Center for Weather Forecasting and Climate Studies/National Institute for Space Research (CPTEC/INPE), São Paulo, Brazil

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 34: (2014) Published online 26 July 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: /joc.3729 The relationship between the Antarctic oscillation and blocking events over the South Pacific and Atlantic Oceans Monica Cristina Damião Mendes* and Iracema Fonseca Albuquerque Cavalcanti* Center for Weather Forecasting and Climate Studies/National Institute for Space Research (CPTEC/INPE), São Paulo, Brazil ABSTRACT: It is known that blocking events have impacts on the behaviour of transient systems and affect the weather in southern continental regions of the Southern Hemisphere. Since the main mode of variability in Southern Hemisphere, the annular mode, or Antarctic Oscillation (AAO), changes the extratropical atmospheric circulation, the blocking characteristics over Pacific and Atlantic Oceans are analysed in this study, in terms of the two opposing phases of the AAO. The characteristics of atmospheric flow patterns and blocking events are analysed as a function of season during the period The relative frequency of blocked days is different for each AAO phase. Maximum duration and number of events over the Southwestern Pacific and South Atlantic regions occur in the positive phase, while the blocking activity over the Southeastern Pacific region is similar in the two phases but the persistence is higher in the negative phase. The atmospheric anomalous centres associated with middle latitude and polar AAO signals, as well as the behaviour of subtropical and polar jets are discussed in relation to the blocking characteristics. Convection over South America is analysed for blocking periods in each AAO phase and is discussed based on anomalous circulation in each phase. Wavetrain patterns over the South Pacific and variations in intensity of jet streams induce anomalous circulation over South America, which are associated with the track and persistence of transient systems over Southeastern South America and with convection over the continent. KEY WORDS blocking events; Antarctic Oscillation; Southern Hemisphere Received 16 February 2011; Revised 18 April 2013; Accepted 27 April Introduction Blocking events are characterized by intense and persistent anticyclones centered between 50 and 65 in both hemispheres. These systems are important because they induce adverse weather conditions associated with persistent atmospheric circulation in some regions and large displacements of meridional transient eddies. In recent years, a growing number of studies have been developed with the aim of providing blocking climatologies for the entire Southern Hemisphere (SH), or for specific sectors of the SH. From the methodological point of view, most of these analyses have used blocking definition algorithms based on either (1) geopotential height gradients (Lejenas, 1984; Tibaldi et al., 1994; Marques and Rao, 2000, 2001; Wiedenman et al., 2002; Adana and Colucci, 2005; Mendes et al., 2008), or (2) persistent positive anomalies of pressure (Trenberth and Mo, 1985; Ruthland and Fuenzalida, 1991; Sinclair, 1996; Renwinck, 1998, Renwinck and Revell, 1999; Montecinos and Aceituno, 2003; Renwinck, 2005). Berrisford et al. (2007) mention that the term blocking is generally used * Correspondence to: M. C. D. Mendes and I. F. A. Cavalcanti, Instituto Nacional de Pesquisas Espaciais, Centro de Previsão de Tempo e Estudos Climáticos, Rodovia Presidente Dutra, SN, Km 40, Cachoeira Paulista, São Paulo, Brazil. s: monica.damiao@gmail.com.br; iracema@cptec.inpe.br to describe an atmospheric phenomenon in which a large, quasi-stationary anticyclone develops in midlatitudes and persists for several days or longer, blocking the ambient westerly winds and weather systems. Previous works dealing with the whole hemisphere (Lejenas, 1984; Trenberth and Mo, 1985; Kayano and Kousky, 1990; Ruthland and Fuenzelida, 1991; Renwinck and Revell, 1999) agree on the predominance of blocking events over the Australian-New Zealand sector, followed by cases over the Southeastern Pacific (SEP) and a considerably smaller number of events occurring over the Southern Atlantic and over the Indian Oceans. More intense and persistent anticyclonic blockings were found southeast of New Zealand and southwest of South America (Sinclair, 1996). Mendes et al. (2008), using NCEP/NCAR reanalysis data for 41 years, concluded that the largest impacts of blocking events in the SEP and Southern Atlantic sectors on temperature and precipitation of South America were observed over the southern regions of the continent. Southeastern Pacific and Atlantic blockings presented opposite influences on temperature and precipitation over southern South America. During SEP blocking events [South Atlantic (SAT)], temperatures were significantly higher (lower) than the climatological values in southern Brazil, Northern Argentina, Uruguay and Paraguay, and lower (higher) than climatology in the extreme Southern America region. The presence of blocking in both areas, favours negative anomalous precipitation over extreme 2013 Royal Meteorological Society

