Does the El Niño-Southern Oscillation Affect the Combined Impact of the Atlantic Multidecadal Oscillation and Paciﬁc Decadal Oscillation on the Precipitation and Surface Air Temperature Variability over South America?

: Previous studies have shown that the Atlantic Multidecadal Oscillation (AMO) and Paciﬁc Decadal Oscillation (PDO) have combined effects on the precipitation (PRP) variability over South America. The combined impacts have been assessed considering four mean states as the averages of the variable anomalies during sub-periods overlapping time intervals of the PDO and AMO phases. Since these sub-periods include years under El Niño-Southern Oscillation (ENSO) extremes, the extent to which these years’ occurrence affects the averaged anomaly patterns during different mean states is investigated. The analyses are done for the PRP and surface air temperature (SAT) during the austral winter (June to August) and summer (December to February) of the 1901–2014 period using a composite technique. The nonlinear ENSO response in each mean state for a variable corresponds to the sum of the anomaly composites of the El Niño and La Niña events. In each mean state, the nonlinear PRP and SAT anomalies are not negligible and show similar patterns of the corresponding mean state, with larger magnitudes. For both seasons and all mean states, these similarities are more pronounced for SAT than for PRP. Thus, the ENSO variability affects the mean state’s PRP and SAT anomaly patterns in different ways. As far as we know, analyses of the nonlinear ENSO response of the South American climate during distinct mean states were not performed before. Our results also indicate that the ENSO variability should be considered in the studies of the low-frequency modes and their effects on the mean state over South America. The results presented could be relevant for climate monitoring and modeling studies.


Introduction
South America presents diverse regional climates characterized by distinct seasonal precipitation (PRP) and surface air temperature (SAT) distributions. The interannual climate variability in this region is primarily modulated by the El Niño-Southern Oscillation (ENSO) through large-scale atmospheric teleconnections, which modulate PRP and SAT in distinct From the above, it is clear that most studies on the combined ENSO, AMO, and PDO effects focused on the influence of the low-frequency variability modes in the ENSO effects. In addition, ENSO extremes are not evenly distributed during sub-periods, overlapping time intervals of the PDO and AMO phases. Considering the mean state for a given variable as the averages of the variable anomalies during these sub-periods, a question that arises is in which measure the ENSO extremes affect the anomaly patterns during distinct mean states? This is a question not dealt with before and is addressed here, with a focus on the South American climate reflected in the PRP and SAT. Recalling that the summation of the EN and LN anomaly composites for a given variable and time interval represents the nonlinear ENSO response for that variable and time interval [32], the present analysis examines the role of the ENSO nonlinearity on the low-frequency-related mean states for atmospheric circulation, PRP, and SAT over South America during the austral winter (June to August-JJA) and summer (December to February-DJF) of the 1901-2014 period.

Materials and Methods
The data used consist of monthly gridded SST, vertical velocity in pressure coordinate at 17 pressure levels, PRP, and SAT. The SST data were obtained from the extended reconstructed SST version V5 dataset available at the National Oceanic and Atmospheric Administration website (https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html, accessed on 17 May 2021) [33]. The vertical velocity data were obtained from the NOAA/CIRES Twentieth Century Reanalysis (20CR) Project version V2C (NOAA/CIRES Twentieth Century Reanalysis, 2017) [34]. The SAT data were obtained from the version 5.01 dataset produced by the University of Delaware [35]. The PRP data were obtained from the Global Precipitation Climatology Centre Full Data Reanalysis V.8 version dataset [36,37]. The vertical velocity, PRP, and SAT data were provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.esrl.noaa.gov/psd/ (accessed on 17 May 2021). The horizontal resolution grids of the data are 2 • for SST, 1 • for vertical velocity and PRP, and 0.5 • for SAT. All data were obtained for the 1901-2014 period, which is the common period of the datasets.
In each mean state, the austral winter and summer variable anomaly composites were calculated separately over all (ALL-) and ENSO neutral (NEU-) years. Kayano et al. [29] already analyzed the austral summer PRP and WVEL anomaly composites of ALL-years during the CAWP, CACP, and WACP mean states, which they determined in the 1901-2011 period. Nonetheless, to facilitate the results' interpretation, they were recalculated here, but used the 1901-2014 period data. Following up Hoerling et al. [32], the nonlinear responses to the ENSO variability were estimated for each variable by the summation of the EN and LN anomaly composites (EN + LN) and were calculated in each mean state. For comparison purposes, the nonlinear PRP and SAT responses to ENSO were also obtained considering all ENSO events in the 1901-2014 period, and are shown in Section 3.1. The statistical significance of the composites was assessed with the Student's t-test for the mean at a 90% confidence level [38].
