negative correlations (less precipitation in the positive phase) occurred associated with the presence of anticyclonic anomaly over the region, which would reduce Copyright 2010 Royal Meteorological Society

22 F. C. Vasconcellos and I. F. A. Cavalcanti the cyclonic synoptic activity and precipitation. They also mention that no significant correlation was found in the summer in that region. An increased (weak) intraseasonal activity in the SH circulation from the tropics to midlatitudes was related to negative (positive) AAO phases in Carvalho et al. (2005). They discuss the observation that cyclones tend to form and to move to lower latitudes during negative phases of the AAO. Rao et al. (2003) examined how the AAO is related to the interannual variability of the storm tracks in the SH. They found an increase of baroclinic eddies growth in midlatitudes, during positive phases of AAO, and a reduction in the negative phases. Thus, it is seen that South America is affected by teleconnection patterns which produce circulation intensification associated with the anomalous precipitation over the continent. The objective of the present study is to analyze atmospheric patterns associated with extreme wet and dry summers over a very populated region of Southeastern Brazil. 2. Data and method The analyzed area (25 20 S, 50 45 W) in the southern sector of Southeastern Brazil, also comprises a mountainous area close to the sea, named Serra do Mar, vulnerable to landslides and floods. In the current study, monthly precipitation data from GPCP (Adler et al., 2003) and monthly 500 hpa and 700 hpa geopotential height, zonal and meridional wind at 200 hpa from NCEP NCAR Reanalysis 2 (Kanamitsu et al., 2002) for December, January, February (DJF) of 1979 2006 were used. The analyses are based on composites of rainfall, geopotential height and wind fields anomalies for the five rainiest and the five driest SH summers, in the analyzed area, obtained from a quintile analysis (Table I). Correlation map between AAO index and monthly precipitation anomalies shows consistent results. The AAO index was calculated from the principal component time series of empirical orthogonal function analysis leading mode of 700 hpa geopotential height monthly mean anomalies poleward of 20 S. Positive (negative) values of AAO index are associated with negative (positive) geopotential height anomalies over Antarctica and positive (negative) geopotential height anomalies at midlatitudes. This methodology was obtained from Climate Prediction Center/NCEP (available on line at http://www.cpc.noaa.gov). 3. Extreme summers precipitation analysis The composite of anomalous precipitation for the rainiest summers, in the analyzed region, shows the positive rainfall anomalies in a northwest southeast band over Southeastern Brazil, typical of the SACZ occurrences, and weaker negative precipitation anomalies to the northeast and to the southwest of this Table I. Classification of the rainiest and the driest summers in the southern sector of Southeastern Brazil (25 20 S, 50 40 W), based on GPCP data. classification Period Rainiest December 1986 February 1987 December 1994 February 1995 December 1998 February 1999 December 2001 February 2002 December 2002 February 2003 Driest December 1983 February 1984 December 1985 February 1986 December 1989 February 1990 December 1990 February 1991 December 1991 February1992 band (Figure 1(a)). These signs reverse in the driest summers, presenting negative precipitation anomalies over SACZ region and opposite anomalies on both sides (Figure 1(b)). Thus, SACZ frequency and/or intensity seems to be responsible for extreme precipitation over this region during the summer season. The geopotential height anomaly composites at 500 hpa for the extreme wet and dry summers indicate a seesaw between polar region and middle latitudes, typical of the AAO pattern (Figure 2). Although the anomalies are not very large, due to the composite mean that smoothes the field, the patterns are recognizable and show significant values. Negative anomalies over Antarctic and positive anomalies around 30 50 S occur in the rainiest summers (positive AAO phase). Opposite configuration is seen in the driest summers (negative AAO phase). The significant correlation between AAO and monthly precipitation anomaly in DJF over part of Southeastern Brazil and over the oceanic SACZ region reinforces the composite pattern results (Figure 3). Other contributions for high precipitation in this region in DJF are related to features of the South America monsoon system (Jones and Carvalho, 2002; Gan et al., 2004; Raia and Cavalcanti, 2008). In the present study, features of the extratropical region were identified. The extreme wet cases in the analyzed region occur when the jet stream is strong at high latitudes (westerlies forced by cyclonic circulation at high latitudes and anticylonic circulation at middle latitudes) during positive phases of the AAO. The extreme dry cases occur when the westerlies at high latitudes are reduced and the jet stream is displaced to middle latitudes (anticyclonic circulation at high latitudes and cyclonic circulation at middle latitudes), during negative phases of the AAO. The relation between AAO phases and upper level jets was discussed by Carvalho et al. (2005) who showed the weakening (strengthening) of the subtropical jet at high levels and strengthening (weakening) of the polar jet in the positive (negative) AAO phase. They also showed a modulation of the extratropical cyclones by the AAO with respect to their latitude of origin and their maximum equatorward displacement. The displacement of the subtropical jet to lower latitudes

