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1OCTOBER 2012 B E R M A N E T A L . 6781

Eastern Seasonal : Influence of Southern Hemisphere Circulation and Links with Subtropical American Precipitation

ANA LAURA BERMAN AND GABRIEL SILVESTRI Centro de Investigaciones del Mar y la Atmo´sfera, Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas and Departamento de Ciencias de la Atmo´sfera y los Oce´anos, Facultad de Ciencias Exactas y Naturales, FCEN, Universidad de , and Unidad Mixta Internacional: Instituto Franco-Argentino sobre Estudios de Clima y sus Impactos,* Buenos Aires,

ROSA COMPAGNUCCI Departamento de Ciencias de la Atmo´sfera y los Oce´anos, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, and Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Buenos Aires, Argentina

(Manuscript received 8 2011, in final form 8 2012)

ABSTRACT

Some aspects of the seasonal precipitation over eastern Patagonia, the southernmost area of east of the Cordillera, are examined in this paper. Results indicate that the central-north areas, the southern continental , and the southernmost islands are three independent of seasonal pre- cipitation, and that each of them is associated with specific patterns of atmospheric circulation. Precipitation over the central-north region is significantly related to the precipitation over a wide area of southern South America east of the Andes during the four . Enhanced (reduced) precipitation over this area is as- sociated with weakened (intensified) westerly flow in the region. Precipitation over the southern continental area has a close connection with the dipolar pattern of precipitation over subtropical South America during , , and . The anomalies of atmospheric circulation at low and upper levels associated with the subtropical dipole are also able to modulate the intensity of the over the south of eastern Patagonia, affecting the regional precipitation. Precipitation over the islands of the southernmost part of eastern Patagonia is connected with subtropical precipitation in summer and . The activity of frontal systems associated with migratory perturbations moving to the east along the Southern Hemisphere storm tracks modulates the variability of seasonal precipitation over this region.

1. Introduction the Southern Hemisphere (e.g., Paruelo et al. 1998; Garreaud 2009; Garreaud et al. 2009). The zonal atmo- Patagonia is a wide region extended over southern spheric flow and transport of humid air from the Pacific South America on both sides of the Andes Cordillera are blocked by the Andes Cordillera, provoking (Fig. 1). The regional atmospheric circulation at lower an important contrast between the dry conditions in and upper levels is strongly influenced by the typical the Argentinean region east of the Andes, referred to westerly flow of subtropical and subpolar of as eastern Patagonia (EPAT) and the humid Chilean side (Prohaska 1976). Regional precipitation is mostly associated with the activity of frontal systems linked * The Unidad Mixta Internacional (3351): Instituto Franco- with migratory surface moving to the east Argentino sobre Estudios de Clima y sus Impactos is sponsored by along the Southern Hemisphere storm tracks (e.g., the Centre National de la Recherche Scientifique, the Consejo Na- Trenberth 1991; Berbery and Vera 1996). Although cional de Investigaciones Cientı´ficas y Te´cnicas, and the Uni- versidad de Buenos Aires. precipitation takes place over the western slope of the Andes, it can over the mountains and fall over the eastern slope and adjacent areas (e.g., Hoffmann 1975). Corresponding author address: Ana Laura Berman, DCAO, FCEN, Universidad de Buenos Aires, Intendente Guiraldes 2160, Furthermore, part of the total amount of precipitation is Ciudad Universitaria, Buenos Aires C1428EGA, Argentina. of the orographic type induced by the upward move- E-mail: [email protected] ment of the low-level flow from the Pacific across the