The austral winter and summer variable composites of ALL-, EN + LN-, and NEUyears in each mean state are analyzed in Sections 3.2 and 3.3. These composites are referred to with the mean state acronyms, and whenever necessary, followed by ALL, EN + LN, and NEU. The anomalies corresponding to the ALL, EN + LN, and NEU are referred to as mean, nonlinear, and NEU anomalies, respectively. For the WVEL and HVEL, only the composites over EN + LN-years are shown. The negative (positive) WVEL and HVEL anomalies represent anomalous ascending (descending) motion associated with the anomalous Walker and regional Hadley cells, respectively. to southeastern Brazil during DJF (Figure 1c,d). During DJF, the opposite sign PRP and SAT anomalies in central South America are consistent. The magnitude of the significant nonlinear PRP and SAT anomalies varies from 0.1 to 0.2 SD units in both seasons. Thus, considering all ENSO events over the period in which the climatologies are based, the nonlinear PRP and SAT responses to the ENSO are weak in South America (Figure 1). This can be the main reason why this aspect of the ENSO responses in South America has been overlooked. and southern (positive) Brazil during JJA; negative anomalies are observed in relatively larger areas of central South America, from northwestern to southeastern Brazil around 20° S, southwestern Bolivia, and positive ones in central-southeastern Brazil (east of 60° W) and central-eastern Argentina during DJF (Figure 1a,b). Regarding the SAT, the significant nonlinear anomalies are dominantly positive and found in the central South American sector extending from 10° S to 20° S during JJA and from the central Amazon to southeastern Brazil during DJF (Figure 1c,d). During DJF, the opposite sign PRP and SAT anomalies in central South America are consistent. The magnitude of the significant nonlinear PRP and SAT anomalies varies from 0.1 to 0.2 SD units in both seasons. Thus, considering all ENSO events over the period in which the climatologies are based, the nonlinear PRP and SAT responses to the ENSO are weak in South America ( Figure 1). This can be the main reason why this aspect of the ENSO responses in South America has been overlooked.   negative PRP anomalies appear in small scattered areas in western South America around 20° S, as well as in northern and central-eastern South America (Figure 2a). During the CAWP, significant mean PRP anomalies occur in small areas, being negative in western South America (northern Peru, northwestern Bolivia, south of 35° S) and in the extreme northern and northeastern areas of Brazil, and are positive in equatorial Brazil east of 60° W and southern Brazil (Figure 2b). Otherwise, the WACP mean state features significan negative mean PRP anomalies in several areas of South America west of 50° W, spread to the south of 20° S and the southern and southeastern Amazon; the akin positive PRP anomalies are centered at 15° S in central-eastern Brazil (Figure 2c). Meanwhile, significan negative mean PRP anomalies extend in central-eastern South America east of 65° W and north 25° S as well as small areas of equatorial South America, and the counterpar positive PRP anomalies appear in small areas of South America to the south of 20° S including northwestern Peru and Suriname during WAWP (Figure 2d).  Thus, comparisons of the three composites (ALL, EN + LN, and NEU) indicate that the ENSO nonlinearities strongly modulate the mean PRP anomalies during the CACP. This is also confirmed by comparing the pattern correlations between ALL and EN + LN composites as well as between ALL and NEU composites of 0.91 and 0.27, respectively (Table 3). During the CAWP, the significant negative NEU PRP anomalies occur in almost all of the small areas with the same sign mean PRP anomalies, plus areas in southern Brazil and Uruguay, and the positive NEU PRP anomalies appear in central Venezuela, the central Amazon (around 60 • W), central-eastern South America (7 • S-20 • S), and southern Bolivia (Figure 2b,j). The opposite sign significant NEU and nonlinear PRP anomalies contrast in eastern South America between 10 • S and 30 • S and in western South America around 20 • S, and justify the reduced mean PRP anomalies (Figure 2b,f,j). As such, the ENSO-and ENSO-neutral-years modulate the mean PRP anomalies during CAWP (Figure 2b,f,j). This is confirmed by comparable pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.60 and 0.71, respectively (Table 3).

