South Atlantic Convergence Zone Region Research Articles

South American Monsoon Lifecycle Projected by Statistical Downscaling with CMIP6-GCMs

This study analyzed the main features (onset, demise, and length) of the South American Monsoon System (SAMS) projected in different time slices (2020–2039, 2040–2059, 2060–2079, and 2080–2099) and climate scenarios (SSP2–4.5 and SSP5–8.5). Eight global climate models (GCMs) from the Coupled Model Intercomparison Project Phase 6 (CMIP6) that perform well in representing South America’s historical climate (1995–2014) were initially selected. Thus, the bias correction–statistical downscaling (BCSD) technique, using quantile delta mapping (QDM), was applied in each model to obtain higher-resolution projections than their original grid. The horizontal resolution adopted was 0.5° of latitude × longitude, the same as the Climate Prediction Center precipitation analysis used as a reference dataset in BCSD. The QDM technique improved the monsoon onset west of 60° W and the simulated demise and length in southwestern Amazonia. Raw and BCSD ensembles project an onset delay of approximately three pentads compared to the historical period over almost all regions and a demise delay of two pentads northward 20° S. Additionally, the BCSD ensemble projects a reduced length with statistical significance in most South Atlantic Convergence Zone regions and a delay of three pentads in the demise over the Brazilian Amazon from the second half of the 21st century.

Future Projections of Extreme Precipitation Climate Indices over South America Based on CORDEX-CORE Multimodel Ensemble

Climate change is undeniable, and its effects, like droughts and intense precipitation, evidence this reality. However, many questions remain unanswered, such as climate extremes’ future frequency and intensity. International collaboration projects such as the Coordinated Regional Climate Downscaling Experiment (CORDEX)-Coordinated Output for Regional Evaluations (CORE) have emerged to address these questions, creating protocols to facilitate the intercomparison among simulations obtained by different regional climate models (RCMs). Given this context, this study aims to describe the projected changes in the extreme precipitation events over the South America domain by using a set of projections of three RCMs (Eta, RegCM, and REMO) under the CORDEX-CORE framework (except Eta). These models were nested in different global climate models (GCMs) from Coupled Model Intercomparison Project phase 5 (CMIP5) under the RCP8.5 scenario and integrated with a grid space of 25 km. In addition, six climate indices (PRCPTOT, SDII, R95p, P95, CDD, and CWD) were computed by season for the historical period (1995–2014) and four future time-slices (2020–2039, 2040–2059, 2060–2079, and 2080–2099). For brevity, this study focuses on the austral summer. RCMs are generally capable of representing the spatial pattern of climate indices but with differences in intensity. The ensemble with all RCMs shows a slightly better spatial correlation with the reference data than the ensemble of each RCM. The signal of the projections is similar between RegCM and REMO and, in general, opposite to Eta (mainly in the South Atlantic Convergence Zone region). Of the six climate indices analyzed, the ensemble performed with the three RCMs projects statistically significant negative (positive) trends for four indices in the Amazon (La Plata Basin), indicating a signal of dry (wet) conditions increasing towards the end of the century. Furthermore, both regions have significant positive (negative) trends of consecutive dry (wet) days. The region with higher uncertainties lies over southeastern Brazil, where Eta projects a dry climate and RegCM and REMO, a wet one.

Assessment of Tropical Atlantic ocean variability and associated rainfall sensibility due to atmospheric ozone changes

Considering the impacts of atmospheric ozone changes on climate, its effects on Tropical Atlantic Variability (TAV) since 1955 are investigated. Two ensembles of simulations from the Community Earth System Model Large Ensemble Project are examined. One of them (varO3) considers historic tropospheric and stratospheric ozone levels while the other (fixO3) keeps ozone fixed at the 1955 levels. Results for varO3 reveal a warming in the northern Tropical Atlantic (TA) when compared to fixO3, which is more pronounced in boreal spring, and a cooling of the southern TA in early summer. A dipole-like pattern was also observed in the mean zonal wind stress field, with a weakening of the trade winds in northern TA and strengthening in southern TA, although the strongest difference is observed in the southern westerlies, which are intensified. As for the impact on the main rainfall regions, results show differences between the experiments in the depiction of the Atlantic Ocean convergence zones. The fixed ozone experiment shifts the ITCZ position to the south, which leads to an increase in precipitation over most of Northeast Brazil and western South Africa, in addition to simulating lower precipitation in the South Atlantic Convergence Zone region. However, although there are differences in SST and precipitation between varO3 and fixO3, the ozone influence on TAV is not significant.

