The role of SST on the South American atmospheric circulation during January, February and March...

Anita Rodrigues de Moraes Drumond Æ Tercio Ambrizzi

The role of SST on the South American atmospheric circulationduring January, February and March 2001

Received: 20 August 2003 / Accepted: 31 August 2004 / Published online: 13 May 2005� Springer-Verlag 2005

Abstract Precipitation deficits were observed oversoutheastern, northeastern and Central Brazil during the2001 Austral Summer. They contributed to the wors-ening of the energy crisis that was occurring in thecountry. A low-level anomalous anticyclonic circulationobserved over eastern Brazil enhanced the deviation ofmoisture transport that usually occurs from the AmazonBasin to southeastern Brazil and inhibited the occur-rence of South Atlantic Convergence Zone events in thatperiod. However, an anomalous low-level northerlymoisture flux was observed over the La Plata Basin, andpositive precipitation anomalies occurred over Bolivia,Paraguay, northeastern Argentina and southern Brazil.Using the ensemble technique, a numerical study wascarried out to investigate the role of different sea surfacetemperature (SST) forcings observed over this anoma-lous South American atmospheric circulation. ReynoldsSST monthly means were used as boundary conditionsto study the influence of South Atlantic, South Indian,South Pacific and Equatorial Pacific oceans. The simu-lations were run from September 2000 to April 2001using the Community Climate Model version 3.6 Gen-eral Circulation Model. Ten integrations using differentinitial conditions were done to each experiment.Numerical experiments suggested that the combinedinfluence of South Pacific and Equatorial Pacific oceanscould be responsible for the drought observed overCentral Brazil. These experiments simulated the low-le-vel anticyclonic anomaly observed over eastern Brazil.However, both experiments have poorly reproduced the

intensity of the anomalous low-level northerly moistureflux observed over the La Plata Basin. Therefore, theintensity of the simulated precipitation anomalies overthe subtropical regions was much weaker than observed.

1 Introduction

Several systems are important for the characterization ofprecipitation over South America during the AustralSummer. For example, precipitation over Central andsoutheastern Brazil is modulated by the occurrence ofSouth Atlantic Convergence Zone (SACZ) events(Kodama 1992). SACZ can be identified on satelliteimages as a band of nebulosity that extends fromsouthern Amazon to subtropical South Atlantic orientedin the NW–SE direction (Kodama 1992; Figueroa et al.1995; Liebmann et al. 1999). The mechanisms related tothe onset and persistence of SACZ events are still un-known. However, studies have indicated the importanceof remote factors, such as the Madden-Julian Oscillation(Casarin and Kousky 1986; Kiladis and Weickmann1992; Paegle et al. 2000; Lenters and Cook 1999), localfactors such as the convection over the Amazon (Rochaet al. 1996), the moisture source over Central Brazil(Sakamoto and Silva Dias 1990), the presence of Andes(Satyamurty and Rao 1988) and the penetration offrontal systems (Oliveira 1986). The climatological pre-cipitation patterns can be viewed in Fig. 1a.

On the other hand, Austral Summer precipitationover southern Brazil and the La Plata Basin is stronglymodulated by the variability of the Low-Level Jet (LLJ).LLJ is observed over a region close to eastern Andes andcontributes to the meridional moisture transport fromthe Amazon to the subtropics (Marengo et al. 2002 andreferences therein).

Herdies et al. (2002) verified two main summercirculation patterns over the continent: one associatedwith the occurrence of SACZ events and the other

A. R. M. Drumond (&) Æ T. AmbrizziDepartment of Atmospheric Sciences,Institute of Astronomy,Geophysics and Atmospheric Sciences,University of Sao Paulo, Rua do Matao,1226, 05508-900 Sao Paulo, BrazilE-mail: [email protected].: +55-11-30914713Fax: +55-11-30914714E-mail: [email protected]

Climate Dynamics (2005) 24: 781–791DOI 10.1007/s00382-004-0472-3

with their absence. During SACZ events, they ob-served an intense LLJ carrying moisture from theAtlantic ocean and the Amazon to the subtropics. ThisLLJ is associated with moisture-flux convergence andprecipitation over the Amazon basin and southeasternBrazil, whereas the weakening of the moisture trans-port to the La Plata Basin causes the reduction ofprecipitation over northern Argentina, Paraguay,southern Bolivia and northern Chile. On the otherhand, during the absence of SACZ events, the LLJ isdisplaced westward and the moisture transport overthe SACZ region is interrupted. As a consequence,moisture–flux convergence over southeastern Brazil isweakened and moisture is transported towards the LaPlata Basin.