Nonlinear PRP and SAT Responses to ENSO during the 1901-2014 Period
Significant negative NEU PRP anomalies appear in small areas along two paths, one in northern South America, and the other from southeastern Colombia to northeastern Brazil across central and southeastern Amazonia, subtropical South America (western South America, part of southern Brazil), and the positive ones in central-eastern Brazil during WACP (Figure 2k). Except for those in northern South America, these anomalies have correspondences with mean PRP anomalies, whereas the opposite sign nonlinear and NEU PRP anomalies defined reduced mean PRP anomalies in northern South America (Figure 2g,k). Therefore, the ENSO-and ENSO-neutral-years determine the mean PRP anomalies during WACP (Figure 2c,g,k), as also indicated in Table 3 with pattern correla- tions between ALL and EN + LN composites and between ALL and NEU composites of 0.77 and 0.70, respectively During the WAWP, the significant negative NEU PRP anomalies in central-eastern South America (5 • S-18 • S) are less extensive than the negative mean and nonlinear PRP anomalies (Figure 2d,h,l). Thus, the ENSO-and ENSO-neutral-years have contributed to the mean PRP anomaly pattern (Figure 2d,h,l). Table 3 shows pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.70 and 0.82, respectively.

SAT Anomaly Patterns
Austral winter SAT composites of ALL-years show remarkable differences among the four mean states (Figure 3a The austral winter nonlinear SAT anomaly composites stratified by the mean states are disclosed in Figure 3e-h. These anomalies are stronger and more extensive than those obtained considering all ENSO events during the study period ( Figure 1c). As for the PRP, the austral winter SAT composites of ALL-and EN + LN-years of each mean state show similarities, with larger magnitudes of the nonlinear SAT anomalies (Figure 3a-h). This is conspicuous for the CACP composites (Figure 3a,e). Regarding the mean SAT anomalies during the CAWP, the significant negative nonlinear SAT anomalies appear in a slightly narrower band in western tropical South America and southern South America, and the positive ones appear in a larger tropical area (Figure 3b,f). Similarities are also noticeable during WACP between ALL and EN + LN SAT composites, but the significant negative nonlinear SAT anomalies over western South America extend meridionally from central Colombia to central-eastern Argentina (Figure 3c,g). The WAWP/ALL and WAWP/EN + LN SAT composites also present similar patterns, but the significant positive nonlinear SAT anomalies are more extensive in northern South America and less extensive in subtropical South America, and the negative ones are limited to the east coast of Brazil (5 • S-25 • S) (Figure 3d,h).
The austral winter NEU SAT composites stratified by the mean states are given in Comparisons among ALL, EN + LN, and NEU composites suggest that the nonlinear SAT anomalies are more crucial in defining the mean SAT anomalies during CACP (Figure 3a,e,i). This is confirmed in Table 4 with the pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.93 and 0.54, respectively. other in northern Peru (Figure 3i). Both positive areas have correspondences with those in the ALL composite, and that in western Colombia with the EN + LN composite ( Figure  3a,i). Comparisons among ALL, EN + LN, and NEU composites suggest that the nonlinear SAT anomalies are more crucial in defining the mean SAT anomalies during CACP (Figure 3a,e,i). This is confirmed in Table 4 with the pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.93 and 0.54, respectively.   During the CAWP, significant negative NEU SAT anomalies extend from northern Colombia/western Venezuela to 20 • S, part of northeast Brazil, part of central-western Brazil, southeastern and southern Brazil, and the positive ones occur in the extreme south of South America (Figure 3j). The negative NEU SAT anomalies in western tropical South America are more extensive than the corresponding mean and nonlinear SAT anomalies (Figure 3b,f,j). For this mean state, the mean SAT anomalies are combinations of the NEU and nonlinear SAT anomalies, but the nonlinear SAT anomalies modulate the anomalous warming in central Bolivia, north of Chile, the eastern Amazon, and along the east coast of northeast Brazil (Figure 3b,f,j). The pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.84 and 0.72, respectively, are comparable (Table 4).
During the WACP, the NEU and ALL SAT composites show similarities, but with the more horizontal structure of the significant NEU SAT anomalies such that the positive values occur in reduced areas of northwestern South America, the southeastern Amazon, northern of northeast Brazil, and subtropical eastern South America, and the negative ones appear in western South America between 5 • S and 20 • S (Figure 3c,k). For this mean state, the mean SAT anomalies seem to be mostly modulated by the nonlinear anomalies, particularly the anomalous cooling along western South America and the anomalous warming from southeastern Colombia to central-western Brazil (Figure 3c,g). However, the pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.88 and 0.79, respectively, are comparable ( Table 4). The WAWP/ALL and WAWP/NEU anomaly patterns are very similar but with smaller magnitudes of the NEU SAT anomalies (Figure 3d,l). In fact, pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.88 and 0.90, respectively, are very close (Table 4).

