ENSO-RELATED RAINFALL ANOMALIES IN SOUTH AMERICA AND ASSOCIATED CIRCULATION FEATURES DURING WARM AND COLD PACIFIC DECADAL OSCILLATION REGIMES

Transcription

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 25: (25) Published online in Wiley InterScience ( DOI: 1.12/joc.1222 ENSO-RELATED RAINFALL ANOMALIES IN SOUTH AMERICA AND ASSOCIATED CIRCULATION FEATURES DURING WARM AND COLD PACIFIC DECADAL OSCILLATION REGIMES RITA V. ANDREOLI and MARY T. KAYANO* Instituto Nacional de Pesquisas Espaciais, Centro de Previsão de Tempo e Estudos Climáticos, Avenida dos Astronautas, 1758, São José dos Campos, SP, Brazil Received 27 January 25 Revised 21 April 25 Accepted 26 April 25 ABSTRACT El Niño/Southern Oscillation (ENSO)-related patterns of monthly reanalyzed upper-level circulation data and monthly rainfall time series over South America are revised for the period considering the phases of the Pacific inter- Decadal Oscillation (PDO). The El Niño (EN) related composites show differences relative to the PDO phases as well as seasonal differences. EN signals in the South American rainfall are more conspicuous for the warm PDO (WPDO) regime, when seasonal differences are more pronounced, than for the cold PDO (CPDO). Differences for the 2-hPa stream function composites seem to determine the precipitation composite differences. In fact, the negative precipitation anomalies over northeast Brazil and the excessive rainfall to the south are explained respectively by a cyclonic center over eastern and northeastern Brazil and a weak anticyclonic center over southeastern South America for the WPDO regime. Positive precipitation anomalies over southeastern South America are located in a southern position for the CPDO regime when compared to those for the WPDO regime. These anomalies might be related to a strengthened upper-level subtropical jet stream associated with strong cyclonic circulation extending over southern South America for the CPDO regime. With regard to the linear and nonlinear parts of the precipitation anomaly patterns related to the ENSO, the nonlinear component is considerably smaller than the linear component, in particular, over northern and southeastern South America. This suggests that the linear approach of the South American precipitation response to the ENSO seems to be appropriated. Copyright 25 Royal Meteorological Society. KEY WORDS: ENSO; rainfall anomalies; Pacific Decadal Oscillation; South America; upper-level circulation 1. INTRODUCTION The El Niño/Southern Oscillation (ENSO) phenomenon is the dominant mode of the Pacific climate on an interannual basis and is associated with near-global climate variations (e.g. Rasmusson and Arkin, 1985). The warm ENSO phase or the El Niño (EN) episode features above normal sea level pressure (SLP) over Indonesia, below normal SLP over central and eastern tropical Pacific and an eastward displaced Walker circulation with an anomalous rising motion over central and eastern equatorial Pacific, where above normal sea surface temperature (SST) and anomalous descending motion to the west prevail. Consistently, the lowlevel equatorial Pacific easterlies are weakened, convection is enhanced in the central and eastern tropical Pacific and anomalous upper tropospheric anticyclonic centers are found at the low latitudes (e.g. Rasmusson and Arkin, 1985). The cold ENSO phase or the La Niña (LN) episode features the opposite anomaly patterns (e.g. Kousky and Ropelewski, 1989). ENSO extremes are largely responsible for interannual variations of the tropical climate. This is the case of the South American climate, for which ENSO effects have been detected mainly for precipitation (Walker, * Correspondence to: Mary T. Kayano, Instituto Nacional de Pesquisas Espaciais, Centro de Previsão de Tempo e Estudos Climáticos, Avenida dos Astronautas, 1758, São José dos Campos, SP, Brazil; mary@cptec.inpe.br Copyright 25 Royal Meteorological Society