Evapotranspiration trends and variability in southeastern South America: The roles of land‐cover change and precipitation variability

AbstractSoutheastern South America is subject to considerable precipitation variability on seasonal to decadal timescales and has undergone very heavy land‐cover changes (LCCs) since the middle of the past century. The influence of local LCC and precipitation as drivers of regional evapotranspiration (ET) long‐term trends and variability remains largely unknown in the region. Here, ensembles of stand‐alone dynamic global vegetation models (DGVMs) with different atmospheric forcings are used to disentangle the influence of those two drivers on austral summer ET from 1950 to 2010. This paper examines the influence of both the El Niño‐Southern Oscillation (ENSO) and the dipole‐like first‐mode of southeastern South American precipitation variability (EOF1) on regional ET. We found that in the lower La Plata Basin, ET was driven by precipitation variability and showed a positive summer trend. Moreover, the region showed marked seasonal anomalies during El Niño and La Niña summers but mainly during EOF1 phases. On the contrary, in the upper La Plata Basin, LCCs forced the negative summer ET trend and particularly reduced the summer anomalies of the late 1990s, a period of ENSO and EOF1‐positive phases. In the South Atlantic Convergence Zone region, the high ET uncertainty across ensemble members impeded finding robust results, which highlights the importance of using multiple DGVMs and atmospheric forcings instead of relying on single model/forcing results.

Influence of the Antarctic Oscillation on the South Atlantic Convergence Zone

The South Atlantic convergence zone (SACZ) is the main summer-typical atmospheric phenomenon occurring in South America, and it is of great interest because it regulates the rainy season in the most populated regions of Brazil. Frequency variability, persistence, and geographical position of the SACZ and its relationship with intraseasonal variability is well described in the literature. However, the influence of extratropical forcing on the SACZ is not well understood. Consequently, the aim of this study is to evaluate the role of the Antarctic Oscillation (AAO) in SACZ events. The persistence and frequencies of SACZ events, mean, standard deviation of total precipitation per event, lag composite of daily precipitation and geopotential height anomalies were obtained for each phase of the AAO. Therefore, frequency, persistence and total precipitation of SACZ events were higher in positive AAO (AAO+) than negative AAO (AAO−). A teleconnection mechanism between the extratropics and the SACZ region is evident in AAO+, through intensification of the polar and subtropical jets, in the days preceding SACZ. The same was not observed in the AAO−, where the anomalies were confined in the subtropical region and displaced to the South Atlantic Ocean.

Extended-range prediction of South Atlantic convergence zone rainfall with calibrated CFSv2 reforecast

During the 2010–2011 wet season in Brazil, widespread landslides triggered by heavy rainfall killed hundreds of people and displaced nearly 35,000. The extreme precipitation was associated with the formation of the South Atlantic convergence zone (SACZ). Even though the physical mechanisms behind the formation and persistence of subtropical convergence zones are still unclear, we demonstrate that early predictions of heavy rainfall in the SACZ region are possible. Precipitation rate hindcasts from the NCEP Climate Forecast System version 2 are calibrated with the aid of a gridded precipitation dataset. When the calibration was applied to the 2010–2011 events, the hindcasts were able to depict both active and break phases of the SACZ with up to 2 weeks in advance during a period of relatively weak intraseasonal variability associated with the Madden–Julian Oscillation (MJO).