WVEL and HVEL Anomaly Patterns
The composites of the WVEL and HVEL anomalies over EN + LN-years of each mean state, and the corresponding composites over ALL-and NEU-years, generally show similar patterns. As such, only the WVEL and HVEL composites over EN + LN-years are presented and compared with the corresponding PRP and SAT composites.
During the CACP, the significant negative nonlinear WVEL anomalies (upward motion) occur in most tropospheric levels in the 80 • W-20 • W, with the largest anomalies at 30 • W (Figure 4a). This ascending motion (Figure 4a (Figure 4e). These HVEL anomalies are part of a regional anomalous meridional cell, which is consistent with nonlinear anomalous dryness over northwestern Venezuela and the nonlinear anomalous wetness in the South American area between 5 • N and 15 • S and west of 60 • W (Figures 2e and 4e). In this South American sector, nonlinear SAT anomalies show more horizontal structure, but the prevailing negative values (Figure 3e) are consistent with the meridional cell's rising branch (Figure 4e).
During the CAWP, the significant positive nonlinear WVEL anomalies (downward motion) occur in the 70 • W-55 • W sector from 700 hPa to 400 hPa and in the 40 • W-15 • W sector in most tropospheric levels (Figure 4b). These prevailing downward motions in most South American longitudes along 6 • S is coherent with the anomalous warming of most of tropical South America, except for a narrow, meridional area in its western side (Figure 3f). Additionally, the downward motion over northeast Brazil longitudes justifies the nonlinear negative PRP anomalies in this region (Figures 2f and 4b). The CAWP/EN + LN HVEL composite discloses an anomalous descending motion from lower to middle tropospheric levels between 5 • N and 17 • S and to the south of this latitude from middle to upper tropospheric levels (Figure 4f). This downward motion further justifies the southward extension of anomalous warming in tropical South America, particularly to the west of 60 • W (Figures 3f and 4f).
The WACP/EN + LN WVEL composite shows a well-defined anomalous east-west cell, with its rising and sinking branches, respectively, in the 80 • W-50 • W and 50 • W-20 • W bands (Figure 4c), which seem to be eastward displaced by 10 • in longitude about the maximum anomalous cooling and warming along 6 • S of the corresponding EN + LNSAT composite (Figure 3g). In addition, for the corresponding HVEL composite, the significant nega-tive anomalies (upward motion) in most tropospheric levels in the 5 • N-15 • S sector justify the negative SAT anomalies along the west coast of South America (Figures 3g and 4g). Nevertheless, the WACP/EN + LN PRP composite presents complex horizontal structures so that consistencies with the anomalous east-west and north-south cells are not obvious (Figures 2g and 4c,g). southward extension of anomalous warming in tropical South America, particularly to the west of 60° W (Figures 3f and 4f).
The WACP/EN + LN WVEL composite shows a well-defined anomalous east-west cell, with its rising and sinking branches, respectively, in the 80° W-50° W and 50° W-20° W bands (Figure 4c), which seem to be eastward displaced by 10° in longitude about the maximum anomalous cooling and warming along 6° S of the corresponding EN + LNSAT composite (Figure 3g). In addition, for the corresponding HVEL composite, the significant negative anomalies (upward motion) in most tropospheric levels in the 5° N-15° S sector justify the negative SAT anomalies along the west coast of South America (Figures 3g and  4g). Nevertheless, the WACP/EN + LN PRP composite presents complex horizontal structures so that consistencies with the anomalous east-west and north-south cells are not obvious (Figures 2g and 4c,g). Consistencies among WAWP/EN + LN PRP, SAT, and WVEL composites are not evident (Figures 2h, 3h and 4d). During WAWP, significant negative nonlinear HVEL anomalies prevail in the equator-10° S sector (ascending motion) and the positive ones in the 15° S-25° S band (descending motion) (Figure 4h). This descending motion to the south of 15° S and the nonlinear anomalous warming along western South America are consistent (Figures 3h and 4h). However, the ascending motion to the north of 10° S and the positive nonlinear SAT anomalies are not consistent. The inconsistencies seem to be due to the small number of ENSO events during the WAWP mean state. Consistencies among WAWP/EN + LN PRP, SAT, and WVEL composites are not evident (Figures 2h, 3h and 4d). During WAWP, significant negative nonlinear HVEL anomalies prevail in the equator-10 • S sector (ascending motion) and the positive ones in the 15 • S-25 • S band (descending motion) (Figure 4h). This descending motion to the south of 15 • S and the nonlinear anomalous warming along western South America are consistent (Figures 3h and 4h). However, the ascending motion to the north of 10 • S and the positive nonlinear SAT anomalies are not consistent. The inconsistencies seem to be due to the small number of ENSO events during the WAWP mean state.