Using statistical analysis, Nogues-Paegle and Mo(1997) suggested the existence of a precipitation dipolepattern over South America. During SACZ events, there

is a maximum of precipitation over its domain regionand a minimum over the subtropics. The opposite occursduring the reduction of the SACZ activity.

It is well known that the atmospheric patterns can bemodulated by variations on sea surface temperature(SST) and therefore the precipitation pattern over re-mote regions may be modified. For example, studies ofinterannual climate variability have shown the influenceof Southern Oscillation (SO) extreme events over SouthAmerican climate (e.g., Ropelewski and Halpert 1987;Rao and Hada 1990; Coelho et al. 2002 and referencestherein). Other studies have showed that the TropicalAtlantic can act as a modulator of precipitation overnorthern and northeastern South America even duringSO extreme events, thereby emphasizing the importanceof the Pacific and Atlantic oceans over South Americanclimate (e.g., Pezzi and Cavalcanti 2001; Uvo et al.1998).

Fig. 1 a JFM precipitationclimatological mean (mmday�1). Climatology wasdefined for the period 1982/2001. Only values higher than2 mm day�1 are shown, and thevalues of isolines are equal to 2;4; 6; 8; 10 mm day�1; b JFM2001 precipitation anomaliesobserved (mm day�1). Theabsolute values of isolines are0.3; 1; 2; 4; 6 mm day�1, andnegative values are shown withdashed contours. Anomalieshigher than 0.3 mm day�1 arein dark gray and lower than�0.3 mm day�1 are in lightgray. (Data: CMAP)

782 Drumond and Ambrizzi: The role of SST on the South American atmospheric circulation

However, the role of austral extratropical oceanicregions in the South American climate is still unknown.Coelho (2001) studied the relationship between thesubtropical South Atlantic and South American pre-cipitation during El Nino events. He found a negativecorrelation between the SST anomalies observed insouthwestern Atlantic and precipitation anomalies overParaguay, central Amazon and Brazilian Northeast anda positive correlation over southern and southeasternBrazil.

Some studies have shown the relationship betweenSACZ and the SST near the southeastern South Amer-ican coast. Nobre (1988) suggested that, during SACZepisodes, the warm SST anomaly observed in the regionof their occurrence is, at least partially, an oceanic re-sponse to the atmospheric circulation. Barros et al.(2000) showed that the summer precipitation overnortheastern Argentina, Uruguay and southern Brazil isinfluenced by the subtropical South Atlantic. The posi-tive (negative) SST anomalies observed over this regionare associated with the displacement of SACZ south-ward (northward) and contribute to the increase(reduction) of the precipitation over southeastern SouthAmerica. These results are consistent with Diaz et al.(1998), who observed the occurrence of wet conditionsover Uruguay and southern Brazil associated with po-sitive SST anomalies in the SACZ region during theAustral Summer.

Concerning the South Pacific, Barros and Silvestri(2002) verified that the subtropical south-central Pacific(SSCP) and the central-east equatorial Pacific have asimilar effect on the interannual variability of precipi-tation observed over southeastern South America duringthe Austral Spring. Moreover, they found that inter ElNino variability of precipitation over this region ismodulated mainly by the SSCP. Therefore, the occur-rence of positive (negative) anomalies of precipitationover southeastern South America is associated withnegative (positive) SST anomalies over SSCP.