DOI: 10.1175/JCLI-D-11-00514.1

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FIG. 1. Patagonia region (topography is shaded; units: meters). The five selected meteoro- logical stations and the CMAP grid points are indicated with white circles and black squares, respectively (see the text for more details). western slope of the mountains. The combination of interannual variability of precipitation over EPAT is processes associated with the orography and the atmo- an issue that has still not received adequate attention, spheric dynamics produces both the amount and vari- perhaps because EPAT has been historically a region ability of precipitation near the Andes that are higher little inhabited, where complete and reliable informa- than in areas far from the mountains (e.g., Jobba´gy et al. tion of precipitation is available for only a few meteo- 1995). Barrett et al. (2009) analyzed the influence of the rological stations. To our knowledge, there are few Andes Cordillera on the occurrence of both frontal and studies published in international journals focused on orographic precipitation over southern South America, the variability of precipitation over this region. Aravena finding an increment of total precipitation over the re- and Luckman (2009) made important progress describ- gion due to the high topographic barrier. Moreover, ing some characteristics of the spatiotemporal variability Garreaud (2007) showed negative (positive) correla- of annual precipitation on both sides of the Andes tions between the 850-hPa zonal wind and the monthly Cordillera. Gonzalez and Vera (2010) and Gonzalez precipitation over the east (west) side of southern An- et al. (2010) described the influence of the Indian and des, meaning that precipitation decreases (increases) Pacific on the interannual variability of winter with intensified westerly flow at the Argentinean (Chil- precipitation over northwestern EPAT. These works ean) side and vice versa [see also Fig. 13 in Compagnucci describe Rossby wave trains extending from both trop- (2011), provided by R. D. Garreaud 2011, personal ical basins toward southern South America, affecting communication]. Easterly winds and advection of mois- the precipitation over specific areas adjacent to the An- ture from the adjacent areas of the can des Cordillera. Moreover, precipitation trends on EPAT also produce daily precipitation over EPAT (e.g., Mayr were analyzed by Castan˜eda and Gonzalez (2008), find- et al. 2007a,b). ing that precipitation enhanced in areas of the north and Although the standard deviation of annual precipi- south but reduced in regions of the center and west tation has low magnitudes over EPAT, the interannual during the second half of the twentieth century. standard deviation normalized by the annual mean has Different authors analyzed forcings of precipitation a maximum over this region (see Fig. 7 in Garreaud et al. variability in areas that, among other regions, include 2009). Changes in the occurrence of precipitation in- EPAT. The pioneer study of Pittock (1980) describes duced by atmospheric circulation variability enhance the influence of the Southern Hemisphere circulation the vulnerability of areas, with dry climate like EPAT patterns on the annual precipitation over southern increasing desertification, water erosion, and soil com- South America. Alessandro (2005, 2008) analyzed the paction. Such processes can affect different systems (e.g., increment of precipitation over areas of the south of biosphere) or human activities, producing an important South America by effect of blocking episodes in the socioeconomic damage. However, the analysis of the surrounding oceans. Moreover, Schneider and Gies

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(2004) showed the influence of El Nin˜ o–Southern Os- Merged Analysis of Precipitation (CMAP) described by cillation (ENSO) events on southernmost South Ameri- Xie and Arkin (1997) is also used. The CMAP data can precipitation on both sides of the Andes Cordillera merge observations from gauges with precipitation through changes in the westerlies. Furthermore, different estimates from several satellite-based algorithms cov- authors have demonstrated that the interannual vari- ering the area of EPAT in a uniform grid of 2.58 lat- ability of precipitation over areas of EPAT is modulated itude 3 2.58 resolution (Fig. 1). An additional by the southern annular mode (e.g., Gillett et al. 2006; point that gives singular importance to the CMAP da- Silvestri and Vera 2009) and the Southern Oscillation taset is the fact that it allows the study of connections index (e.g., Pittock 1980; Kiladis and Diaz 1989; Aravena between the precipitation over areas of EPAT and and Luckman 2009). precipitation over other regions of South America. In the light of the previous comments, it is clear that Atmospheric conditions in the Southern Hemisphere there are some aspects of the variability of precipitation are described using monthly-mean fields of geopotential over EPAT that must be investigated to have a more height at 850 hPa (Z850) and 200 hPa (Z200) from complete comprehension of the climate variability in the National Centers for Environmental Prediction– this remote region of the world. In particular, the prin- National Center for Atmospheric Research (NCEP– cipal modes of spatiotemporal variability of seasonal NCAR) reanalysis (Kalnay et al. 1996). precipitation over EPAT, the associated anomalies of The study is made for the period 1979–2009 because it atmospheric circulation, and the relations with the pre- is the period covered by the available dataset of pre- cipitation over other areas of southern South America cipitation (NMSA and CMAP). Moreover, during this have not been analyzed in previous papers. Therefore, period the NCEP–NCAR reanalysis includes informa- the aim of this study is to investigate such characteristics tion from satellites, assuring a better quality of the data of the climate variability in southern South America. at high latitudes of the Southern Hemisphere. Results presented in this paper describe not only aspects Monthly anomalies are defined as departures from of the present climate in this region but also can be an the corresponding monthly means computed over the important contribution to understand the causes of the entire 1979–2009 period. Seasonal means of such monthly notorious climate changes that occurred in the past (e.g., anomalies are calculated for all dataset considering the Gilli et al. 2005; Haberzettl et al. 2005). seasons as in the Southern Hemisphere: – The article is organized as follows: data and method- (summer), – (autumn), – ology are described in section 2, the main results are (winter), and September– (spring). presented in section 3, and the conclusions are summa- b. Methodology rized in section 4. Linear correlations among the time series of seasonal precipitation registered in the five selected meteoro- 2. Data and methodology logical stations are analyzed as a first approach of the spatial homogeneity. a. Data The CMAP dataset was used in several investigations The National Meteorological Service of Argentina of precipitation variability over tropical and subtropical (NMSA) provided the monthly-mean values of pre- areas of South America (e.g., Zhou and Lau 2001; cipitation corresponding to the meteorological stations Berbery and Barros 2002), but there are no anteced- Trelew (TRE), Comodoro Rivadavia (CR), Rı´oGallegos ents about the use of this dataset in studies focused on (RG), and Ushuaia (USH) (Fig. 1). Although there are southern regions. Therefore, seasonal correlations be- more meteorological stations in the region under study, tween the precipitation from the meteorological stations most of them have large gaps or cover short periods. and the CMAP data are analyzed to demonstrate that Only the four selected stations have information with- the precipitation CMAP and the five stations represent out missing values in the daily data with which the the same time variability and spatial relations. Conse- NMSA constructed the monthly means. The Chilean quently, the analysis of the spatiotemporal variations station Punta Arenas (ARE) (Fig. 1) is not located in the of regional precipitation is made considering the area of EPAT, but it is included in the analysis for CMAP data. a better description of the seasonal precipitation over The principal modes of seasonal precipitation vari- southernmost South America. Monthly-mean precipi- ability are obtained by principal component analysis tation corresponding to the station ARE was taken from (PCA) applied to the CMAP data in the area of EPAT the Global Historical Climatology Network-Monthly (see Fig. 1) using a correlation input matrix among the (GHCN-M, version 3). The Climate Prediction Center grid points’ time series that is, namely, the S mode (e.g.,