PRP Anomaly Patterns
The austral summer PRP anomaly patterns stratified by the mean states are illustrated in Figure 5a-d. During CACP, CAWP, and WACP, comparisons with the maps shown in Kayano et al. [29] indicate that the inclusion of three additional years implies only small differences. During the CACP, significant negative mean PRP anomalies extend in areas from the central Amazon  (Figure 5d). The sign reversal of the mean PRP anomalies in western tropical South America between the WACP (positive) and CAWP (negative) during DJF reported by Kayano et al. [29] is also noted here. They argued that this sign reversal is due to a regional Walker cell's multidecadal connection between the Atlantic and Pacific [23][24][25]. Still, for the WACP, the positive mean PRP anomalies over the western Amazon are consistent with the previous finding on the increase of PRP over the 1979-2015 period [39,40]. The austral summer PRP anomaly composites for NEU-years stratified by the mean states are given in Figure 5i-l. During the CACP, the main feature is the significant negative NEU PRP anomalies in the central Amazon (center at 10° S; 65° W), which is slightly southward extended in relation to that of the ALL composite (Figure 5a,i). The austral summer composites of the nonlinear PRP anomalies stratified by the mean states are given in Figure 5e-h. In this season, it is interesting to note the similarities between the nonlinear PRP anomaly patterns of the ENSO-years of the study period and the CACP mean state (Figures 1b and 5e). Thus, the PRP nonlinearity associated with ENSO variability during the CACP strongly modulates the PRP nonlinearity over the study period. Consequently, the PRP anomaly composites of the ALL-and EN + LN-years of the CACP mean state are very similar (Figure 5a,e).
During the CAWP, the EN + LN and ALL PRP anomaly composites show similarities in the South American sector to the north of 10 • S, except for better defined significant negative nonlinear PRP anomalies in the western and central Amazon (Figure 5b,f). Still, during the CAWP, significant negative nonlinear PRP anomalies occur in small areas of northern Chile, southern Peru, western Uruguay and adjacent Argentina, western South America to the south of 30 • S, and the positive ones occur in areas of southern Bolivia, Paraguay, central-western, southeastern, and southern Brazil (Figure 5f). For the WACP, the significant positive nonlinear PRP anomalies appear in some scattered areas in most of South America west of 50 • W, except for two small areas between 30 • S and 40 • S, and the negative ones extend over northeast Brazil (Figure 5g). This negative area in northeast Brazil has correspondence with that in the CAWP/ALL composite (Figure 5c,g). The WAWP/EN + LN and WAWP/ALL composites of the PRP anomalies show similar patterns, except for larger magnitudes of the nonlinear anomalies (Figure 5d,h).
The austral summer PRP anomaly composites for NEU-years stratified by the mean states are given in Figure 5i-l. During the CACP, the main feature is the significant negative NEU PRP anomalies in the central Amazon (center at 10 • S; 65 • W), which is slightly southward extended in relation to that of the ALL composite (Figure 5a,i). Considering the ALL, EN + LN, and NEU PRP anomaly composites, it is clear that the ENSO nonlinearity is crucial in defining the mean PRP anomaly pattern during the CACP (Figure 5a,e,i). In fact, the pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.90 and 0.50, respectively, confirm this aspect (Table 5). During the CAWP, significant positive NEU PRP anomalies appear in small areas of central-eastern South America, from northern Bolivia to central Argentina, and negative ones between 40 • S and 50 • S (Figure 5j). Comparisons of the ALL, EN + LN, and NEU anomaly composites for this mean state suggest that most features of the ALL anomaly composite in the South American sector north of 10 • S and west of 58 • W are modulated by the ENSO nonlinearities, but elsewhere the neutral-ENSO-years seem to be dominant (Figure 5b,f,j). Pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.75 and 0.70, respectively, indicate that both EN + LN and NEU PRP anomalies define the mean PRP anomalies (Table 5).
During the WACP, significant positive NEU PRP anomalies occur in the western and southeastern Amazon, and the negative ones appear in areas of southern Brazil and central Argentina. Analyzing the three composites (ALL, EN + LN, and NEU) of the WACP mean state, similarities between the NEU and the other two composites are identified in specific areas. They refer to the positive values in the western Amazon and negative ones in southern Brazil and central Argentina noted for NEU and ALL composites; the positive values in the southeastern Amazon and the negative ones in northeast Brazil are noted for the NEU and EN + LN composites (Figure 5c,g,k). Both EN + LN and NEU PRP anomalies determine the mean PRP anomalies (Table 5).