2 530 M. C. D. MENDES AND I. F. A. CAVALCANTI southern South America. SEP (SAT) blocking indicated a decrease (increase) of precipitation over southeastern Brazil. The Antarctic Oscillation (AAO) and its Northern Hemisphere analogue, the Arctic Oscillation (Thompson and Wallace, 1998), correspond to an oscillation that is identified between middle and high-latitude largescale pressure and geopotential fields. They are also called the Northern and Southern Annular Modes, due to their zonal annular structure. Several studies have been done to investigate the Southern Annular Mode influence on mid-latitude and subtropical climates of the Southern Hemisphere (Gong and Wang, 1999; Thompson and Wallace, 2000; Carvalho et al., 2005; Vasconcellos and Cavalcanti, 2010). This mode is characterized by a zonally symmetric configuration with alternating positive and negative phases between high and mid latitude regions. Since the positive and negative phases are related to changes in the circulation at middle and high latitudes, the objective of this study is to investigate the relationship between AAO and blocking occurrences over the South Pacific and South Atlantic Oceans. The data and the criteria used to define blocking events are introduced in Section 2. In Section 3 the blockings are analysed in each phase of the AAO, with discussion of their characteristics and influences. Section 4 presents analyses of the atmospheric configurations, and conclusions are shown in Section Data and method The analyses consist on a discussion of the blocking configurations during the four seasons and the frequency of events and blocked days with respect to the annual cycle and interannual variability in the two AAO phases. The blocking detection criterion is applied to daily geopotential height at 500 hpa in the area bounded by 180 W 10 E and 30 S 65 S provided by the ECMWF reanalysis ERA 40 (Uppala, 2002). The ERA 40 resolution is (longitude, latitude) and was regrided to 2.5 (latitude, longitude) to maintain the same criterion of Mendes et al. (2008) who used this resolution to identify the blockings. The diagnostics of blocking compiled in this study consisted on the number of events, their duration and onset location. The study has been carried out using the methodology developed by Tibaldi and Molteni, (1990), Tibaldi et al. (1994) and extended by Trigo et al. (2004) for the Northern Hemisphere and Mendes et al. (2008) for the Southern Hemisphere. Therefore, two 500 hpa geopotential height meridional gradients GHGS (south) and GHGN (north) are evaluated for 2.5 longitude intervals over the areas of Southwestern Pacific (SWP), Southeastern Pacific (SEP) and South Atlantic (SAT) oceans, indicated in Mendes et al. (2008). GHGS = Z (λ, φ S ) Z (λ, φ 02 ) (1) GHGN = Z (λ, φ 01 ) Z (λ, φ N ) (2) where φ N = 40 S+ ; φ 01 = 55 S+ ; φ 02 = 50 S+ ; φ S = 65 S+ ; = 10.0, 7.5, 5.0, 2.5,0. The Z (λ,φ) is 500 hpa height at longitude λ and latitude φ and is the latitudinal interval. Then, following the procedure used in Mendes et al. (2008), a given longitude is defined as blocked at a specific instant in time if the following conditions are satisfied for at least one value of : (1) GHGN > 0 and (2) GHGS < 10 m. A sector (SWP, SEP and SAT) is considered to be blocked on a particular day if three or more adjacent longitudes, within the study area, are blocked (Mendes et al., 2008). This criterion is sufficient to define a local (spatial) blocking pattern. However, the definition of a true synoptic blocking requires the specification of a certain persistence time for the event. The typical duration of blocking events varies between 5 and 30 d (Treidl et al., 1981; Tibaldi and Molteni, 1990). Here, we adopted a threshold of five consecutive days considered as blocked according to the circulation constraints defined by Equations (1) and (2), also used in Mendes et al. (2008). The AAO monthly index was obtained from the Climate Prediction Center (CPC) at NCEP ( cpc.ncep.noaa.gov/products/). This index is obtained by projecting the 700 hpa geopotential height anomaly onto the leading mode (EOF-1) and normalizing by the standard deviation of the monthly index in the period. Outgoing long-wave radiation (OLR) obtained from NOAA (Liebmann and Smith, 1996) and wind fields from ERA40 were used to study the dominant patterns of convection and circulation anomalies in blocking periods of each AAO phase. The blocking events and blocked days for each AAO phase were obtained based on the following steps: i The blocking events and blocked days were identified for each month during ii The events and blocked days were separated into two groups depending on the AAO phase of each case. The selection of blocking events for each phase of the AAO oscillation was obtained by analysing the monthly signal of the AAO index. If the blocking periods (blocked days) occurred during a month with positive (negative) sign of the AAO, the events were classified as positive (negative). The intensity of positive and negative indices was not taken into account. If the blocking event extended between months, it was classified as a single event and assigned the AAO sign of the month in which the majority of blocked days occurred. For example, for the blocking event occurring between 29 July and 14 August 1986 (Marques and Rao, 1999), the event was classified with the AAO sign of August Composites of the anomalous variables were analysed for the blocked days in each season. The anomalies were obtained taking into account the daily climatologies (during 22 years) of each variable. The significance of composites was tested with