2 218 R. V. ANDREOLI AND M. T. KAYANO 1927; Caviedes, 1973; Hastenrath and Heller, 1977; Kousky et al., 1984; Ropelewski and Halpert, 1987, 1989, hereafter RH87 and RH89; Aceituno, 1988; Kayano et al., 1988; Kiladis and Diaz, 1989; Rao and Hada, 199; Barros and Silvestri, 22; Grimm, 23; Kayano, 23; Vera et al., 24). RH87 and RH89 defined three separated areas in South America with ENSO-related rainfall anomaly patterns: (1) western coastal region, including the subtropical areas of Chile, (2) northeastern sector, including Venezuela, Guyana, Surinam, French Guiana and near equatorial regions of Brazil and (3) southeastern region, extending over southern Brazil, Uruguay and parts of northern Argentina. RH87 and RH89 found dry (wet) conditions in the northeastern sector during July March (June March), wet (dry) conditions in the southeastern sector during November February (June December) and excessive (inhibited) rainfall along western subtropical Chile during May October for EN (LN) episodes. The first month of these periods refer to the onset year of ENSO extremes. ENSO-related precipitation anomalies in certain regions of the globe might be modulated by lowerfrequency climate modes (Gershunov and Barnett, 1998; McCabe and Dettinger, 1999; Gutzler et al., 22; Krishnan and Sugi, 23; Brown and Comrie, 24). Among these modes, the Pacific (inter-) Decadal Oscillation (PDO) plays an important role (Mantua et al., 1997). This mode is part of the inter-decadal variability in the Pacific with anomaly patterns of the SST, SLP and wind stress fields similar to those of the ENSO (Nitta and Yamada, 1989; Zhang et al., 1997; Mantua et al., 1997; Zhang et al., 1998). However, the PDO pattern for the SST is less equatorially confined in the eastern Pacific and shows significant structure in the extratropical North Pacific (e.g. Zhang et al., 1997). The high PDO phase (or the warm PDO regime (WPDO)) features an anomalously deep Aleutian low-pressure system, colder than normal waters in the central and western North Pacific and warmer than normal waters along the west coast of the Americas and in the central and eastern tropical Pacific (e.g. Zhang et al., 1997; Mantua et al., 1997; Zhang et al., 1998; Enfield and Mestas-Nuñez, 1999). Nearly reversed patterns of the SLP and SST prevail for the low PDO phase (or the cold PDO regime (CPDO)). The CPDO regime occurred during the and periods, and the WPDO regime occurred during the period and from 1977 to mid-199s (Mantua et al., 1997). The PDO and the ENSO might have combined effects in the precipitation anomalous distributions in some regions, acting constructively (strong and well-defined anomalies) when they are in the same phase and destructively (weak and noisy anomalies) when they are in opposite phases. This is the case of northwestern/southwestern North America and the Indian monsoon region (Gershunov and Barnett, 1998; McCabe and Dettinger, 1999; Gutzler et al., 22; Krishnan and Sugi, 23; Brown and Comrie, 24). These studies provided indications that the knowledge on relationship of the PDO phases and the strength of ENSO effects may have a potential use in improving the climate forecasting. Therefore, the present paper revises the ENSO-related summer rainfall anomaly patterns over South America, taking into account the PDO phases. This implies that the ENSO/PDO combined effect on the rainfall variability over this region is the main interest of the present analysis. Analyses focus on the austral autumn season because a large portion of South America presents its principal rainy regime during this season with seasonal precipitation percentages for the Amazon and northeast sectors varying from 35 to 5% of the annual total (Rao and Hada, 199; Rao et al., 22). In addition, ENSO teleconnections are strong in both hemispheres during this season (Kayano and Andreoli, 1998). The associated large-scale upper-level circulation patterns are also examined. Analyses of circulation fields will provide indications of the large-scale dynamics responsible for rainfall interannual variability. 2. DATA AND METHODOLOGY Monthly global-gridded SSTs used in this paper consist of the extended reconstructed SST (ERSST) at 2 by 2 latitude longitude resolution grid for the period obtained by Smith and Reynolds (23). Monthly reanalyzed 2-hPa velocity potential (2 hpa χ), 2-hPa streamfunction (2 hpa ψ) and 5- hpa vertical velocity (5 hpa ω) data for the period from 1948 to 1999 are also used. These data were produced by the Climate Data Assimilation System (CDAS) Reanalysis Project (Kalnay et al., 1996) and Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

3 RAINFALL ANOMALIES IN SOUTH AMERICA AND PACIFIC DECADAL OSCILLATION 219 have a resolution of 2.5 in latitude and longitude. Analyses of the reanalyzed data are done for the global band between 9 N and 9 S. Reanalyzed precipitation data are not used because values over northeastern Brazil are overestimated; these data are highly correlated with rain gauge precipitation data only over southern and southeastern Brazil and the validity of these data over other Brazilian areas is questionable (Rao et al., 22). So, a total of 576 monthly precipitation series at rainfall stations or grid points in the South American sector between 1 N and 4 S are obtained from several sources for the base period from 1948 to The Brazilian series are obtained from Superintendência do Desenvolvimento do Nordeste (SUDENE), Instituto Nacional de Meteorologia (INMET) and Agência Nacional de Energia Elétrica (ANEEL). Precipitation series for other South American countries are from the gu23wld98.dat (version 1.), constructed and provided by Dr. Mike Hulme. The description of this dataset can be found in Hulme (1992, 1994) and in Hulme et al. (1998). Only rainfall stations and grid points with at least 3 years of the base period with data are selected. Monthly values higher than 2 mm and suspicious values (detected by visual inspections of the series) are replaced by a missing data code. Figure 1 illustrates the location of these stations and grid points. ENSO extreme years are selected from the Niño-3 SST index using the criterion suggested by Trenberth (1997). This index is defined as the 5-month running mean of the averaged SST anomalies in the area bounded at 6 N, 6 S, 15 W and 9 W. An EN (A LN) event is identified when the Niño-3 SST index exceeds (is lower than) the threshold of.5 C (.5 C) for at least six consecutive months. Table I lists the onset years of ENSO extremes for the period. In order to avoid biases due to the phases of the PDO, the SST anomalies used for the Niño-3 SST index are departures from the long-term (1854 2) means. The PDO phases are determined from the PDO index obtained by Mantua et al. (1997). Although this index was obtained for the North Pacific region, positive (negative) values refer to above (below) normal SST along the west coast of North America and in the eastern equatorial Pacific and below (above) normal 2S 4S 9W 75W 6W 45W 3W Figure 1. Locations of the 576 rainfall series Table I. The onset years of the ENSO extremes during the warm and cold PDO regimes ENSO phase WPDO CPDO El Niño 1976, 1979, 1982, 1986, 1987, 1991, 1994, , 1957, 1963, 1965, 1968, 1969, 1972 La Niña 1985, , 195, 1954, 1955, 1967, 197, 1973, 1975 Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