For the WAWP, significant positive NEU PRP anomalies show up in the southeastern Amazon (area centered at 10 • S; 55 • W), two small areas along 18 • S (western Bolivia, eastern Brazil), and the negative ones occur in equatorial eastern South America (around 50 • W) (Figure 5l). These areas also appear in the ALL PRP composite (Figure 5d), but outside these areas, the NEU anomalies are quite reduced and the similarities are more evident between the ALL and EN + LN composites. Thus, except for the mentioned significant NEU PRP anomalies, the mean PRP anomalies are strongly modulated by the ENSO variability during WAWP (Figure 5d,h,l). However, the pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.77 and 0.82, respectively, are comparable (Table 5).

SAT Anomaly Patterns
The austral summer SAT anomaly composites of ALL-years of the four mean states are given in Figure 6a-d. They exhibit differences among them, but similarities with the corresponding austral winter composites, particularly in areas north of 20 • S, are apparent (Figures 3a-d and 6a-d). During the CACP, significant negative mean SAT anomalies appear in northern South America, the eastern Amazon, northeast Brazil, Paraguay, southern Brazil, southern South America south of 35 • S, and narrow areas along the west coast of South America from 5 • S to 35 • S (Figure 6a). During the CAWP, significant negative mean SAT anomalies extend from northern Colombia to southern Peru, from westernsoutheastern Brazil to northeastern Argentina, and northeast Brazil, with the positive ones occurring from the western Amazon to central-western Brazil and adjacent Bolivian areas, part of southeastern Brazil and extreme southern South America (Figure 6b). Meanwhile, during the WACP, significant positive mean SAT anomalies show up in northwestern South America, northeast Brazil, and southeastern Brazil, and the negative ones appear in western South America from the equator to 25 • S. On the other hand, the austral summer and winter SAT anomaly composites of ALL-years during the WAWP mean state show similar patterns, except for the occurrence of the significant positive mean SAT anomalies in northeast Brazil and their absence in southern South America during summer (Figures 3d and 6d).
The austral summer nonlinear SAT anomaly composites for each mean state are shown in Figure 6e-h. These and ALL-year composites exhibit quite similar features with stronger nonlinear SAT anomalies (Figure 6a-h). In particular, the ALL and EN + LN SAT composites during the CACP show striking similarities (Figure 6a,e). During the CAWP, besides the similar patterns of the ALL and EN + LN SAT composites, the significant nonlinear SAT anomalies present less horizontal structure, highlighting the positive area in central South America limited approximately at the equator, 25 • S, 70 • W, 50 • W (Figure 6b,f). During the WACP, regarding the mean SAT anomalies, significant positive nonlinear SAT anomalies are more extensive in the area from the western to central-southeastern Amazon and more intense over northeast Brazil and southeastern Brazil, while the negative ones in western South America are less extensive, remaining north of 18 • S (Figure 6c,g). During the WAWP, the ALL and EN + LN SAT composites also exhibit similarities, such that the significant positive nonlinear SAT anomalies are more extensive in northeastern and subtropical South America, and the negative ones are less extensive, being limited to two small areas (the northwestern Amazon and central South America around 20 • S (Figure 6d,h).
The austral summer NEU SAT composites stratified by the mean states are shown in Figure 6i-l. During the CACP, significant negative NEU SAT anomalies appear in small scattered areas along the west coast of South America south of 25 • S, southeastern Brazil, and the southeastern Amazon (Figure 6i). Considering the three composites for this mean state, it is obvious that the ENSO-years strongly define the mean SAT anomalies during CACP (Figure 6a,e,i). This is confirmed by the pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.94 and 0.61, respectively (Table 6).  The CAWP/NEU SAT composite presents significant negative SAT anomalies in most of tropical South America north of 20° S, except for areas in central, northern, and northeastern South America, and the positive ones appear in the east coast of South America to the south of 35° S (Figure 6j). The negative NEU SAT anomalies along western and eastern tropical South America north of 20° S overlap the same sign mean SAT anomalies (Figure 6b,j). As such, the mean SAT anomalies during CAWP result from a  The CAWP/NEU SAT composite presents significant negative SAT anomalies in most of tropical South America north of 20 • S, except for areas in central, northern, and northeastern South America, and the positive ones appear in the east coast of South America to the south of 35 • S (Figure 6j). The negative NEU SAT anomalies along western and eastern tropical South America north of 20 • S overlap the same sign mean SAT anomalies (Figure 6b,j). As such, the mean SAT anomalies during CAWP result from a combination of the NEU and nonlinear SAT anomalies. The last ones are important in central South America (Figure 6b,f,g). The pattern correlations between ALL and EN + LN composites and between ALL and NEU composites of 0.86 and 0.70, respectively, are comparable (Table 6).