3 THE RELATIONSHIP BETWEEN THE AAO AND BLOCKING EVENTS 531 Figure 1. Composites of 500 hpa anomalous geopotential height (shaded, gpm) for blocking events over the Southwestern Pacific region in: (a, e) summer, (b, f) autumn, (c, g) winter and (d, h) spring. The blocking events of the AAO negative phase are presented in the left-hand side panels (a d) and those of the positive phase in the right-hand side panels (e h). The Southwestern Pacific blocking area is located between 180 W and 120 W. A t-test at the 95% confidence level (shaded) was applied. a t-test hypothesis for the equal means. This test was also used by Hansen et al. (1993) in composite analysis. The annual frequency of blocked days is calculated as: F DAYS = (bdays/ndays) 100, where bdays is the number of blocked days and ndays is 365 or 366 (number of days in a year) discussed in Table 2. The distribution of number of blocked days per year is given by the box plot shown in Figure 4. The monthly frequency of blocked days is calculated as: (total number of blocked days in each month that was in one AAO phase during the 22 years)/(total number of days in each AAO phase) 100, discussed in Figure Blocking features in the two AAO phases Typical patterns of blocking events between 1979 and 2000 are represented in the composites for each AAO phase (Figures 1 3); the regions are defined as SWP, 180 W 120 W; SEP, 120 W 80 W and SAT, 80 W 10 E. During the blockings over the SWP region during the negative AAO phase (Figure 1, left), the positive geopotential height anomalies associated with the blocking anticyclone extend to the pole, except in autumn. The middle latitude opposite signal is seen mainly over the Pacific and Atlantic Oceans. In the positive AAO phase, negative anomalies are located