4 22 R. V. ANDREOLI AND M. T. KAYANO SST in central and western North Pacific around 45 N. This SST anomaly pattern is similar to that obtained considering other indices (including the one for the Pacific area between 2 N and 2 S) as shown by Power et al. (1999). The PDO regime shift of 1977 divides the base period in two subperiods, one before 1977 with the CPDO regime and the other after 1977 with the WPDO regime. Monthly anomalies of the variables are based on departures from normal during ENSO neutral years. Monthly anomalies for the reanalysis variables are computed at each grid point. Monthly precipitation anomaly time series, standardized by the standard deviation of the anomaly time series, are obtained for each rainfall station or grid point. Composite technique is used to obtain the ENSO-related patterns for circulation variables and precipitation over South America. Analyses are done for the periods from November () to December () (ND () ) and from January (+) to February (+) (JF (+) ), with the symbols () and (+) referring to onset years of the ENSO extremes and the following year respectively. This temporal stratification is based on the results by Berri and Bertossa (24) who found the strongest (weakest) association of the southeastern South America precipitation anomalies with the SST variations in the tropical and subtropical Atlantic and eastern Pacific Oceans during the period November December (January February). The EN-related anomaly patterns of the variables are obtained for the two PDO regimes separately. The LN analyses are not done because the number of events during the WPDO regime is not enough for the statistical significance of the composites. The linear (nonlinear) part of the climate response to ENSO extremes is estimated by the difference between (summation of the) EN and LN composites (Hoerling et al., 1997). These analyses are done only for the CPDO regime. Statistical significance of the composites is assessed by assuming that the number of degrees of freedom is the number of events and by using the Student s t-test (Press et al., 1986). A statistical confidence level of 95% is used. 3. RESULTS 3.1. EN-related patterns during ND ( ) : differences between WPDO and CPDO regimes The EN-related rainfall anomaly composites over South America for the WPDO and the CPDO regimes during ND () are displayed in Figures 2 and (b) respectively. Significant anomalies are found in small areas EI Nino (WPDO) ND() (b) EI Nino (CPDO) ND() 2S 2S 4S 4S 8W 6W 4W 8W 6W 4W Figure 2. El Niño-related mean standardized rainfall anomalies during ND () for WPDO regime and (b) CPDO regime. Contour interval is.2 SD, with negative (positive) contours being dashed (continuous). Shading encompasses values that are, according to the Student s t-test, significant at the 95% confidence level Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

5 RAINFALL ANOMALIES IN SOUTH AMERICA AND PACIFIC DECADAL OSCILLATION 221 in northeastern and northern Brazil (negative) and in southern Brazil (positive) for the WPDO composite (Figure 2). On the other hand, significant anomalies are noted over Guiana and Surinam (negative) and in eastern and central Argentina (positive) for the CPDO composite (Figure 2(b)). So, EN has different effects on the South American rainfall anomaly pattern under the two phases of the PDO. The EN-related 2-hPa χ anomaly composite for the WPDO regime during ND () shows convergent flow between 9 E and 12 W and divergent flow in the complementary longitudinal sector, which resembles a zonal wave number one pattern. Upper-level maxima divergence and convergence are found approximately along the equator over the western Indian Ocean and over Papua/New Guinea respectively (Figure 3). Consistently, the associated 5-hPa ω anomaly composite reproduces an anomalous Walker circulation in the tropical Pacific with its upward branch over central and eastern equatorial Pacific and its downward branch to the west (Figure 3(b)). The associated 2-hPa ψ anomaly composite shows anticyclones over the 9N 6N 3N 3S 6S EI Nino (WPDO) ND() (d) EI Nino (CPDO) ND() 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W (b) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W 9S 6E 12E 18 12W 6W (e) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W (c) 9N (f) 9N 6N 3N 3S 6S 6N 3N 3S 6S 9S 6E 12E 18 12W 6W 9S 6E 12E 18 12W 6W Figure 3. El Niño-related mean 2-hPa χ anomalies during ND () for WPDO regime and (d) CPDO regime. El Niño-related mean 5 hpa ω for (b) WPDO regime and (e) CPDO regime. El Niño-related mean 2 hpa ψ for (c) WPDO regime and (f) CPDO regime. Contour intervals are m 2 /s in and (d),.1 Pa/s in (b) and (e), and m 2 /s in (c) and (f). Negative (positive) contours are dashed (continuous). Shading encompasses values that are, according to the Student s t-test, significant at the 95% confidence level Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