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TABLE 1. Correlations between the time series of precipitation corresponding to the five meteorological stations depicted in Fig. 1. One (two) asterisk(s) indicate correlations that are statistically significant at the 95% (99%) level for a Student’s t test. There are no values significant at the 90% level.

Summer Autumn TRE CR RG USH TRE CR RG USH CR 0.65** CR 0.44* RG 20.10 20.15 RG 0.05 0.27 USH 20.07 0.03 0.03 USH 0.13 20.07 20.15 ARE 20.22 20.28 0.53** 0.25 ARE 20.20 0.10 0.56** 0.00

Winter Spring TRE CR RG USH TRE CR RG USH CR 0.58** CR 0.49** RG 0.58** 0.41* RG 0.05 0.08 USH 20.21 20.17 20.26 USH 20.10 0.26 20.12 ARE 20.01 0.01 0.48** 0.08 ARE 0.03 0.07 0.51** 20.15

Richman 1986). The analytic formulation as well as Regressions with Z850 and Z200 are performed with specific characteristics of this procedure were presented the standardized series of both variables because the PC and discussed by Compagnucci and Richman (2008). scores are standardized series. Therefore, regressions This methodology defines the pattern time series for presented in this paper do not have dimensions. A linear clusters of points with similar covariability [principal trend was removed from all analyzed time series. Special component (PC) scores] and the concomitant areas care was taken in the calculation of all correlations and where these pattern time series are representative (PC regressions, ensuring that neither was dominated by few loadings) (e.g., Castan˜ eda and Compagnucci 2005). specific extreme cases. Therefore, the PC scores of each mode, which are non- dimensional time series, can be considered as a time 3. Results index representing the precipitation variability in a spe- cific area of EPAT. Correlations among the five selected series of pre- The varimax orthogonal rotation (Kaiser 1958, 1959) cipitation describe a common pattern of spatial relations is applied to eliminate possible degeneration in the so- during summer, autumn, and spring (Table 1). In fact, lutions due to eigenvectors’ sampling errors (North et al. three common characteristics are detected in these 1982) and to improve the accuracy of the solutions seasons: 1) significant linear relations among the (Richman 1986). The number of retained components is northeastern stations TRE and CR, but both are not preview by the Scree test (Cattell 1966) and determined connected with the other stations; 2) significant re- through an iterative rotation from two-component re- lations among the southern stations RG and ARE, but tention and adding one more in each step until the both are independent of the other ones; and 3) the Tucker’s congruence coefficients, which test the result- southernmost station USH has no connection with any ing matches between components and the dataset, give other station. A similar pattern is observed in winter, values in the range of 0.92–0.82, considered the bor- with the only exception that the southern station RG derline match (Richman 1986). has a significant connection with the northern stations To capture both the explained variance and the rate of TRE and CR. These relations for seasonal precipi- variability, we calculate the correlation and regression tation are, in general, in agreement with the conclu- coefficients between the PC scores obtained from the PCA sions of Aravena and Luckman (2009) for annual of precipitation CMAP over EPAT (referred as PCA- means. The poor relation of the precipitation in Us- CMAP) and the precipitation over the South American huaia with the nearby stations Rio Gallegos and Punta south of 108S. The same relations are calculated Arenas could be associated with the very particular with the fields of Z850 and Z200 to analyze the anomalies location of this station. In fact, Ushuaia is located in the of atmospheric circulation at lower and upper levels, re- area of the Cordillera Darwin (see Fig. 1 in Schneider spectively. The analysis for the upper troposphere was also and Gies 2004), which means that very special condi- performed considering the streamfunction at 200 hPa tions can affect the local precipitation. from the NCEP–NCAR reanalysis, but the results are Relations mentioned in the previous paragraph are similar to those of Z200 even in tropical latitudes. suitably represented by the CMAP data. The spatial