4 532 M. C. D. MENDES AND I. F. A. CAVALCANTI Figure 2. Composites of 500 hpa anomalous geopotential height (shaded, gpm) for blocking events over the Southeastern Pacific region in: (a, e) summer, (b, f) autumn, (c, g) winter and (d, h) spring. The blocking events of the AAO negative phase are presented in the left-hand side panels (a d) and those of the positive phase in the right-hand side panels (e f). The Southeastern Pacific blocking area is located between 120 Wand 80 W. A t-test at the 95% confidence level (shaded) was applied. over the pole and the blocking anticyclone is part of a wavenumber 3 pattern (Figure 1, right). The composite of blockings over the SEP region during negative AAO phase (Figure 2) also shows the highest positive geopotential anomalies associated with the blocking anticyclone. Although there are also negative anomalies at middle latitudes, there is less annular configuration than in the SWP blocking. In this case the blocking has a dipole pattern with cyclonic anomalies to the north, and also to the west and east of the blocking ridge. In the positive phase the blocking anticyclone is closer to the extreme south of South America than in the negative phase and the cyclonic center to the east extends to the pole. The blocking patterns over the SAT region in each AAO phase are presented in Figure 3. During all seasons with negative AAO (Figure 3, left), negative anomalies of geopotential at 500 hpa over southeastern South America, which represent the cyclonic center of the blocking dipole pattern, can be associated with the passage of

5 THE RELATIONSHIP BETWEEN THE AAO AND BLOCKING EVENTS 533 Figure 3. Composites of 500 hpa anomalous geopotential height (shaded, gpm) for blocking events over the South Atlantic region in: (a, e) summer, (b, f) autumn, (c, g) winter and (d, h) spring. The blocking events of the AAO negative phase are presented in the left-hand side panels (a d) and those of the positive phase in the right-hand side panels (e h). The South Atlantic blocking area is located between 80 W and 10 E. A t-test at the 95% confidence level (shaded) was applied. transient systems over the region. In the positive phase, the negative geopotential anomalies (associated with transient systems) affect the southwest coast of South America during spring and summer; while in the autumn and winter, affect southeastern South America (Figure 3, right). This behaviour is related to the position of the blockings that are identified close to the southwestern South America coast in spring and summer and close to the southeastern coast in autumn and winter. In the negative AAO phase of winter, the blocking high is close to the southeastern coast of South America and it is stronger than in the positive phase. The number of events and blocked days in the two phases are shown in Table 1 for the three regions (SWP, SEP and SAT). The maximum number of events and blocked days over the SWP and SAT occur in the positive phase, when a wavenumber three is observed. The presence of a wave three pattern related to blockings was discussed by Trenberth and Mo (1985) and Cavalcanti (2000). However, this pattern is not present during the negative phase of AAO. In SEP, although the persistence is higher in the negative phase (higher number of blocked days), the difference in number of events between the two phases is not high. The similar number of events can be explained by the similar configuration close to South America in the two phases (blocking highs over the Southeast Pacific region and troughs to the west and east of the continent). The higher persistence in the negative