Análise de um Vórtice Ciclônico e Mesoescala Associado a ZCAS em Janeiro de 2009

Resumo Este trabalho objetiva analisar a condição termodinâmica durante a atuação de um vórtice ciclônico de mesoescala (VCM) embebido na Zona de Convergência do Atlântico Sul (ZCAS), no mês de janeiro de 2009. Os VCMs possuem características similares: encontram-se na baixa e média troposfera associado a altos índices pluviométricos, ciclo de vida inferior a 24 h, escala espacial de aproximadamente 200 km2 e o descolamento do sistema na mesma direção do fluxo predominante na baixa troposfera. Além disso, há uma relação direta entre estes máximos pluviométricos e a circulação ciclônica dos vórtices. O VCM simulado através do modelo de mesoescala BRAMS se formou no Estado de Mato Grosso e esteve associada à ocorrência de chuvas intensas. Além das características típicas de caracterização, este sistema também apresentou vorticidade relativa da mesma ordem de magnitude do parâmetro de Coriolis (10−4 s−1), núcleo quente acima do nível de máxima intensidade e um rápido crescimento do centro de vorticidade ciclônica principalmente nos baixos níveis. Pela comparação da chuva simulada com o produto Hidroestimador, sugere-se que o modelo BRAMS, com uma alta resolução espacial e temporal, melhora a representação do VCM, comparado com os dados da reanálise CFSR do NCEP.

Influence of South America orography on summertime precipitation in Southeastern South America

Impacts of the main South American orographic structures (the Andes, the Brazilian Plateau and the Guiana shield) on the regional climate and associated global teleconnection are investigated through numerical experiments in which some of these features are suppressed. Simulations are performed with a ‘‘two-way nesting’’ system coupling interactively the regional and global versions of the LMDZ4 atmospheric general circulation model. At regional scale, the simulations confirm previous studies, showing that both the Andes and the Brazilian Plateau exert a control on the position and strength of the South Atlantic convergence zone (SACZ), mainly through their impact on the low-level jet and the coastal branch of the subtropical anticyclones. The northern topography of South America appears to be crucial to determine the leading mode of rainfall variability in eastern South America, which manifests itself as a dipole-like pattern between Southeastern South America and the SACZ region. The suppression of South America orography also shows global-scale effects, corresponding to an adjustment of the global circulation system. Changes in atmospheric circulation and precipitation are found in remote areas on the globe, being the consequences of various teleconnection mechanisms. When the Brazilian Plateau and the Andes are suppressed, there is a decrease of precipitation in the SACZ region, associated with a weakening of the large-scale ascendance. Changes are described in terms of anomalies in the Walker circulation, meridional displacements of the mid-latitude jet stream, Southern annular mode anomalies and modifications of Rossby wave train teleconnection processes.

The role of synoptic and intraseasonal anomalies in the life cycle of summer rainfall extremes over South America

The main goal of this study is to describe the role of synoptic and intraseasonal anomalies during the life cycle of summer rainfall extremes over South America. Eastward-propagating synoptic-scale midlatitude waves are the main drivers of extreme precipitation events south of the Amazon and their interaction with intraseasonal anomalies over South America is important for heavy rainfall over the South Atlantic convergence zone (SACZ) region and the La Plata basin. Madden–Julian Oscillation (MJO) convective activity in the western Pacific (phases 6 and 7) leads 31 out of 81 extremes over the SACZ region by nearly 10 days. The connection between the MJO and rainfall extremes in other regions is less robust. During El Nino seasons extremes are more frequent in the La Plata basin, with decreased importance of intraseasonal anomalies. Precipitation extremes over the La Plata basin tend to be less frequent and also shorter during La Nina summers and, consequently, less hazardous. In the SACZ and the southeastern Brazilian coast, heavy rainfall is also more frequent under El Nino conditions, while La Nina episodes also increase extreme events in the southeastern coast. Extremes over the southeastern coast during El Ninos are favored by strong intraseasonal anomalies flanking the subtropical jet, while during La Ninas intraseasonal anomalies are not significant.

Influence of the Madden Julian Oscillation on precipitation and surface air temperature in South America

The regional influence of the Madden–Julian oscillation (MJO) on South America is described. Maps of probability of weekly-averaged rainfall exceeding the upper tercile were computed for all seasons and related statistically with the phase of the MJO as characterized by the Wheeler–Hendon real-time multivariate MJO (RMM) index and with the OLR MJO Index. The accompanying surface air temperature and circulation anomalies were also calculated. The influence of the MJO on regional scales along with their marked seasonal variations was documented. During December–February when the South American monsoon system is active, chances of enhanced rainfall are observed in southeastern South America (SESA) region mainly during RMM phases 3 and 4, accompanied by cold anomalies in the extratropics, while enhanced rainfall in the South Atlantic Convergence Zone (SACZ) region is observed in phases 8 and 1. The SESA (SACZ) signal is characterized by upper-level convergence (divergence) over tropical South America and a cyclonic (anticyclonic) anomaly near the southern tip of the continent. Impacts during March–May are similar, but attenuated in the extratropics. Conversely, in June–November, reduced rainfall and cold anomalies are observed near the coast of the SACZ region during phases 4 and 5, favored by upper-level convergence over tropical South America and an anticyclonic anomaly over southern South America. In September–November, enhanced rainfall and upper-level divergence are observed in the SACZ region during phases 7 and 8. These signals are generated primarily through the propagation of Rossby wave energy generated in the region of anomalous heating associated with the MJO.