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FIG. 2. Correlations between the time series of precipitation corresponding to the five meteorological stations depicted in Fig. 1 and the CMAP data during summer. Areas where positive (negative) values are statistically significant at the 90%, 95%, and 99% level of a Student’s t test are shaded in light, medium and dark gray, respectively. Contours: 60.2, 60.3, 60.37, 60.47, 60.60, and 60.80. Negative contours are dashed. correlation fields of each station’s precipitation time are significant because they are clearly outside of the series with CMAP data show that CMAP is consistent noise tail (not shown). Furthermore, the congruence with precipitation measured at the five meteorological coefficients of the iteratively varimax rotated solution stations. Figure 2 describes these relations during sum- are lower than the 0.82 limit if four or more component mer, and similar features are detected in autumn, winter, are retained. Therefore, we considered the first three and spring (figures not shown). Precipitation registered varimax rotated components of the PCA-CMAP, which at the stations TRE and CR have significant positive explain almost 70% of the total variance and define correlation with CMAP over a wide area of northern similar homogenous of precipitation vari- EPAT (Figs. 2a and 2b), precipitation at the stations RG ability in the four seasons (Fig. 3). These components and ARE have significant positive correlation with subdivide the precipitation CMAP over EPAT in re- CMAP over the southern portion of EPAT (Figs. 2c and gions centered near the location of the meteorological 2d), and precipitation at USH has significant positive stations Trelew, Rio Gallegos, and Ushuaia, coinciding correlation with CMAP over the island of Tierra del with the subregions inferred from Table 1 and Fig. 2. Fuego (Fig. 2e). Moreover, correlation among time series The corresponding PC scores are depicted in Fig. 4. of precipitation at the meteorological stations and CMAP These time series represent the temporal variability of corresponding to the closest point to each station (Fig. 1) standardized anomalies of seasonal precipitation in results, in all cases, higher than 0.85. Such specific series each . In particular, the occurrence of ex- of CMAP precipitation have among themselves the same treme seasonal precipitation can be detected consid- patterns of relation asthose described by Table 1 for the ering such series. meteorological stations (not shown). A more detailed Center and north of EPAT is a homogeneous area analysis is performed in the following subsection, but this defined by the component that explains more variance in preliminary description indicates that the gridded CMAP summer, winter, and spring, whereas it is the second in precipitation reproduces the main characteristics of time order of explained variance in autumn (pattern 1 in Fig. 3). variability and spatial relations detected by the meteo- This component has strong positive correlation with the rological observations in EPAT. Therefore, the leading precipitation over a broad area of the surrounding At- modes of seasonal precipitation over EPAT and the lantic, the Pampa region (center of Argentina), and the correlations extended over the South American conti- eastern Pacific in 258S–408S. nent are obtained from the CMAP dataset. These modes The variability of precipitation over most of the of precipitation variability and the associated anomalies continental area of EPAT south of 458S corresponds to of atmospheric circulation are described for each the second component in order of explained variance in the following subsections. in all seasons except in autumn, when it is the com- ponent of higher variance (pattern 2 in Fig. 3). Except a. Subregions of EPAT precipitation and links with in winter, this subregion has strong connections with subtropical areas the dipolar pattern of precipitation typical of sub- The Scree test for the eigenvalues of the PCA- tropical South America east of the Andes. In fact, CMAP suggests that only the first three components significant positive values of correlation are observed

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FIG. 3. Patterns of seasonal correlations between the first three PC score time series of the precipitation in EPAT and the CMAP data. Areas where positive (negative) values are statistically significant at the 90%, 95%, and 99% level of a Student’s t test are shaded in light, medium, and dark red (green), respectively. Contours: 60.2, 60.3, 60.37, 60.47, 60.60, and 60.80. The variance explained by each PCA- CMAP mode is indicated in the corresponding panel.