6 534 M. C. D. MENDES AND I. F. A. CAVALCANTI Table 1. Total number of blocked days and events over the Southwestern Pacific, Southeastern Pacific and South Atlantic regions for each AAO phase. (a) Sectors Blocked days Events 1979/2000 AAO AAO AAO AAO SWP 989 * * 92 SEP * * SAT 303 * * 31 * Refers to the maximum number in each phase. phase may be related to the AAO configuration (positive geopotential anomaly at high latitudes), which supports the occurrence of a blocking high. The frequency distribution of number of blocked days/year for each area is shown in Figure 4. Over SWP both phases show similar values for the median (around 9) and for the upper quartile (around 11). However, the distribution is larger in the positive phase. For the SEP blocking the median duration is around 8 (7) in the negative (positive) phases and the upper quartile is around 11 (9) days. Over SAT the median is 6 (5) in the negative (positive) AAO phase and the upper quartile is 7 in both phases. These distributions confirm that events over the SEP region tend to persist more in the negative than in the positive AAO phase, although longer durations are found in the SWP area. Figure 5 shows the number of events and percentage of blocked days relative to the total number of days for the whole period in each month, in each AAO phase. The number of events over the SWP presents monthly variations, with maximum in June (September) in the negative (positive) phase (Figure 5(a)). However, in the negative (positive) phase the maximum number of blocked days occurs in June and August (August and September), Figure 5(b). The AAO also affects differently the annual cycle of SEP blocking, i.e. the negative phase favours maximum number of events and blocked days in June while the positive phase favours both maximum in July (Figure 5(c) and (d)). It is seen that the largest number of events and longer durations in winter occur in the negative phase, in the SEP. High negative correlation of wave breaking index, which can be related to blocking, with the AAO index was found in the winter season by Berrisford et al. (2007) near New Zealand, but not over SEP. Over the SAT there is larger number of events in July in the positive phase, but blocking occurrence is less frequent than in the other two regions, in both phases. In the negative phase, the maximum number of events occurs in May, June and October, while in the positive phase, larger number is observed in July (Figure 5(e)). The number of blocked days is high in May, June, August and October during the negative phase and in May, July and September during the positive phase (Figure 5(f)). The interannual distribution of number of blocked days and events presented high variability for both negative and positive phases (Table 2). It is seen a large number (b) (c) Figure 4. Box-plot diagram of statistical properties of number of blocked days/year in both AAO phases for (a) Southwestern Pacific, (b) Southeastern Pacific and (c) South Atlantic regions. of blocked days and events in El Niño years in SWP and SEP in both AAO phases. The El Niño (EN), La Niña (LN) and Neutral (N) years with moderate to strong intensity were extracted from CPC-NCEP-NOAA, for the period of June 1979 to July Therefore, the El Niño years are 1982, 1983, 1986, 1987, , 1997; the La Niña years are 1984, 1988, 1995, and the Neutral years are , 1985, , The highest numbers of blocked days (above the 75th percentile of days in each AAO phase) are highlighted in Table 2 with bold italic. El Niño and La Niña years are represented by light and dark gray, respectively. In the SWP region, there were more blocked days in 1980, 1983, 1986, 1987, 1991 and 1994 in the negative phase, while more blocked days (above 75% percentile) occurred in 1979, 1982, 1990, 1992, 1993 and 1997 in the positive phase. For the SEP the number of blocked days was above the 75th percentile in 1986, 1987, 1991, 1992, 1994 and 1997 in the negative phase, and 1979, 1982, 1986, 1993, 1997 and 1999 in the positive phase. Over the SAT Ocean the years with number of blocked days above the 75th percentile were 1981, 1984, 1988,

7 THE RELATIONSHIP BETWEEN THE AAO AND BLOCKING EVENTS 535 Figure 5. Annual cycle of the blocking events (left) and frequency of the blocked days (right) for (a, b) Southwestern Pacific, (c, d) Southeastern Pacific and (e, f) South Atlantic regions in the period. The negative phase of the AAO is represented by black columns; the positive phase by gray columns. 1992, 1996 and 1997 in the negative phase, while in the positive phase they occurred in 1982, 1989, 1993, 1996, 1998 and During the negative AAO phase, the maximum number of blocked days occurred over the SWP and SEP during the same years of 1986, 1987, 1991 and 1994 while in the positive phase the maximum activity of blocking occurred in 1979, 1982, 1993 and 1997 in both sectors. In 1982 and 1993, there was also maximum number of blocked days during the positive phase over the SAT region. The increase of Southern Hemisphere blockings in El Niño years have been discussed by others authors such as Renwinck (1998), Renwinck and Revell (1999), Wiedenman et al. (2002), Mendes et al. (2008). The mechanisms of this relation were the modification of the atmospheric circulation in the extratropics by Rossby wave propagation from the equatorial anomalous convection during El Niño. On the other hand, the negative (positive) AAO phase was related to El Niño (La Niña) events in Carvalho et al. (2005). 4. Atmospheric characteristics of blocking composites in the two AAO phases The patterns of circulation and convection for the blocking periods over SWP, SEP and SAT are shown in Figures Figures 6(a) (h) and 8(a) (h) present the zonal wind anomalies and wind anomalies at 200 hpa, and Figures 7(a) (h) and 9(a) (h) show wind anomalies at 850 hpa and AOLR of blocking events composites that occurred in the SWP (SEP) regions during negative (left) and positive (right) AAO phases, between 1979 and