6 222 R. V. ANDREOLI AND M. T. KAYANO central and eastern tropical Pacific centered in the latitudinal bands between 15 and 2 of both hemispheres (Figure 3(c)). These large-scale atmospheric circulation patterns accompany the warm surface waters in the eastern equatorial Pacific and are consistent with the developing stage of EN events. With regard to the regional characteristics of the upper-level circulation, the cyclonic center over northeastern Brazil and the anticyclonic center over southern Brazil are consistent with the observed dry and wet conditions respectively in these regions (Figures 2 and 3(c)). These centers seem to be the weakest part of a Rossby wave train pattern stretching from the heating source in the equatorial eastern Pacific to northeastern Brazil through the Southern Hemisphere (SH) midlatitudes (Figure 3(c)). The EN-related 2-hPa χ anomaly composite for the CPDO regime during ND () shows a more horizontal structure with a strong divergence over tropical Pacific to the east of 16 E, a weak divergence over the western Indian Ocean and three convergence centers, one over southern Africa, one over the eastern Indian Ocean and one over the Amazon (Figure 3(d)). In agreement, the associated 5-hPa ω pattern shows ascending motions over the central and eastern equatorial Pacific and descending motions over the western tropical Pacific, over northern/northeastern South America and along the South Atlantic convergence zone (Figure 3(e)). The associated 2-hPa ψ anomaly composite features a strong Rossby wave train pattern extending from the central and eastern equatorial Pacific to the southeastern Pacific and adjacent South American areas, where a cyclonic center prevails (Figure 3(f)). Another feature of the 2-hPa ψ composite is the belt of cyclonic centers in the SH midlatitudes. This feature is typical of the negative phase of the Antarctic Oscillation and has been related to EN conditions in the eastern equatorial Pacific during the austral summer (Carvalho et al., 25). The EN-related circulation composites for the WPDO and CPDO regimes show differences. Among them, the differences for the 2-hPa ψ composites for the two PDO regimes seem to determine the differences noted for the precipitation composites. In fact, the cyclonic center over eastern and northeastern Brazil and the weak anticyclonic center over southeastern South America are consistent with negative precipitation anomalies over northeast Brazil and excessive rainfall to the south for the WPDO regime. On the other hand, the strengthened upper-level subtropical jet stream associated with strong cyclonic circulation extending over southern South America explains the positive precipitation anomalies located further south over southeastern South America for the CPDO regime EN-related patterns during JF (+) : differences between WPDO and CPDO regimes The EN-related precipitation anomaly composite over South America for the WPDO regime during JF (+) shows significant anomalies (negative) in a large area, including southern Venezuela, Guyana, Surinam, French Guiana and most of the northern Brazil area to the west of 55 W, and over northern Chile (Figure 4). The corresponding composite for the CPDO regime features significant anomalies (negative) only in a small area over central Chile (Figure 4(b)). The EN effects in the rainfall over South America show differences relative to the PDO phases, in particular, in the Amazon. The EN-related 2-hPa χ composite for the WPDO regime during JF (+) shows convergent flow between 9 E and 15 W (centers near Papua/New Guinea and over western North Pacific) and divergent flow in the complementary band (centers over western equatorial Indian Ocean and over Central America) (Figure 5). Consistently, the associated 5-hPa ω pattern shows descending motions over the subtropical western North Pacific and in an area extending zonally from the north of Papua/New Guinea to the central and eastern Pacific, and ascending motions off the west coast of the United States and over the central equatorial Pacific (Figure 5(b)). In addition, ascending motions are noted in the western tropical North Atlantic and descending motions in the Atlantic sector along the coast of Guyana, Surinam, French Guiana and northern Brazil. The descending motions are in agreement with the rainfall deficit in northern South America during JF (+).The 5-hPa ω pattern illustrates changes in the Walker and Hadley cells in the tropical Pacific and Atlantic Oceans. The associated 2-hPa ψ anomalous pattern features alternated anticyclonic and cyclonic centers in the central and eastern Pacific, extending in both hemispheres from approximately 15 in latitude to the midlatitudes (Figure 5(c)). The upper-level circulation features are consistent with the mature stage of EN events. Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