During the WACP, significant positive NEU SAT anomalies appear in small areas of northwestern, northeastern, and southwestern South America, as well as central-western Brazil (south of 20 • S), and the negative ones occur from northern Peru to Bolivia (Figure 6k). The positive areas north of 20 • S occur in the three composites of this mean state (Figure 6c,g,k). Therefore, the mean SAT anomaly pattern during WACP is due to both ENSO and ENSO neutral years (Figure 6c,g,k). The WAWP/NEU and WAWP/ALL SAT composites show similar patterns but with smaller magnitudes of the NEU SAT anomalies (Figure 6d,l). For the WAWP, the similarities among the ALL, EN + LN, and NEU composites indicate that the corresponding mean SAT anomaly composite results from combinations of the nonlinear and NEU SAT anomalies (Figure 6d,h,l). For WACP and WAWP, Table 6 shows almost the same magnitude of the pattern correlations between ALL and EN + LN composites and between ALL and NEU composites.

WVEL and HVEL Anomaly Patterns
As for the austral winter, only the WVEL and HVEL composites over EN + LN-years are presented and compared with the corresponding PRP and SAT composites The CACP, CAWP, and WAWP of the WVEL anomaly composites over the EN + LN-years feature a significant anomalous east-west dipole along 6 • S with the separation line of the nodes approximately at 60 • W (Figure 7a-c). During the CACP, this dipole features its sinking and rising branches, respectively, to the west and east of 60 • W (Figure 7a), and at the same time, significant positive nonlinear HVEL anomalies occur in the 5 • N-15 • S band (Figure 7e). Therefore, the descending motion occurs to the west of 60 • W and between 5 • N and 15 • S and is consistent with the negative nonlinear PRP anomalies in the central Amazon to the west of 60 • W during CACP (Figures 5e and 7a,e). On the other hand, the ascending motion to the east of 60 • W is concordant with the negative nonlinear SAT anomalies in the eastern Amazon and northeast Brazil (Figures 6e and 7a). between the equator and 15° S is concordant with the anomalous warming in western South America (Figures 6h and 7d,h). There is also a noticeable descending motion around 40° W (Figure 7d), which agrees with the predominantly nonlinear dryness and warming over northeast Brazil (Figures 5h and 6h).

Conclusion and Summary
Recently, Kayano et al. [28,29] examined the SST, PRP, and vertical velocity mean anomaly patterns during the mean states defined by the overlapping periods of AMO and PDO phases. Nevertheless, they did not determine if the ENSO variability affects these During the CAWP, the dipole presents its rising and sinking branches, respectively, in the western and eastern nodes (Figure 7b). This anomalous rising-sinking cell is consistent with the nonlinear anomalous cooling over western South America (northern Colombia to southern Peru), and the akin warming in central South America (Figures 6f and 7b). The associated HVEL anomaly composite indicates an upward motion at most tropospheric levels in the 5 • N-12 • S band, and a downward motion in the 12 • S-17 • S (Figure 7f). The ascending motion to the north of 12 • S is consistent with nonlinear anomalous cooling in Colombia and the descending motion between 12 • S and 17 • S is consistent with nonlinear anomalous warming and dryness in central South America west of 60 • S (Figures 6f and 7f).
During the WACP, the nodes of the east-west dipole of the nonlinear WVEL anomalies have the same signs as those of that during CAWP, but with weaker anomalies (Figure 7b,c). During the WACP, the maximum descending motion of the eastern node appears in longitudes (just to the east of 60 • W and 40 • W) of anomalously warmed areas in the centralsoutheastern Amazon and northern of northeast Brazil, with this last area also presenting anomalous dryness (Figures 5g, 6g and 7c). This confirms the consistencies of the nonlinear WVEL, PRP and SAT anomalies in these areas. In addition, the ascending motion of the western node and the prevailing upward motion in the equator-18 • S band associated with the HVEL/EN + LN composite are in agreement with the nonlinear anomalous cooling in western South America (5 • N-20 • S band) (Figures 6g and 7c,g).
The WAWP/EN + LN WVEL and HVEL composite do not show well-defined zonal and meridional cells (Figure 7d,h). However, there are clear indications of the consistencies of these composites and the corresponding SAT composite (Figures 6h and 7d,h). In fact, the descending motion associated with the WVEL around 70 • W and HVEL between the equator and 15 • S is concordant with the anomalous warming in western South America (Figures 6h and 7d,h). There is also a noticeable descending motion around 40 • W (Figure 7d), which agrees with the predominantly nonlinear dryness and warming over northeast Brazil (Figures 5h and 6h).