During the 2001 Austral Summer, defined here as theperiod between January and March of 2001, precipita-tion deficits over northeastern, southeastern and CentralBrazil were observed (Fig. 1b). On the other hand, therewere positive rainfall anomalies over northern Argen-tina, southern Brazil and Uruguay. This anomalousprecipitation pattern, associated with the deficit ofrainfall along the 2000 year, triggered an energy crisis inBrazil. There can be many mechanisms responsible forthe modification of the atmospheric circulation overSouth America during this period. For instance, Cav-alcanti and Kousky (2001) suggested a possible remoteinfluence of the Indonesian convection over the anom-alous activity of the upper tropospheric cyclonic vortices(UTCV) over northeastern Brazil (Kousky and Gan1981), as well as the Madden–Julian Oscillations. Otherimportant mechanisms are the SST forcings, which willbe the main focus of this paper. It is our intention toshow that remote SST anomalies were responsible forthe modification of the atmospheric circulation patterns

over South America and for the generation and main-tenance of the drought observed in the period. Section 2presents the data, methodology and a brief descriptionof the numerical model used to test the above hypoth-eses. Section 3 presents the main atmospheric featuresobserved during the 2001 Austral Summer and discussesthe numerical simulations. Section 4 summarizes themain results found.

2 Data and methods

Monthly precipitation data from Climate PredictionCenter (CPC) merged analysis of precipitation (CMAP)(Xie and Arkin 1997, 1998) and monthly NationalOceanic and Atmospheric Administration (NOAA)interpolated outgoing longwave radiation (OLR)(Liebmann and Smith 1996) were used for the observa-tional analysis. Both fields are available on a grid of2.5�·2.5� horizontal resolution.

Specific humidity and wind monthly means from theNational Center for Environmental Prediction–NationalCenter for Atmospheric Research (NCEP–NCAR)Reanalysis (Kalnay et al. 1996) were also used. Thehorizontal resolution is 2.5� latitude by 2.5� longitude.Reynolds SST monthly means (Reynolds and Smith1994) are available on a grid of 1�·1� horizontal reso-lution. These data sets are available on the ClimateDiagnostics Center (CDC) NOAA—Cooperative Insti-tute for Research in Environmental Sciences (CIRES)Web page. Seasonal analyses were done for January–February–March (JFM) of 2001. Climatology was de-fined for the period from 1982 to 2001.

The Atmospheric General Circulation Model (GCM)used is the NCAR Community Climate Model 3.6(CCM3.6). It is a spectral global model, and T42L18resolution was used for this study. A full description ofthe model can be found in Kiehl et al. (1998) and Hurrellet al. (1998).

The numerical study consisted of the realization of sixexperiments to investigate the impact of different SSTforcings observed over the 2001 Austral Summeranomalous South American atmospheric circulation.The experiments were performed from September 2000until April 2001. One experiment used global SSTmonthly data observed (hereafter referred to as theGSST experiment), and the other five used the SSTobserved in the regions indicated in Fig. 2 and clima-tological SST outside. The regions selected were: SouthAtlantic (SASST) (90�S–5�N; 70�W–10�E), South In-dian (SISST) (90�S–10�S; 20�E–110�E), South Pacific(SPSST) (90�S–10�S; 120�E–75�W), Equatorial Pacific(EPSST) (15�S–15�N; 120�E–75�W) and SouthPacific + Equatorial Pacific (SEPSST) (90�S–15�N;120�E–75�W). Table 1 summarizes the experimentsperformed.

Ten integrations using ten different initial conditions(IC) were performed for each experiment. These ICswere extracted from a 10-year-long integration, which

Drumond and Ambrizzi: The role of SST on the South American atmospheric circulation 783

used the climatological annual cycle of SST as aboundary condition. This integration will be hereafterreferred to as the control integration (CI). In this way,the anomaly calculated for the numerical experimentswas defined as the difference between each ensemblemean and the average of the 10-year-long CI. Thismethodology has been used in many other studies (e.g.,Hoerling et al. 1997).

Precipitation anomalies, 200 h Pa anomalous zonallyasymmetric component of streamfunction, anomalousmoisture flux and the anomalous divergence of moistureflux at 925 h Pa were plotted for the observational andnumerical results. To assess the statistical significance ofthe numerical results, a two-sided Student’s t test wasapplied to each experiment according to (1)

t ¼ XM � R

S2M=Nð Þ1=2

ð1Þ

where XM and SM2 are (respectively) the ten-member

ensemble mean and the variance of the members of theensemble, R is the 10-year CI mean and N is the size ofthe ensemble. The test was computed with 95% confi-dence and with N�1 degrees of freedom. Only the resultsstatistically significant will be discussed.

3 Results

3.1 Atmospheric features observed during 2001Austral Summer

During JFM 2001, negative precipitation anomaliesoccurred over northeastern, southeastern and CentralBrazil, whereas positive precipitation anomalies wereobserved over southern Brazil, Uruguay, northernArgentina and northern Brazil (Fig. 1b).