Extreme summers precipitation over Southern Brazil and the SH annular mode 23 (a) (b) Figure 1. Composites of precipitation anomaly (mm/day) in DJF (shaded). (a) Extreme wet summers and (b) extreme dry summers over part of Southeastern Brazil. Areas with 90% significance are shown in contours (t-student test). in the negative phase induced cyclone initiation and dissipation also in lower latitudes. This can explain the negative correlation between AAO and precipitation over Southeastern South America (SESA) in November/December identified by Silvestre and Vera (2003). Those results were also consistent with Rao et al. (2003) who found a negative correlation between AAO and the storm tracks at high and subtropical latitudes, and a positive correlation at middle latitudes.

24 F. C. Vasconcellos and I. F. A. Cavalcanti (a) (b) Figure 2. Composites of geopotential height anomaly at 500 hpa (m) in DJF (shaded). (a) Extreme wet summers and (b) extreme dry summers over part of Southeastern Brazil. Areas with 90% significance are shown in contours (t-student test). In the present analysis, anomalous positive precipitation occurs in the SACZ region during the positive AAO phase, when anomalous negative rainfall occurs to the south. When the AAO phase is negative, higher than normal precipitation occurs over Uruguay and Northeastern Argentina and there is an extreme deficit of precipitation over Southeastern Brazil. In the extreme wet composite two wavetrains are directed to Southern South America in a PSA-type configuration, one in the subtropical region and another at higher latitudes (Figure 4(a)). The latter wavetrain has a strong cyclonic center anomaly around 120 W, near Antarctica, which seems to be intensified by the polar signal of AAO. These wavetrains induce an anomalous cyclonic anticyclonic circulation pair behind ahead of the analyzed area in the wet summers, affecting the convection over the region. The influence of two upper level wavetrains over Southern Pacific on frost cases in South America during the wintertime was discussed by Müller and Berri (2007). In the driest summers (Figure 4(b)), the configuration of streamline anomaly composites at 200 hpa shows the circulation pair over Southeastern Brazil in opposite phase to the wet case, with an anticyclonic cyclonic circulation pair behind ahead of the analyzed region. In this case the PSA-like wavetrain pattern originates from tropical Indian Ocean. The middle latitude signal of AAO seems to affect the cyclonic circulation anomaly close to the Southwestern South America coast contributing to the wavetrain over the continent. Rossby wavetrain propagation from the tropical Indian Ocean directed to South America was discussed by Saji et al. (2005) when analyzing relations between the Indian Ocean Dipole and surface air temperature. The wavetrain pattern from Southeastern Pacific to the continent is similar to that found in blocking events over Southeastern Pacific (Sinclair 1996; Pezzi and Cavalcanti 1998). Therefore, in the present study, besides the influence of AAO in the position of subtropical and polar jets

Extreme summers precipitation over Southern Brazil and the SH annular mode 25 Figure 3. Correlation map between AAO index and monthly precipitation anomalies (DJF) with 90% significance (t-student test). (a) (b) Figure 4. Composites of streamline anomaly at 200 hpa in DJF. (a) Extreme wet summers and (b) extreme dry summers over part of Southeastern Brazil. The shaded area has 90% significance (t-student test) for meridional plus zonal wind. and in the storm tracks and cyclones behavior, previously analyzed, it is shown that the polar and middle latitude signals of AAO affect the anomalous cyclonic center over Southeastern Pacific in wet and dry cases, respectively. Thus, these patterns contribute to the anomalous circulation intensification over Southeastern Brazil, responsible for the extreme precipitation cases.

26 F. C. Vasconcellos and I. F. A. Cavalcanti 4. Summary and concluding remarks The focus of this work is on the atmospheric patterns associated with extreme precipitation over Southeastern Brazil, a very populated and industrialized region. The anomalous precipitation was associated with the SACZ occurrences and the atmospheric patterns were related to PSA-type wavetrains, with the Southeast Pacific cyclonic center intensified by the SH annular mode. Excessive precipitation over Southeastern Brazil occurs in the positive AAO phase and a PSA pattern with strong cyclonic center at 120 W near Antarctica. Extremely dry conditions over Southeastern Brazil display negative AAO phase and also a PSA-type pattern but with a significant cyclonic anomaly center close to SESA, in the middle latitudes AAO signal region. 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