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FIG. 4. PC scores corresponding to the PCA-CMAP patterns depicted in Fig. 3 for pattern 1: blue line, pattern 2: red line, and pattern 3: green line. in the northeast of Argentina, southern , west- nonsignificant relations over most of subtropical South ern , and north of [the region re- America and the adjacent Pacific Ocean during sum- ferred to as southeastern South America (SESA)] mer (Fig. 5a1). Positive relations are observed at mid- during the period spring–summer–autumn. In con- and subpolar latitudes being significant in areas of the trast, significant negative correlations are detected in southern Pacific and Atlantic Oceans. A similar structure an elongated band extending from the coast of Brazil of atmospheric circulation over Patagonia is detected in at 208S to, at least, 358S in the surrounding Atlantic. the upper troposphere, where the magnitudes in the Negative correlations with precipitation over the center of negative correlations over the southern con- eastern part of the central Andes onto the plateau tinent are significant (Fig. 5b1). Significant relations are known as the Bolivian are also detected in also detected at all levels between 700 and 200 hPa (not summer and autumn. shown), indicating that weakened westerlies take place in The variability of precipitation over the surroundings the entire troposphere. These relations imply weakened oftheislandofTierradelFuego at the southernmost part (intensified) westerly flow at low levels over southern of EPAT defines the third subregion explaining less vari- South America associated with more (less) precipitation ance in the four seasons (pattern 3 in Fig. 3). During over the center-north of EPAT. summer, inverse connections are observed with the var- In autumn, the relations of this subregion with the iability of precipitation over the surroundings of the Andes atmospheric circulation at low and upper levels are in 208–408S and over the Bolivian Altiplano. Moreover, different from those observed in summer. In fact, the inverse relations are detected with precipitation over the negative relations at low levels over the continental Pampa region and the adjacent Atlantic in winter. subtropical region are located over the western sector of the continent and over the Pacific, shifted to the west b. Anomalies of atmospheric circulation related to with respect to the position in summer (Fig. 5a2). Fur- EPAT precipitation thermore, the center of positive relations located at high latitudes over the Pacific is shifted to the east, forming 1) PATTERN 1 a coupled meridional dipole with the subtropical nega- Correlations between precipitation over the center- tive center extended across the entire troposphere north of EPAT (pattern 1 of Fig. 3) and the atmo- (Fig. 5b2). These significant anomalies imply weakened spheric circulation at low levels (Z850) show negative low-level westerlies over Patagonia, producing more

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FIG. 5. Seasonal correlations between the first PC score time series of the precipitation CMAP in EPAT (corre- sponding to pattern 1 in Fig. 3) and (left) Z850 and (right) Z200. Areas with statistically significant positive (negative) values at 90%, 95%, and 99% level for a Student’s t test are shaded in light, medium, and dark red (green), respectively. Solid (dashed) black lines indicate positive (negative) regressions (contours: 0.1; see text for more details). precipitation over northern EPAT. Moreover, the neg- of the Andes that could reach the north of EPAT, ative correlations with low-level circulation in north- producing more precipitation in this region. Inverse western Argentina suggest an intensification of the anomalies of atmospheric circulation are associated with northerly transport of humid air over areas to the east less precipitation over EPAT.