Over the Central East Equatorial Pacific, negativeSST anomalies (�1�C) are observed and they are asso-ciated with the occurrence of a weak La Nina event(Fig. 2). Positive anomalies are seen in the WesternEquatorial Pacific and Northern and Southern Sub-tropical Pacific. In these subtropical regions, the positiveSST anomalies reached values up to 1�C. In the SouthPacific, positive SST anomalies reach 0.5�C nearnortheastern Australia and reach around 1�C close tothe south of the continent and over the eastern sub-tropical oceanic regions. In the South Indian Ocean,1.5�C positive SST anomalies were observed nearsoutheastern Africa, while �1�C negative anomaliesoccurred close to southwestern Australia. A dipolestructure of SST anomalies was observed over theextratropical South Atlantic. Positive anomalies around1�C occurred near 30�S and negative anomalies of �1�Cwere observed near the Argentinean coast.

At low levels, an anomalous moisture flux withanticyclonic curvature is observed over eastern Brazil(Fig. 3). The northeasterly anomalous moisture flux seenover southern Amazon region and the northerly anom-alous flux observed over Paraguay and southern Brazilwere possibly associated with the inhibition of theoccurrence of South Atlantic Convergence Zone (SACZ)events (Nogues-Paegle and Mo 1997). One of themechanisms associated with the manifestation of the

Fig. 2 JFM 2001 SSTanomalies observed (�C) andthe regions used as boundaryconditions in the numericalexperiments. The names andboundaries of the boxes areindicated. For the SSTanomalies, the interval betweenisolines is 0.5 �C, and valueshigher than 0.5�C are in darkgray and lower than –0.5�C arein light gray. Climatology wasdefined for the period 1982/2001. (Data: Reynolds)

Table 1 Numerical experiments performed

Experiment Acronym Region used as forcing

Global SST GSST AllSouth Atlantic SASST 90�S–5�N; 70�W–10�ESouth Indian SISST 90�S–10�S; 20�E–110�ESouth Pacific SPSST 90�S–10�S; 120�E–75�WEquatorial Pacific EPSST 15�S–15�N; 120�E–75�WSouth Pacific +Equatorial Pacific

SEPSST 90�S–15�N; 120�E–75�W

Each experiment was forced by the SST observed in the regionindicated


SACZ is the low-level moisture transport from theAmazon to southeastern Brazil (Kodama 1992). Con-sequently, large 925 h Pa anomalous moisture-fluxdivergence occurred over Central Brazil and over thesoutheastern Brazilian coast, whereas anomalous mois-ture-flux convergence was observed over the extremesouthern Brazil, Uruguay and northeastern Argentina(Fig. 3).

This low-level anomalous anticyclonic circulation isassociated with an upper-level anomalous cyclonic cir-culation over Central Brazil (Fig. 4a). As suggested byCavalcanti and Kousky (2001), this anomalous circula-tion may be linked to the anomalous enhanced activityof UTCV over northeastern Brazil, which could haveaffected southeastern and Central Brazil. The authorsdiscussed that this anomalous UTCV inhibited theoccurrence of SACZ events and the formation of deepclouds over tropical South America associated with theconfiguration of the Bolivian High. In addition, theactivity of UTCV enhanced the blocking of frontalsystems over southern Brazil, increasing the precipita-tion over the region (e.g., Kousky and Gan 1981).

Figure 4a shows an alternation between high-levelcyclonic and anticyclonic anomalies over the tropicaland extratropical Southern Hemisphere, suggesting awave train pattern reaching northeastern Brazil. Theenhanced convection over Indonesia observed throughOLR anomalies (Fig. 4b) may be the disturbance source.Anticyclonic anomalies over Australia and southeasternAsia are also indicated. Circulation anomalies seem toemanate from these anomalous centers towards SouthAmerica via the South Pacific. Cavalcanti and Kousky(2001) suggested that the intense convective activityobserved over the Indonesian region and the re-intensi-fication of a La Nina event may have contributed to thedrought in eastern Brazil, through a teleconnection