7 RAINFALL ANOMALIES IN SOUTH AMERICA AND PACIFIC DECADAL OSCILLATION 223 EI Nino (WPDO) JF(+) (b) EI Nino (CPDO) JF(+) 2S 2S 4S 8W 4S 6W 4W 8W 6W 4W Figure 4. As in Figure 2, except during JF (+) The EN-related 2-hPa χ anomaly composite for the CPDO regime during JF (+) (Figure 5(d)) shows divergent and convergent centers located almost in the same positions of those in Figure 3(d). The ENrelated 5-hPa ω pattern for the CPDO regime during JF (+) shows an anomalous Walker circulation in the tropical Pacific with its upward branch over the central and eastern equatorial Pacific and its downward branch over the western tropical Pacific (Figure 5(e)). The associated 2-hPa ψ composite features a strong Rossby wave train pattern in the central and eastern Pacific, extending from tropical to extratropical areas of both hemispheres, and a belt of robust and well-defined cyclonic centers in the SH midlatitudes, which are characteristic of the Antarctic Oscillation negative phase (Figure 5(f)). Similar to the analysis for ND (), the EN-related circulation composites for JF (+) show differences between the two phases of the PDO. Differences for the 5-hPa ω and for the 2-hPa ψ composites between the two PDO regimes might explain the differences noted for the precipitation in the Amazon. Indeed, negative precipitation anomalies for the WPDO regime in this region are consistent with the descending motion and cyclonic circulation over northern South America and the adjacent Atlantic sector. On the other hand, the relatively weaker circulation patterns in these sectors result in smaller magnitude precipitation anomalies in the Amazon for the CPDO Linear and nonlinear components of precipitation and upper-level circulation related to the ENSO for the CPDO regime during ND ( ) The linear part of the precipitation anomaly composite related to the ENSO over South America for the CPDO regime during ND () shows significant values to the north of the equator (negative) and to the south of 3 S (positive) (Figure 6). The northern pattern in this analysis and that for LN years of the CPDO during ND () (Figure not shown), except for the sign, show similar configurations. So, LN events have a larger contribution than EN events in this region. On the other hand, the corresponding nonlinear part of the precipitation anomaly composite related to the ENSO shows significant values only in small areas in South America (Figure 6(b)). The linear part of 2-hPa χ anomalous pattern related to the ENSO for the CPDO regime during ND () shows divergent flow over the central and eastern tropical Pacific (strong) and over India and the western Indian Ocean (weak) and convergent flow centered over Indonesia and over South America (Figure 7). This configuration resembles a zonal wave number two pattern. Consistently, the associated 5-hPa ω pattern shows ascending motions in an area extending from the central equatorial Pacific eastward to the coast of Central America and southeastward to the subtropical eastern Pacific, including southern South Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

8 224 R. V. ANDREOLI AND M. T. KAYANO 9N 6N 3N 3S 6S EI Nino (WPDO) JF(+) (d) 9N 6N 3N EI Nino (CPDO) JF(+) 3S 6S 9S 6E 12E 18 12W 6W (b) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W (e) 9N 6N 3N 3S 6S 9S 9S 6E 12E 18 12W 6W 6E 12E 18 12W 6W (c) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W (f) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W Figure 5. As in Figure 3, except during JF (+) America (Figure 7(b)). Descending motions are noted over Indonesia, to the east of Australia, over the central tropical North Pacific, over eastern South America and over an area in the central tropical North Atlantic (Figure 7(b)). The associated 2-hPa ψ composite features a strong Rossby wave train pattern over the central and eastern Pacific with its centers extending towards the extratropics of both hemispheres and a belt of quite robust and well-defined cyclonic centers in the SH midlatitudes associated with the negative phase of the Antarctic Oscillation (Figure 7(c)). The linear part of the precipitation composite related to the ENSO shows negative values over northwestern South America, which are consistent with large-scale upper-level convergence, descending motions at 5 hpa and upper-level cyclonic circulation. On the other hand, the positive precipitation anomalies over southern South America are associated with a strengthened upper-level subtropical jet stream in the southeastern Pacific. The nonlinear component of the 2-hPa χ anomalous pattern related to the ENSO for the CPDO regime during ND () shows strong divergent flow over the central and eastern Pacific and convergent flow centered over the eastern Indian Ocean, over southern Africa and over northwestern South America (Figure 7(d)). These features mostly reflect the differences in the longitudinal locations of the opposite sign centers for EN and LN composites. The associated 5-hPa ω pattern shows small values in most of the study domain, Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