8 536 M. C. D. MENDES AND I. F. A. CAVALCANTI Table 2. Total number of annual events and blocked days over the Southwestern Pacific, Southeastern Pacific and South Atlantic regions for each AAO phase (italic numbers represent numbers greater than 75th percentile in each phase). El Niño (La Niña) years are indicated by light (dark) gray box. Years SWP SEP SAT Days Fdays Event Days Fdays Events Days Fdays Event Days Fdays Events Days Fdays Event Days Fdays Events TOTAL PERCENTILE 75 (P75) PERCENTILE 90 (90) El Niño years are indicated by Italic values and La Niña years are indicated by bold values.

9 THE RELATIONSHIP BETWEEN THE AAO AND BLOCKING EVENTS 537 Figure 6. Composites of zonal wind anomalies at 200 hpa (shaded, m s 1 ) and wind anomalies at 200 hpa (vector, m s 1 ) for the blocking events of the Southwestern Pacific region during (a d) negative phase and (e h) positive phase of AAO, in summer (a, e), autumn (b, f), winter (c, g), spring (d, h) Similar fields for blocking events in the SAT Ocean are shown in Figures 10 and 11. Anomalous winds at 850 and 200 hpa were used to investigate variations in the atmospheric circulation and the intensities and position of jet streams, whereas the anomalies of outgoing longwave radiation (AOLR) were used to study relationships between the AAO phases and the convective activity over South America during the blocking periods. The fields of the wind anomaly composites are consistent with the blocking patterns observed in distinct phases of the AAO (composites of geopotential anomalies at 500 hpa, Figures 1 3). The configuration in the blocking anticyclonic region of SWP is represented by weaker winds bounded by strengthened westerlies at 200 hpa, which are stronger, at the equatorward side, in the negative AAO phase than in the positive phase in summer, spring and winter (Figure 6(a) (d)). This implies strengthening of the subtropical jet in the negative phase. However, in the positive phase, there is a strengthening of the polar jet in the four seasons (Figure

10 538 M. C. D. MENDES AND I. F. A. CAVALCANTI Figure 7. Composites of Outgoing Long-wave Radiation anomalies (shaded, Wm 2 ) and wind anomalies at 850 hpa (vector, m s 1 ) for the blocking events of the Southwestern Pacific region during (a d) negative phase and (e h) positive phase of AAO, in summer (a, e), autumn (b, f), winter (c, g), spring (d, h). A t -test at the 95% confidence level (shaded) was applied are shown. 6(e) (h)), consistent with the circulation anomalies associated with the AAO pattern. The circulation anomalies, associated with the blocking activity over SWP, extend barotropically from the low to high levels of the atmosphere. Similar blocking features are seen at 850 hpa (Figure 7), 500 hpa (Figure 1) and 200 hpa (Figure 6). During summer and autumn, AOLR over southern/southeastern South America are opposite in each phase, when there are blocking patterns over SWP (Figure 7(a), (b), (e) and (f)). Convection occurs over these regions in the negative phase (negative AOLR) and dry conditions occur in the positive phase (positive AOLR). Although the blocking configuration is similar in the SWP region in both phases, the atmospheric circulation close to South America is different in each phase. In the positive phase, a PSA-type wavetrain, in the summer and autumn (Figure 6(e) and (f)) induces anticyclonic/cyclonic anomalies over the continent, which result in the northern-southern OLR dipole in Figure 7(e) and (f) (dry in the south and wet in the north). This result