pattern similar to the Pacific-South America (PSA)pattern discussed in several studies (e.g., Cavalcanti2000, Mo 2000). Previous works (e.g., Kousky and Gan1981) have already discussed the interaction between theteleconnection patterns over eastern South Pacific andthe formation of UTCV over northeastern SouthAmerica. OLR anomalies also indicate the intensifiedconvective activity over the South Pacific ConvergenceZone (SPCZ), which was slightly displaced westward ofits climatological position. It is interesting that Grimmand Silva Dias (1995) showed that the displacement ofthe SPCZ southeastward of its climatological positionduring El Nino events is linked to enhanced convectionin the SACZ. In this way, maybe the displacement of theSPCZ westward of its climatological position can beconnected to the inhibition of the convection in theSACZ.

3.2 Numerical simulations with a GCM

The GSST experiment fairly reproduced the anomalousprecipitation pattern observed over tropical SouthAmerica (Fig. 5a). However, this experiment had somedeficiency in reproducing the positive anomalies ob-served over southern Brazil, northern Argentina andUruguay. For the SASST experiment (Fig. 5b), theintensity of the anomalies is small, and the anomalouspattern observed over Brazil is only reproduced near thecoast. The numerical results using the SISST experimentdid not show any significant anomalies over the conti-nent (Fig. 5c). Considering the impact of SPSST(Fig. 5d), the negative anomalies observed over Brazilwere fairly simulated. The positive anomaly patternobserved over subtropical regions was reproduced onlyover northern Argentina. Considering the role of EPSST

Fig. 3 JFM 2001 anomalousmoisture flux (g kg�1 m s�1)(vectors) and anomalousdivergence of moisture flux(·10�5 g kg�1 s�1) (shaded andcontour lines) at 925 h Pa.For divergence, the absolutevalues of isolines are 0.5; 1;2; 4; 6·10�5 g kg�1 s�1.Anomalies higher than0.5·10�5 g kg�1 s�1 are in darkgray and lower than�0.5·10�5 g kg�1 s�1 are inlight gray. Climatology wasdefined for the period 1982/2001. (Data: NCEP/NCARReanalysis)


(Fig. 5e), the drought observed over Tropical Brazil wasreproduced by the model. In the subtropical regions, thepositive precipitation pattern was not statistically sig-nificant. For the SEPSST experiment (Fig. 5f), part ofthe negative anomalies simulated over Central Brazil arestatistically significant. In general, the patterns obtainedby the SEPSST show a mix between the South Pacificand Equatorial Pacific experiments (Fig. 5d, e).

At low levels, the model forced by GSST simulatedthe anticyclonic curvature of the anomalous moistureflux over eastern Brazil fairly well (Fig. 6a). However,the anomalous meridional flux over northeasternArgentina was not reproduced correctly and the modelsimulated a western flux over southeastern Brazil, dif-fering from the observed pattern. As a result, theanomalous moisture-flux convergence observed at925 h Pa over southern Brazil was not well reproduced(Fig. 6a). One should notice that due to the smallmagnitude of some results, the scale used for theanomalous moisture-flux vector was multiplied by afactor of 2 in order to better emphasize the circulationpattern.

For the SASST experiment, there is statistical sig-nificance for the chosen level only in the Northeastern

Brazil. The results indicate a displacement of the ITCZto the north of its climatological position (Fig. 6b).

The moisture-flux results from the SISST experimentare mainly characterized by low-level cyclonic anomaliesover southeastern Brazil and its associated moisture-fluxconvergence over the region (Fig. 6c), which is differentfrom the expected pattern.

When the South Pacific SST forcing was considered,low-level anticyclonic anomalies were configured oversoutheastern Brazil, being associated with the anoma-lous moisture-flux divergence over the region (Fig. 6d).However, the northerly anomalous moisture flux ob-served over northeastern Argentina and southern Brazilis weaker, explaining why the intensity of the precipi-tation anomalies simulated by the model was also belowthe observed pattern.

For the Equatorial Pacific experiment, a low-levelanomalous eastern moisture flux and divergence overEastern Brazil are observed (Fig. 6e). The low-levelnorthwesterly anomalous flux and anomalous moisture-flux convergence over southern Brazil were not signifi-cantly at the 95% level. The patterns obtained in theSEPSST experiment (Fig. 6f) are quite similar to theEquatorial Pacific simulation.