9 RAINFALL ANOMALIES IN SOUTH AMERICA AND PACIFIC DECADAL OSCILLATION 225 EI Nino La Nina (CPDO) ND() (b) EI Nino + La Nina (CPDO) ND() 2S 2S 4S 8W 6W 4W 4S 8W 6W 4W Figure 6. ENSO-related rainfall anomalies for the CPDO regime during ND () : linear component and (b) nonlinear component. Display is the same as in Figure 2 except over Amazon and over northeast Brazil, where significant positive values (Figure 7(e)), which are mostly due to the EN composite, prevail. The associated 2-hPa ψ pattern shows significant centers zonally distributed along the 3 N-equator, equator 3 S and 3 S 6 S bands. The nonlinear component of the 2-hPa ψ pattern related to the ENSO along the 3 S 6 S band reflects the belt with cyclonic centers in the SH midlatitudes for the EN composite not noted for the LN composite (Figure 7(f)) Linear and nonlinear components of precipitation and upper-level circulation related to the ENSO for the CPDO regime during JF (+) The linear part of precipitation composite related to the ENSO over South America for the CPDO regime during the JF (+) period is similar to the corresponding precipitation composite during ND (), except for the positive values to the south of 3 S, which are not significant (Figure 8). This might be due to the fact that LN events have stronger effects in this region during ND () than during JF (+) (RH89). LN events show a larger contribution than EN events to the linear part of the precipitation composite related to the ENSO in northern South America. The nonlinear part of precipitation composite related to the ENSO for the CPDO regime during JF (+) presents significant values only in small areas (Figure 8(b)). The linear part of the 2-hPa χ anomaly composite related to the ENSO for the CPDO regime during JF (+) shows divergent flow over the central and eastern tropical Pacific and convergent flow centers located over tropical South America and over Indonesia and the western Pacific (Figure 9). The associated 5- hpa ω composite (Figure 9(b)) in the Pacific sector is similar to that displayed in Figure 6(b). An interesting feature of this composite is the occurrence of alternating sign centers in southeastern United States (positive), in the Caribbean Sea (negative) and in the coastal region of northern South America (positive) (Figure 9(b)). This latest characteristic is more closely related to the LN events (Figure not shown), and seems to be associated with a short Rossby wave train pattern. The linear part of the 2-hPa ψ composite related to the ENSO during JF (+) for the CPDO features a strong Rossby wave train pattern in the central eastern Pacific with centers in the tropics and in the Northern Hemisphere (NH) midlatitudes (Figure 9(c)). The upper-level circulation in the SH midlatitudes does not show closed cyclonic centers in some regions (Figure 9(c)). The nonlinear 2-hPa χ anomalous pattern related to the ENSO for the CPDO regime during JF (+) shows divergent flow over the central and eastern tropical Pacific and convergent flow in the eastern Indian Ocean, Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

10 226 R. V. ANDREOLI AND M. T. KAYANO 9N 6N 3N EI Nino La Nina (CPDO) ND() (d) EI Nino + La Nina (CPDO) ND() 9N 6N 3N 3S 6S 3S 6S 9S 6E 12E 18 12W 6W (b) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W (c) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W 9S 6E 12E 18 12W 6W (e) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W (f) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W Figure 7. ENSO-related 2-hPa χ anomalies for the CPDO regime during ND () : linear component and (d) nonlinear component. ENSO-related 5-hPa ω anomalies for the CPDO regime during ND () : (b) linear component and (e) nonlinear component. ENSO-related 2-hPa ψ anomalies for the CPDO regime during ND () : (c) linear component and (f) nonlinear component. Display is thesameasinfigure3 in southern Africa and in northern South America (Figure 9(d)). These features reflect the differences in the longitudinal locations of opposite sign centers for EN and LN composites. The corresponding patterns of the 5 and the 2 hpa ψ show small values in most of the study domain (Figures 9(e) and (f)). 4. CONCLUSIONS Fifty-two years ( ) of monthly reanalyzed 2-hPa velocity potential (2 hpa χ), 2-hPa streamfunction (2 hpa ψ) and 5-hPa vertical velocity (5 hpa ω) data and monthly rainfall time series over South America are used to explore ENSO-related patterns. This study also considers the phases of the PDO. The EN and LN years are selected using the criterion suggested by Trenberth (1997) for the Niño-3 SST index, and the selections of the WPDO and the CPDO regimes are based on the PDO index Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