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During winter, the anomalies of atmospheric circula- average subtropical and a reinforced north- tion and associated processes are similar to those de- westerly flow of warm and humid air toward SESA (e.g., scribed in autumn (Figs. 5a3 and 5b3). The structure of Vera et al. 2006 and references herein). Similar char- circulation in the lower troposphere corresponds to the acteristics were detected by Doyle and Barros (2002) in pattern described by Rutllant and Fuenzalida (1991) and the analysis of low-level water vapor transport during Compagnucci and Vargas (1998) associated with abun- anomalous precipitation events connected with the dant and frequent snowfalls over the central Andes the SACZ-SESA dipole. The studies of Doyle and Barros winter after an El Nin˜ o year. (2002) and Diaz and Aceituno (2003) were focused on The connection between this mode of precipitation the variability of convective cloudiness and precipitation variability and a dipolar structure of atmospheric cir- over subtropical South America east of the Andes. In culation extended through the whole troposphere with contrast, our analysis has been focused on the variability centers at subtropical and subpolar latitudes of the of precipitation over the southernmost areas of the eastern Pacific persists during spring but with lower South American continent. However, we found that the values of correlation (Figs. 5a4 and 5b4). Therefore, circulation patterns associated with the precipitation enhanced (reduced) precipitation over this subregion of over EPAT are similar to those suggested by the other EPAT is associated with weakened (intensified) west- authors for the area SESA-SACZ. These connections erlies in the area. between EPAT and SESA-SACZ have not been pre- viously addressed in the scientific literature. 2) PATTERN 2 The anomalies of low-level circulation in autumn are During summer, the low-level circulation associated quite similar to those observed in summer (Fig. 6a2). with precipitation over the southern continental area of The main differences are the lower magnitudes of neg- EPAT (pattern 2 in Fig. 3) is characterized by a cyclonic ative correlations over the continent and the increment center covering almost all of the South American con- of positive correlations at polar latitudes. The most tinent south of 208S (Fig. 6a1). Anticyclonic centers at prominent feature of the anomalous circulation at upper subpolar latitudes over the Amundsen and Bellings- levels is the structure of opposite phases such as those hausen and over the subtropical Atlantic near the typically associated with the southern annular mode South American coast are also observed. This pattern of (SAM) (Fig. 6b2). A wave train similar to the Pacific– correlation implies weakened (intensified) westerly flow South America 1 pattern (PSA1; Mo and Higgins 1998) over southern EPAT associated with enhanced (re- is also detected. These influences of the SAM and PSA1 duced) precipitation over the region. The structure of on the variability of precipitation over the southernmost upper-level circulation resembles the Pacific–South areas of South America east of the Andes during au- America 2 pattern (PSA2; e.g., Mo and Higgins 1998) tumn have not been described in previous studies. extending from the east of to southern South During winter, the anomalies of atmospheric circula- America and western Atlantic (Fig. 6b1). Previous tion associated with precipitation over the south of studies demonstrated the influence of PSA2 on the EPAT are different from those observed in summer and summer precipitation over tropical and subtropical autumn. In fact, the anomalous low-level circulation is South America (e.g., Mo and Paegle 2001), but our re- characterized by an anticyclonic center over the Drake sults show that this pattern of atmospheric circulation Passage and a cyclonic center extended in the southern also affects the precipitation over the southernmost Pacific reaching the Chilean coast (Fig. 6a3). Such areas of the continent. Moreover, this structure of low- a pattern of atmospheric circulation can be associated and upper-level circulation agrees with the pattern de- with weakened westerlies over southernmost South scribed by Diaz and Aceituno (2003) for episodes of America, favoring the increment of precipitation over enhanced convective cloudiness over SESA. In fact, southern areas of EPAT. Inverse anomalies of circu- they demonstrated that enhanced over SESA lation imply intensified westerlies associated with less during austral spring and summer is associated with precipitation. The weak connection between precipi- weakened convection over the South Atlantic conver- tation over the south of EPAT and the regional at- gence zone (SACZ). The atmospheric conditions during mospheric circulation can be the cause of the lack of such episodes are mainly characterized by anomalous relation between precipitation over this region and over anticyclonic circulation at the middle and upper tropo- SESA (see Fig. 3b3). Moreover, the upper-level circula- sphere centered over the subtropical western Atlantic. tion does not show wave trains (PSA2, etc.) acting as This anomaly is part of a wavelike quasi-barotropic remote forcing of the variability of precipitation over structure extending across the Pacific from Australia to southern EPAT (Fig. 6b3). This characteristic could be , and it is connected with a stronger-than- explained by the results of Berbery et al. (1992), who

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FIG. 6. As in Fig. 5, but for the second PC score time series (corresponding to pattern 2 in Fig. 3). demonstrated that the latitudinal gradient of absolute In spring, the anomalies of atmospheric circulation at vorticity hinders the meridional propagation of waves both low and upper levels recover the characteristics emanating from the western tropical Pacific during aus- observed in summer and autumn. In fact, a center of tral winter. negative correlation (cyclonic anomalies) over southern