Fig. 4 a JFM 2001 200 h Paanomalous zonally asymmetriccomponent of stream functionobserved (·106 m2 s�1). Theabsolute values of isolines are0.5; 1; 2; 4; 6; 8·106 m2 s�1, andnegative values are representedby dotted lines; b JFM 2001OLR anomalies (W m�2).Anomalies higher than5 W m�2 are in light shadingand lower than �5 W m�2 arein dark shading. For both fields,climatology was defined for theperiod 1982/2001. (Data: (a)NCEP/NCAR Reanalysis and(b) NOAA)


At high levels, the experiment forced by GSSTsimulated a PSA-like wave train propagation and thecyclonic anomaly observed over Tropical SouthAmerica (Fig. 7a). However, the anticyclonic anoma-lies observed over the subtropics were displacedsouthward. In the SASST experiment, a wave trainpropagation pattern is clearly seen along the Southern

Hemisphere mid latitudes reaching South America,where a cyclonic center is located over eastern Brazil(Fig. 7b). This pattern resembles the observed field(Fig. 4a), but it has a more pronounced arch orienta-tion. The SISST experiment shows weak amplitudes allover the Southern Hemisphere (Fig. 7c) and no sig-nificant wave pattern.

Fig. 5 Simulated precipitation anomalies from a the GSSTexperiment; b the SASST experiment; c the SISST experiment; dthe SPSST experiment; e the EPSST experiment; f the SEPSSTexperiment. The anomalies were calculated with respect to the CI

mean. The absolute values of isolines are 0.3; 1; 2; 4; 6 mm day�1,and negative values are shown with dashed contours. Dark (light)shaded areas denote positive (negative) anomalies that aresignificant at the 95% level


For the South Pacific experiment, an anomalous cy-clonic circulation dominates Southeastern Brazil(Fig. 7d). The experiment using the Equatorial Pacific

forcing (Fig. 7e) shows a wave pattern emanatingfrom Indonesia towards South America. There is acyclonic anomaly over tropical South Atlantic and the

Fig. 6 Simulated anomalous moisture flux (g kg�1 m s�1)(vectors) and anomalous divergence of moisture flux(·10�5 g kg�1 s�1) (shaded) at 925 h Pa from a the GSSTexperiment; b the SASST experiment; c the SISST experiment;d the SPSST experiment; e the EPSST experiment; f the SEPSST

experiment. The anomalies were calculated with respect to the CImean. Divergence anomalies higher than 0.5·10�5 g kg�1 s�1 arein dark gray and lower than �0.5·10�5 g kg�1 s�1 are in lightgray. For both fields, only significant anomalies at the 95% levelare plotted


extratropical anticyclonic pattern observed over SouthAmerica was displaced to the southeast. Finally, theSouth and Equatorial Pacific experiment shows anatmospheric circulation pattern as a combination be-tween the results obtained for the SPSST and EPSSTexperiments (Fig. 7f).

From the results discussed in this section, there issome indication that the South Pacific and the Equa-torial Pacific anomalous SST may have played themost important role to modulate the drought observedover Central Brazil during the 2001 Austral Summer.The overall difficulties in the model to put the upper

Fig. 7 Simulated 200 h Pa anomalous zonally asymmetric compo-nent of stream function (·106 m2 s�1) from a the GSST experi-ment; b the SASST experiment; c the SISST experiment; d theSPSST experiment; e the EPSST experiment; f the SEPSST

experiment. The anomalies were calculated with respect to the CImean. The absolute values of isolines are 0.5; 1; 2; 4; 6;8·106 m2 s�1, and negative values are represented by dotted lines.Shaded areas denote anomalies that are significant at the 95% level


and lower level atmospheric stationary waves in theright position must be recalled, though this is a com-mon deficiency in GCMs (e.g., Hoerling and Kumar2002). This problem has some impact on the precipi-tation patterns simulated. The general response foundfor the different SST forcings used in the experimentsindicates that the drought is statistically significant.However, none of the simulations were able toreproduce the precipitation over the La Plata basin.