11 RAINFALL ANOMALIES IN SOUTH AMERICA AND PACIFIC DECADAL OSCILLATION 227 El Nino La Nina (CPDO) JF(+) (b) El Nino + La Nina (CPDO) JF(+) 2S 2S 4S 4S 8W 6W 4W 8W 6W 4W Figure 8. Linear component of the ENSO-related rainfall anomalies for the CPDO regime during JF (+) and (b) nonlinear component of the ENSO-related rainfall anomalies for the CPDO regime during JF (+). Display is the same as in Figure 2 obtained by Mantua et al. (1997). Analyses are done for the November () December () (ND () ) and the January (+) February (+) (JF (+) ) periods, with the symbols () and (+) referring to the ENSO extremes of the onset year and the following year respectively. These analyses are based on the composite technique, which is applied for EN years of the WPDO and the CPDO regimes separately. Similar analysis for the LN years is not done because of the small number of LN events for the WPDO regime. Linear and nonlinear components of precipitation and upper-level circulation patterns related to the ENSO are obtained for the CPDO regime. The EN-related composites for the WPDO and CPDO regimes during ND () and JF (+) show differences relative to the PDO phases as well as seasonal differences. EN signals in the South American rainfall for both periods are more conspicuous for the WPDO than for the CPDO. The seasonal differences are more pronounced for the WPDO regime. In fact, the largest rainfall anomalies are found in southern South America (positive) during ND () and in northern South America (negative) during JF (+). In addition, EN composites show differences relative to the PDO phases, which might lead to important regional features. Differences for the 2-hPa ψ composites seem to be crucial in determining the differences for the precipitation composites. In fact, negative precipitation anomalies over northeast Brazil and excessive rainfall to the south are explained by the presence of the cyclonic center over eastern and northeastern Brazil and the weak anticyclonic center over southeastern South America for the WPDO regime. On the other hand, the positive precipitation anomalies located further south over southeastern South America might be related to a strengthened upperlevel subtropical jet stream associated with strong cyclonic circulation extending over southern South America for the CPDO regime. The small number of the LN events during the WPDO regime did not allow a more comprehensive study of the differences of LN-related circulation patterns relative to the PDO phases. However, the results of EN analyses provide strong indications that the ENSO-related climate response in South America is dependent on the PDO phases. So, ENSO-based climate monitoring and forecasts should be considered with caution. With regard to the linear and nonlinear parts of the precipitation anomaly patterns related to the ENSO, the available data allowed us to investigate only the cases for the CPDO regime. In most of the study domain, the nonlinear component is considerably smaller than the linear component, in particular, over northern and southeastern South America during ND and JF. So, this suggests that the linear approach relative to the ENSO phases seems to be appropriated. However, it is worthwhile to note that our analysis Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

12 228 R. V. ANDREOLI AND M. T. KAYANO 9N EI Nino La Nina (CPDO) JF(+) (d) 9N EI Nino + La Nina (CPDO) JF(+) 6N 6N 3N 3N 3S 3S 6S 6S 9S 6E 12E 18 12W 6W (b) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W (e) 9N 6N 3N 3S 6S 9S 6E 12E 18 12W 6W 9S 6E 12E 18 12W 6W (c) 9N 6N (f) 9N 6N 3N 3S 6S 3N 3S 6S 9S 6E 12E 18 12W 6W 9S 6E 12E 18 12W 6W Figure 9. ENSO-related 2-hPa χ anomalies for the CPDO regime during JF (+) : linear component and (d) nonlinear component. ENSO-related 5-hPa ω anomalies for the CPDO regime during JF (+) : (b) linear component and (e) nonlinear component. ENSO-related 2-hPa ψ anomalies for the CPDO regime during ND () : (c) linear component and (f) nonlinear component. Display is thesameasinfigure3 was restricted to the austral summer months, and due to the data limitations, analyses for the WPDO regime were not done. ACKNOWLEDGEMENTS The authors are grateful to the two anonymous reviewers for their useful comments. The authors were partially supported by the Conselho Nacional de Desenvolvimento Científico and Tecnológico of Brazil. Thanks are due to Dr Mike Hulme for the provision of the gu23wld98.dat (version 1.) constructed at the Climatic Research Unit, University of East Anglia, Norwich, UK. The NCEP Reanalysis data were provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, USA, from their web site at Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