11 THE RELATIONSHIP BETWEEN THE AAO AND BLOCKING EVENTS 539 Figure 8. Composites of zonal wind anomalies at 200 hpa (shaded, m s 1 ) and wind anomalies at 200 hpa (vector, m s 1 ) for the blocking events of the Southeastern Pacific region during (a d) negative phase and (e h) positive phase of AAO, in summer (a, e), autumn (b, f), winter (c, g), spring (d, h). is consistent with the occurrence of this dipole in the positive AAO phase in DJF, discussed by Vasconcellos and Cavalcanti (2010) in cases of extreme precipitation in the SACZ region. In the negative phase of the summer season (Figure 6(a)) the atmospheric circulation anomalies are not well organized in a wavetrain pattern during SWP blocking. Instead, negative OLR anomalies over southern South America (Figure 7(a)) are associated with the path of transient systems following the trough and the subtropical jet. In autumn (Figure 6(b) and (f)) there are organized PSA-type wavetrains in both phases, but the anomalous centres close to South America display different configurations, which imply opposite anomalous convection over the continent (Figure 7(b) and (f)). In winter the differences of subtropical jet intensity between the two phases (Figure 6(c) and (g)) explain the convective activity over the continent in the negative phase, also related to the persistence of synoptic systems in that region (Figure 7(c) and (g)). The synoptic systems tend to follow the baroclinic regions that are identified by the jet

12 540 M. C. D. MENDES AND I. F. A. CAVALCANTI Figure 9. Composites of Outgoing Long-wave Radiation anomalies (shaded, Wm 2 ) and wind anomalies at 850 hpa (vector, m s 1 ) for the blocking events of the Southeastern Pacific region during (a d) negative phase and (e h) positive phase of AAO, in summer (a, e), autumn (b, f), winter (c, g), spring (d, h). A t -test at the 95% confidence level (shaded) was applied are shown. regions. The subtropical jet intensified over the Pacific (in the negative phase) is the guide to the position of systems at lower latitudes, driving them over the continent. In the positive phase, when the polar jet is stronger, the systems tend to move at higher latitudes. In spring (Figure 7(d) and (h)) both phases display convective activity over southern but there are opposite conditions over tropical South America. During SEP blocking episodes, the subtropical jet extends over South America and is stronger in the negative phase, in the autumn and spring (Figure 8(a) (d) and (e) (h)). Similar to the SWP case, the wind pattern extends barotropically from the low to high levels of the atmosphere. The southerly winds at low levels, derived from the blocking high, are farther west in the negative phase (Figure 9(a) (d)) in relation to the positive phase composites, except in the winter (Figure 9(e) (h)). Therefore, the circulation anomalies are situated in different positions, influencing the ascending motion in different places. In the summer there is close to climatological convection over the continent in the negative phase, but in the other seasons negative OLR anomalies are seen over southern and southeastern South America, consistent with the jet stream and passage of

13 THE RELATIONSHIP BETWEEN THE AAO AND BLOCKING EVENTS 541 Figure 10. Composites of zonal wind anomalies at 200 hpa (shaded, m s 1 ) and wind anomalies at 200 hpa (vector, m s 1 ) for the blocking events of the South Atlantic region during (a d) negative phase and (e h) positive phase of AAO, in summer (a, e), autumn (b, f), winter (c, g), spring (d, h). synoptic systems to the north of the blocking high (Figure 9(b) (d)). During this phase, reduction of convection or clear skies occurs in the tropical continental areas of South America. This is related to the strong subtropical jet pattern (northwest-southeast) that prevents frontal system penetration into the central regions of South America. However, it favours convective activity associated with synoptic systems in the southern regions. Except in the summer, a wavetrain pattern from the blocking high over the SEP region establishes a cyclonic anomaly close to southern South America, favouring convection there (Figures 8(b) (d) and 9(b) (d)). In the positive phase, convection occurs during summer and spring in the southern regions (Figure 9(e) and (h)), when the blocking dipole is close to the continent, while positive OLR anomalies are seen in