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South America and the adjacent Atlantic together with Although the correlations are not significant, the inverse a center of positive correlation (anticyclonic anomalies) relation between the precipitation and the upper-level over the Amundsen and Bellingshausen Seas are circulation over the Drake Passage and the southern detected at low levels (Fig. 6a4). A PSA1 is clearly de- Atlantic is also detected (Fig. 7b2). tected at upper levels as well as the typical structure of The cyclonic center extending into the entire tropo- the SAM (Fig. 6b4). Although the influence of the SAM sphere over the Drake Passage and the surrounding on the variability of precipitation over areas of southern South Atlantic is more intense during winter (Figs. 7a3 South America during austral spring was suggested by and 7b3). The frontal activity (cold fronts) associated Silvestri and Vera (2009), the simultaneous impact of with this anomalous circulation can explain the occur- this atmospheric mode and the PSA1 on EPAT has not rence of more precipitation over southernmost EPAT. been described in previous studies. The consequences of At the same time, the cyclonic circulation can reduce the these atmospheric anomalies on the precipitation over flow of moisture from the subtropical Atlantic to eastern EPAT and over subtropical areas of South America South America, explaining the inverse relation between have been discussed in the previous paragraphs. The precipitation over southernmost EPAT and precipi- dynamic mechanisms connecting the variability of pre- tation over SESA (see Fig. 3c3). cipitation over both regions of the continent (see Fig. The inverse relations between precipitation over this 3b4) are clear. subregion and the atmospheric circulation over the Drake Passage and the surrounding Atlantic persist 3) PATTERN 3 during spring (Figs. 7a4 and 7b4). As was previously During summer, the anomalies of atmospheric circu- described, this structure of anomalous circulation can lation associated with precipitation over the islands of produce an increment of precipitation over southern- southernmost EPAT (pattern 3 in Fig. 3) have character- most EPAT due to the activity associated with the istics opposite of those detected in the other two modes passage of cold fronts. of precipitation. In fact, precipitation over southernmost c. Relations between precipitation over the east and EPAT has a positive relation with the low-level circu- west sides of the southern Andes lation over most of the South American continent south of 308S, whereas negative relations are detected at sub- Section 1 mentioned that the low-level zonal wind in polar and polar latitudes (Fig. 7a1). The anomalous cir- Patagonia is negatively (positively) correlated with culation extends to the upper levels (Fig. 7b1), generating monthly precipitation variations to the east (west) of the reinforced westerly flow in the entire troposphere over Andes Cordillera. These relationships are also clearly the south of South America. These conditions favor the detected in the seasonal time scale considering indexes passage of migratory perturbations that move to the east, of zonal wind and precipitation on both sides of the increasing the precipitation in the region. The lack of mountains. In fact, the low-level zonal wind averaged in significant correlations could be a consequence of strong the northern and southern areas of Patagonia (Fig. 8) interdiurnal changes affecting the seasonal means or has significant negative (positive) correlation with pre- strong interannual changes, which are not analyzed in cipitation over regions to the east (west) of the Andes this study. Moreover, the anticyclonic center over the during the four seasons (Table 2). However, poor re- central Andes can inhibit the upward movements over lations are found among precipitation registered at the the western slopes of the mountains, reducing the pre- same over both sides of the mountains, de- cipitation in these regions. It could be the mechanism that scribing a lack of west–east dipole between the hyper- explains the inverse relation between precipitation over humid conditions to the west of the Andes and the southernmost EPAT and precipitation over the central semiarid conditions to the east. This characteristic is also Andes and the Bolivian Altiplano (see Fig. 3c1). observed in the seasonal correlations of CMAP data In autumn, the anomalies of atmospheric circulation with the PC scores of the PCA-CMAP (see Fig. 3). The associated with precipitation over this subregion are zonal wind has influence on the seasonal variability of different from those observed in summer, and the EPAT precipitation, but the effects of other forcings can anomalous pattern suggests the important influence of produce precipitation over EPAT that does not vary in cold fronts on the precipitation variability. In fact, the phase with that registered to the west of the Andes. inverse relations between the precipitation and the low- Results presented in the previous section suggest that level circulation in areas of southern South America and precipitation over northern EPAT can be influenced by the southern Atlantic configure a structure of atmo- the northerly transport of humid air over areas to the spheric anomalies favorable to outbreaks of cold air east of the Andes in autumn and winter. Blocking anti- from polar latitudes to the south of EPAT (Fig. 7a2). cyclones at higher latitudes over the western South

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FIG. 7. As in Fig. 5, but for the third PC score time series (corresponding to pattern 3 in Fig. 3).

Atlantic produce easterly winds and advection of precipitation over EPAT (e.g., Prohaska 1976; Mayr moisture from the adjacent areas of the Atlantic Ocean et al. 2007b). Intense daily precipitation over different that penetrates into the eastern continent up to hun- regions of EPAT is also associated with persistence of dreds of kilometers, producing stratiform daily strong easterly winds and advection of moisture from the

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TABLE 2. Correlations between the time indexes of zonal wind and precipitation corresponding to the areas depicted in Fig. 8. One (two) asterisk(s) indicate correlations that are statistically signifi- cant at the 95% (99%) level for a Student’s t test. There are no values significant at the 90% level.