13 RAINFALL ANOMALIES IN SOUTH AMERICA AND PACIFIC DECADAL OSCILLATION 229 REFERENCES Aceituno P On the functioning of the southern oscillation in the South American sector. Part 1: surface climate. Monthly Weather Review 116: Barros VR, Silvestri GE. 22. The relation between sea surface temperature at the subtropical south-central Pacific and precipitation in southeastern South America. Journal of Climate 15: Berri GJ, Bertossa GI. 24. The influence of the tropical and subtropical Atlantic and Pacific oceans on precipitation variability over southern central South América on seasonal time scales. International Journal of Climatology 24: Brown DJ, Comrie AC. 24. A winter precipitation dipole in the western United States associated with multidecadal ENSO variability. Geophysical Research Letters 31: L9 23, DOI: 1.129/23GL Carvalho LMV, Jones C, Ambrizzi T. 25. Opposite phases of the Antarctic oscillation and relationships with intraseasonal to interannual activity in the tropics during the austral summer. Journal of Climate 18: , DOI /JCLI Caviedes CN Secas and El Niño: two simultaneous climatical hazards in South America. Proceedings of the Association of American Geographers 5: Enfield DB, Mestas-Nuñez A Multiscale variabilities in global sea surface temperatures and their relationships with tropospheric climate patterns. Journal of Climate 12: Gershunov A, Barnett TP Interdecadal modulation of ENSO teleconnections. Bulletin of the American Meteorological Society 79: Grimm AM. 23. The El Niño impact on the summer monsoon in Brazil: regional processes versus remote influences. Journal of Climate 16: Gutzler DS, Kann DM, Thornbrugh C. 22. Modulation of ENSO-based long-lead outlooks of southwestern U.S. winter precipitation by the Pacific decadal oscillation. Weather and Forecasting 17: Hastenrath S, Heller L Dynamics of climatic hazards in northeast Brazil. Quarterly Journal Royal Meteorological Society 13: Hoerling MP, Kumar A, Zhong M El Niño, La Niña, and the nonlinearity of their teleconnections. Journal of Climate 1: Hulme MA global land precipitation climatology for the evaluation of general circulation models. Climate Dynamics 7: Hulme MA Validation of large-scale precipitation fields in general circulation models. In Global Precipitations and Climate Change, NATO ASI Series, Desbois M, Desalmand F (eds). Springer-Verlag: Berlin, 466. Hulme MA, Osborn TJ, Johns TC Precipitation sensitivity to global warming: comparison of observations with HadCM2 simulations. Geophysical Research Letters 25: Kalnay E, Kanamitsu M, Kistler R, Collins W, Deaven D, Gandin L, Iredell M, Saha S, White G, Woollen J, Zhu Y, Chelliah M, Ebisuzaki W, Higgins W, Janowiak J, Mo KC, Ropelewski C, Wang J, Leetmaa A, Reynolds R, Jenne R, Joseph D The NCEP/NCAR 4-year reanalysis project. Bulletin of the American Meteorological Society 77: Kayano MT. 23. A note on the precipitation anomalies in southern South America associated with ENSO variability in the Tropical Pacific. Meteorology and Atmospheric Physics 84: Kayano MT, Andreoli RV Interanual variability of the upper tropospheric circulation. Meteorology and Atmospheric Physics 68: Kayano MT, Rao VB, Moura AD Tropical circulations and the associated rainfall anomalies during two contrasting years. Journal of Climatology 8: Kiladis G, Diaz HF Global climatic anomalies associated with extremes in the Southern oscillation. Journal of Climate 2: Kousky VE, Ropelewski CF Extremes in the Southern oscillation and their relationship to precipitation anomalies with emphasis on the South American region. Revista Brasileira de Meteorologia 4: Kousky VE, Kayano MT, Cavalcanti IFA A review of the Southern oscillation: oceanic-atmospheric circulation changes and related rainfall anomalies. Tellus 36A: Krishnan R, Sugi M. 23. Pacific decadal oscillation and variability of the Indian summer monsoon rainfall. Climate Dynamics 21: Mantua NJ, Hare SR, Zhang Y, Wallace JM, Francis RC A Pacific interdecadal climate oscillation with impacts on salmon production. Bulletin of the American Meteorological Society 78: McCabe GJ, Dettinger MD Decadal variations in the strength of ENSO teleconnections with precipitation in the western United States. International Journal of Climatology 19: Nitta T, Yamada S Recent warming of tropical sea surface temperature and its relationship to the Northern Hemisphere circulation. Journal of the Meteorological Society of Japan 67: Power S, Casey T, Folland C, Colman A, Mehta V Inter-decadal modulation of the impact of ENSO on Australia. Climate Dynamics 15: Press WH, Flannery BP, Teukolsky SA, Vetterling WT Numerical Recipes: The Art of Scientific Computing. Cambridge University Press: Cambridge. Rao VB, Hada K Characteristics of rainfall over Brazil: annual and variations and connections with the Southern oscillation. Theoretical Applied Climatology 42: Rao VB, Santo CE, Franchito SH. 22. A diagnosis of rainfall over South America during the El Niño event. Part I: Validation of NCEP-NCAR reanalysis rainfall data. Journal of Climate 15: Rasmusson EM, Arkin PA Interannual climate variability associated with the El Niño/Southern oscillation, Coupled Ocean- Atmosphere Models, Nihoul JCJ (ed.). Elsevier Science Publishers. B.V, Amsterdam, Netherlands: Ropelewski CF, Halpert MS Global and regional scale precipitation patterns associated with the El Niño/Southern oscillation. Monthly Weather Review 115: Ropelewski CF, Halpert MS Precipitation patterns associated with the high index phase of the Southern oscillation. Journal of Climate 2: Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)

14 23 R. V. ANDREOLI AND M. T. KAYANO Smith TM, Reynolds RW. 23. Extended reconstruction of global sea surface temperatures based on COADS data ( ). Journal of Climate 16: Trenberth KE The definition of El Niño. Bulletin of the American Meteorological Society 78: Vera C, Silvestri GE, Barros VR, Carril A. 24. Differences in El Nino response over the Southern Hemisphere. Journal of Climate 17: Walker GT World weather III. Memorie of the Royal Meteoreorological Society, Washington, Smithsonian Miscellaneous Collections, V Zhang Y, Wallace JM, Battisti D ENSO-like interdecadal variability: Journal of Climate 1: Zhang Y, Norris J, Wallace JM Seasonality of large-scale atmosphere-ocean interaction over the North Pacific. Journal of Climate 11: Copyright 25 Royal Meteorological Society Int. J. Climatol. 25: (25)