14 542 M. C. D. MENDES AND I. F. A. CAVALCANTI Figure 11. Composites of Outgoing Long-wave Radiation anomalies (shaded, Wm 2 ) and wind anomalies at 850 hpa (vector, m s 1 )forthe blocking events of the South Atlantic region during (a d) negative phase and (e h) positive phase of AAO, in summer (a, e), autumn (b, f), winter (c, g), spring (d, h). A t -test at the 95% confidence level (shaded) was applied are shown. autumn and winter in central and southeastern South America (Figure 9(f) and (g)). In general, the wavetrain from the blocking high is displaced eastward, in the positive phase, compared to the negative phase (Figure 8). The SAT blockings configuration in the high wind fields are more organized in the positive phase and the westerlies are weaker than in the negative phase over southern South America, except in autumn. The blocking favours a more northward position of the subtropical jet over South America compared to the climatological position in both phases of the AAO (Figure 10). The region of weak westerlies (negative zonal anomalies) is located over southern South America in both phases, and the positive OLR anomalies in extreme southern South America are consistent with clear skies in the blocking area (Figure 11). Convection over the continent in the two phases can be related to the persistence of systems

15 THE RELATIONSHIP BETWEEN THE AAO AND BLOCKING EVENTS 543 in those regions, favoured by the jet stream position and related to blocking conditions to the south. 5. Conclusion The blocking patterns and frequencies during opposite AAO phases were investigated for the period 1979 to The results show that the AAO influences the atmospheric configurations associated with blocking events and frequency of blocked days. There is a maximum activity (blocked days and events) in the positive phase of the AAO over the SWP and SAT regions, while the frequency of days over the SEP region is similar in the two phases but the number of blocked days is higher in the negative phase. The blocking highs in the SWP and SAT regions in the positive AAO phase are located in the middle latitude band of positive geopotential anomaly and display a wavenumber three pattern, which is one of the preferred modes of variability in the Southern Hemisphere. Therefore, the frequency of events and blocked days is higher during this phase. However, the higher persistence of a blocking high in the SEP region in the negative rather than the positive phase is supported by the extension of the polar positive geopotential anomaly, which occurs during this phase. In general, in the positive phase, blocking highs are located in the middle latitude AAO region and in the negative phase they are extensions of the polar AAO region. The highest frequencies of blocking events during the year occur in winter and beginning of spring in the three regions. The events have higher frequencies later in the positive phase than in the negative phase in SWP and SEP. There was large interannual variability in the number of blocked days in the three regions, and maximum numbers tend to occur in El Niño years, consistent with findings of previous studies. Although there are occurrences in both phases, there are more blocked days in the negative phase, consistent with Carvalho et al. (2005). The SWP blocking events impacts on South America are identified in summer and autumn, mainly in the positive phase, when convection anomalies are of opposite sign in OLR over southeastern (positive) versus eastern (negative) South America. During SEP blocking events, the impacts are larger in the spring, different from the SWP analysis that did not show impact during this season. In SEP blocking events, differences in circulation in the two AAO phases result in different positions of anomalous convection over the continent. During SAT blocking, impacts are seen over extreme southern South America (dry conditions) and central or southeastern South America (wet conditions) in all seasons, stronger in the negative phase. In opposite AAO phases, blockings in the three regions affect convection over South America in different manners. The differences are related to the intensity of subtropical jet and wavetrains from the blocking area to South America. The subtropical jet is more intense in the negative phase than in the positive, while the polar jet is stronger in the positive phase, consistent with the AAO configuration. The jet positions have influence on the track of synoptic systems that affect the precipitation over the continent. The anomalous centres of the wavetrains in different positions over South America in each AAO phase also affect the development of transient systems and convection over the region. Acknowledgements This work was supported by FAPESP through the project Influence of Antarctica Oscillation on atmospheric blockings and projections for future scenarios (08/ ). We thank NCEP/NOAA (National Centers for Environmental Prediction) for OLR data and AAO and ENSO information. Thanks also to ECMWF (European Centre for Medium Range Weather Forecasts) for providing the reanalysis data. 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