Summer

NWPP NEPP SWPP SEPP

NWIND 0.43* 20.42* SWIND 0.60** 20.50** NWPP 20.03 SWPP 20.09

Autumn

NWPP NEPP SWPP SEPP

NWIND 0.55** 20.58** SWIND 0.44* 20.59** NWPP 0.02 SWPP 20.14

Winter

NWPP NEPP SWPP SEPP

NWIND 0.54** 20.44* SWIND 0.51** 20.46* NWPP 20.03 SWPP 20.08

FIG. 8. Areas considered for indexes of 850-hPa zonal wind and Spring precipitation in Patagonia. The two boxes limited by solid lines NW NE SW SE indicate the areas where the zonal wind is averaged to define the PP PP PP PP 2 2 indexes for northern (NWIND) and southern (SWIND) Patagonia. NWIND 0.57** 0.48** SWIND 0.63** 0.48** The four shaded boxes indicate the areas where the CMAP data NWPP 0.09 SWPP 20.11 are averaged to define the indexes of precipitation for northwest- ern (NWPP), northeastern (NEPP), southwestern (SWPP), and southeastern (SEPP) Patagonia. seasonal precipitation. Moreover, each subregion is as- sociated with specific anomalies of atmospheric circu- Atlantic (Frumento 2000; Mayr et al. 2007a). Although lation and has particular links with precipitation over these daily events are not detected with the seasonal other areas of South America. correlations analyzed in this paper, they are additional Precipitation over the center-north region of EPAT local processes that explain part of the precipitation has significant positive correlation with precipitation variability in areas of EPAT contributing to the lack of over the surrounding Atlantic and the Pampa region relation with precipitation variability registered to the (center of Argentina) during the four seasons. The en- west of the southern Andes. Such local forcings can also hancement (reduction) of seasonal precipitation over contribute to the different variability of precipitation this part of EPAT is associated with weakened (en- over the northern and southern continental areas of hanced) westerly flow over the region. Enhanced EPAT (patterns 1 and 2 of Fig. 3; see Fig. 4), even though (weakened) northerly transport of humid air over areas both regions are influenced by the westerlies. to the east of the Andes can also produce more (less) precipitation in the region during autumn and winter. Although the westerlies have influence on precipitation 4. Concluding remarks over both sides of the Andes Cordillera, the character- Different aspects of the variability of seasonal pre- istics of the low-level flow in autumn–winter affecting cipitation over EPAT, the South American region south the precipitation over the north of EPAT contribute to of 408S east of the Andes, have been described in this the lack of negative correlation between precipitation paper. Connections between precipitation over this re- over this region and that registered over areas west mote region of the world and precipitation over other of the mountains. Daily precipitation associated with areas of South America were studied as well as the as- easterly winds and strong advection of moisture from sociated anomalies of atmospheric circulation. the adjacent Atlantic toward the north of EPAT de- Three homogeneous subregions of precipitation scribed in previous studies can contribute to the lack of extending over almost similar areas of EPAT are inverse relation among precipitation over both sides of detected in the four seasons. In fact, the center-north the Andes during the four seasons. region, the southern continental area, and the south- Precipitation over the southern continental subregion ernmost islands are independent regions of variability of has a strong connection with precipitation over

Unauthenticated | Downloaded 10/01/21 07:50 AM UTC 6794 JOURNAL OF CLIMATE VOLUME 25 subtropical areas east of the Andes during spring, sum- Results presented in this paper contribute to a better mer, and autumn. In fact, positive correlations are knowledge of the variability of seasonal precipitation detected with precipitation over the SESA region, while over southern South America. This issue is very im- negative connections are observed in an elongated band portant not only for descriptions of the present climate extended along the coast of Brazil and the surrounding but also to infer past conditions. In fact, the information Atlantic in 208S–358S. The connection between pre- from paleoclimatic deposits located in southernmost cipitation over subpolar and subtropical regions is estab- South America reveals important climate changes in the lished via an atmospheric pattern constituted by region (e.g., Boninsegna et al. 2009). Our results can anomalous cyclonic circulation over most of southern help to understand the possible conditions in this remote South America at low levels associated with the Pacific– region of the world during specific past periods as well as South America wave train extending from the western the causes of such climate fluctuations. Pacific at upper levels. A similar structure of atmospheric circulation was presented by Diaz and Aceituno (2003) for Acknowledgments. Comments and suggestions pro- episodes of enhanced convective cloudiness over sub- vided by three anonymous reviewers were very helpful tropical areas, but our results indicate that this atmo- in improving this paper. The study was financed by spheric pattern is also able to connect the variability of Grants UBACYT EX016, UBACYT 20020100101049, precipitation over subpolar and subtropical regions. In AGENCIA-MINCYT PICT-2007-00438, PICT-2010- Patagonia, the anomalous cyclonic (anticyclonic) circu- 2110, CONICET-PIP-114-201001-00250, and MINCYT- lation implies weakened (enhanced) westerly flow that MEYS-ARC/11/09. increases (reduces) the precipitation over southern EPAT. In subtropical latitudes, strong (weak) subtropical jet REFERENCES stream and reinforced (weakened) northwesterly flow Alessandro, A., 2005: Bloqueos simultaneos en el Atlantico y Pa- of warm and humid air produce more (less) precipitation cifico Sur y sus influencias sobre la Republica Argentina. Rev. over SESA but a weakened (reinforced) SACZ, re- Bras. Meteor., 20, 277–300. ducing (increasing) the precipitation farther to the ——, 2008: Temperature and precipitation conditions in Argen- tina associated with strong westerly mid-latitute. Rev. Bras. north. This subpolar–subtropical precipitation link and Meteor., 23, 126–142. the influence of the Pacific–South America patterns on Aravena, J.-C., and B. H. 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