4 Discussion and conclusion

The drought that occurred in eastern Brazil and thepositive precipitation anomalies observed over Para-guay, northern Argentina and southern Brazil duringthe 2001 Austral Summer were associated with a low-level anomalous anticyclonic circulation over easternBrazil. This anomalous circulation caused the devia-tion of the moisture transport from the Amazon basintowards La Plata basin and inhibited the occurrence ofSACZ events in that period. This anomalous flux wasaccompanied by an enhanced activity of the UTCV, assuggested by Cavalcanti and Kousky (2001).

In particular, the relationship between the low-levelcirculation pattern and the SACZ has been confirmed bymany previous works. Early studies by Sugahara et al.(1994) have analyzed circulation features in southeasternSouth America during eight consecutive summers andidentified that the South American LLJ is intense whenthe SACZ is weak and vice versa. This was later con-firmed by Nogues-Peagle and Mo (1997), Seluchi andMarengo (2000), Herdies et al. (2002), Berbery andBarros (2002) and Nieto-Ferreira et al. (2003) usingobservational data. However, there has been no studydirectly linking the SST remote forcings to this anoma-lous South American circulation pattern.

Observational results showed high-level anticyclonicanomalies over Australia and southeastern Asia associ-ated with the enhanced convection over Indonesia.Circulation anomalies seem to emanate from theseanomalous centers towards South America via SouthPacific. Cavalcanti and Kousky (2001) suggested thatthe intense convective activity observed over the Indo-nesian region and the re-intensification of a La Ninaevent may have contributed to the drought in easternBrazil, through a teleconnection PSA-like pattern. Theintensified convective activity over the SPCZ and itsdisplacement westward of its climatological position canalso be connected to the inhibition of the convection inthe SACZ. According to Grimm and Silva Dias (1995),the displacement of the SPCZ southeastward of its cli-matological position observed during El Nino events islinked to enhanced convection in the SACZ. In this way,there is a possibility that the opposite pattern can occur.

A numerical study was carried out with the CCM3.6GCM to investigate the impact of different SST forcingsobserved over the anomalous South American circula-

tion. Six experiments were performed: one global andfive for different areas (South Atlantic, South Pacific,South Indian and from the equator to South PacificOcean). Ten integrations using ten different ICs weredone for each experiment. These ICs were extractedfrom a 10-year CI, which used the climatological annualcycle of SST as a boundary condition.

The numerical experiments suggested that the com-bined influence of South Pacific and Equatorial Pacificforcings played the most important role in the droughtobserved over the Tropical South America during the2001 Austral Summer. Both experiments simulated thehigh-level circulation anomalies initiating over Indone-sia and reaching South America via the tropics andsubtropics. Consequently, the low-level anticyclonicanomaly observed over eastern Brazil was simulated.However, they poorly reproduced the anomalousamplitudes of the low-level northerly moisture flux ob-served over the La Plata Basin. Therefore, the positiveprecipitation anomalies simulated over subtropical re-gions were much weaker than the observed ones. Thisresult may be related to the problems that GCMs stillhave in simulating the position and amplitude of plan-etary waves, often misrepresenting regional precipitationpatterns. However, the general response found for thedifferent SST forcings indicates that the UTCV is sta-tistically significant. Although the precipitation anoma-lies generated by the South Atlantic experiment weresimilar to the ones observed near the South Americaneastern coast, the anomalies over Central Brazil and theLa Plata river basin region were very small and withoutany statistical significance at a level of 95%. The SouthIndian anomalies had no impact over the anomalousSouth American circulation observed during this event.

In summary, our results suggest that it was the re-mote influence of the anomalous SST from the Equa-torial and South Pacific regions which played the mainrole for the drought observed over Central Brazil duringthe 2001 Austral Summer. Of course, this conclusion isbased on one event only. Therefore, generalization mustbe viewed with caution. Composites of similar anoma-lous precipitation events and other numerical experi-ments are currently in progress and they will bepresented elsewhere.

Acknowledgements We would like to thank Fundacao de Amparoa Pesquisa do Estado de Sao Paulo—FAPESP (proc. 01/06842-6and 01/13816-1) for the support. TA has also been supported by theConselho Nacional de Desenvolvimento Cientıfico e Tecnologi-co—CNPq (proc. 302459/2002-2 and 475281/03-9). We also wish tothank the Inter America Institute for Global Change Re-search—IAI (proc. IAI - CRN 055) and the anonymous reviewersfor their useful comments and suggestions.

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The role of SST on the South American atmospheric circulation during January, February and March...

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