Conclusions and Summary
Recently, Kayano et al. [28,29] examined the SST, PRP, and vertical velocity mean anomaly patterns during the mean states defined by the overlapping periods of AMO and PDO phases. Nevertheless, they did not determine if the ENSO variability affects these anomaly patterns. This aspect was investigated here for WVEL, HVEL, PRP, and SAT over South America using 114 years of data (1901-2014) and considering the CACP, CAWP, WACP, and WAWP mean states. Considering the EN-and LN-years during the study period (Tables 1 and 2), analyses were done for the austral winter and summer and based on variable anomaly composites obtained over ALL-, EN + LN-, and NEU-years in each mean state, with the corresponding anomalies referred to as mean, nonlinear, and NEU.
The nonlinear PRP and SAT anomalies considering all ENSO events during the study period are almost negligible (Figure 1). Notwithstanding, the nonlinear PRP and SAT anomalies of the mean states are not negligible, and their patterns are similar to those of the corresponding mean anomaly composites (Figures 2a-h, 3a-h, 5a-h and 6a-h). For both seasons, the similarities between ALL and EN + LN composites are more pronounced for SAT than for PRP (Figures 2a-h, 3a-h, 5a-h and 6a-h). However, the ALL PRP and SAT anomaly patterns also show some features of the corresponding NEU anomaly composites during the CAWP, WACP, and WAWP (Figures 2b-d, 2j-l, 3b-d, 3j-l, 5b-d and 6j-l). In contrast, during both seasons of the CACP, and for both variables, the similarities between ALL and EN + LN composites and the dissimilarities between these two composites and the NEU composites are remarkable (Figures 2a,e,i, 3a,e,i, 5a,e,i and 6a,e,i). Thus, the mean PRP and SAT anomaly patterns are strongly modulated by the ENSO years during the CACP.
Moreover, ALL, EN + LN, and NEU SAT composites of each mean state present similarities between the two analyzed seasons, implying, in general, a weak seasonal dependence of these composites; in contrast, the corresponding PRP composites show much more seasonal variations (Figures 2, 3, 5 and 6). Additionally, consistencies be-tween the nonlinear vertical velocity and SAT anomalies, with anomalous downward (upward) motion associated with warming (cooling), are conspicuous for both seasons and most mean states (Figures 3e,f, 4a-h, 6e,f and 7a-h). Nevertheless, the consistencies between the nonlinear vertical velocity and PRP anomalies are less evident, except during CACP, with an anomalous downward (upward) motion associated with dryness (wetness) (Figures 2e,f, 4a-h, 5e,f and 7a-h).
These results indicate that the ENSO-and ENSO-neutral-years affect the variable anomaly mean states in different ways depending on the season and variable. As such, the studies on the low-frequency variability should consider not only the variable mean states but also that the ENSO variability might modulate them. It is also worth recalling that the physical consistencies among variable anomaly composites guarantee the robustness of the results. Furthermore, we calculated the 200 hPa stream function composites (Figures  not shown). However, after analyzing the figures, we did not find a clear signature of the Rossby-wave trains because we are dealing with the averages of the variable anomalies during ALL-, EN + LN-, and NEU-years. The Rossby-wave train pattern should appear when analyzing EN and LN events separately. From another point of view, Enfield and Mestas-Nunez [41] applying the empirical orthogonal function analysis on the global residual SST anomalies (after removing the ENSO signal), found that the first three non-ENSO modes represent the low-frequency variability modes such that the first two modes have the strongest decadal to multidecadal signature in the Pacific and the third one in the extratropical North Atlantic. These modes correspond to the PDO and AMO. So, the results here indicated that although these are non-ENSO modes, the occurrences of ENSO extremes during their phases affect the PRP and SAT mean states in South America.
The results here have an essential implication for climate studies, in which the monthly anomalies are calculated concerning monthly climatologies based on a fixed 30-year period, which generally spans over three complete decades such as 1961-1990, 1971-2000, and 1981-2010. These 30-year periods combine distinct mean states. In the present analysis, we did not deal with the nature of the ENSO nonlinearities when the nonlinear ENSO response was considered as the summation of the EN-and LN-years [32] in each mean state. Nonetheless, we are conscious that a more detailed study evaluating the ENSO nonlinearities due to types and intensities [42] is still necessary to complement the present analysis.