A study of the air-sea interaction in the South Atlantic Convergence Zone through Granger causality

Air-sea interaction in the region of the South Atlantic Convergence Zone (SACZ) is studied using Granger causality (GC) as a measure of directional coupling. Calculation of the area weighted connectivity indicates that the SACZ region is the one with largest mutual air-sea connectivity in the south Atlantic basin during summertime. Focusing on the leading mode of daily coupled variability, GC allows distinguishing four regimes characterized by different coupling: there are years in which the forcing is mainly directed from the atmosphere to the ocean, years in which the ocean forces the atmosphere, years in which the influence is mutual and years in which the coupling is not significant. A composite analysis shows that ocean-driven events have atmospheric anomalies that develop first and are strongest over the ocean, while in events without coupling anomalies develop from the continent where they are strongest and have smaller oceanic extension.

Sea ice concentration variability over the Southern Ocean and its impact on precipitation in southeastern South America

ABSTRACTThis article quantifies the relationship between sea ice cover (SIC) on the Southern Ocean and precipitation and river discharges over southeastern and eastern South America. The period of analysis covers from 1981 to 2008 and includes monthly mean data of SIC over the Ross, Amundsen–Bellingshausen and Weddell Seas, as well as atmospheric variables and discharges of selected rivers in eastern South America. To isolate the effect of known modes of climate variability on SIC, the signals of El Niño‐Southern Oscillation and the Southern Annual Mode are removed from all time series through regression analysis. Composites of precipitation differences over South America reveal a pattern of enhanced rainfall activity over the South Atlantic Convergence Zone (SACZ) in summer (JFM) when SIC is above average particularly over the Weddell Sea area, while winter (ASO) SIC anomalies show negative and significant correlations with rainfall over much of South America. Moisture fluxes patterns in summer reveal increased moisture transport towards the SACZ region and decreased low‐level jet activity (and precipitation) over northern Argentina. In particular a large dry bias over southeastern South America associated to positive SIC anomalies on the Weddell Sea in September is shown to impact the discharges of the Uruguay and Iguazú Rivers, with the largest effects found two months after the SIC anomaly (November). Although less robust, a relationship with the Paraná River is also found. These results suggest that increased SIC over the Weddell Sea during September can help to forecast drier conditions particularly on the Uruguay basin and somewhat wetter conditions in the SACZ region within the following season.

Impact of projected SST changes on summer rainfall in southeastern South America

Recent studies have shown that global warming and associated sea-surface temperature (SST) changes may trigger an important rainfall increase in southeastern South America (SESA) during the austral summer (December–January–February, DJF). The goal of this paper is to provide some insight into processes which may link global and SESA changes. For this purpose, a “two-way nesting” system coupling interactively the regional and global versions of the LMDZ4 atmospheric model is used to study the response to prescribed SST changes. The regional model is a variable-grid version of the global model, with a zoom focused over South America. An ensemble of simulations forced by distinct patterns of DJF SST changes has been carried out using a decomposition of full SST changes into their longitudinal and latitudinal components. The full SST changes are based on projections for the end of the twenty-first century from a multi-model ensemble of WCRP/CMIP3. Results confirm the presence of a major rainfall dipole structure, characterized by an increase in SESA and a decrease in the South Atlantic Convergence Zone region. Rainfall changes found in the WCRP/CMIP3 models are largely explained as a response of this dipole structure to the zonally-asymmetric (or longitudinal) component of SST changes. The rainfall response to the zonal-mean (or latitudinal) SST changes (including the global warming signal itself) shows an opposite contribution. The processes explaining the role of zonally-asymmetric SST changes involve remote effects of SST warming over the equatorial Indian and Pacific oceans inducing an atmospheric wave-train extended across the South Pacific into South America.

From drought to flooding: understanding the abrupt 2010–11 hydrological annual cycle in the Amazonas River and tributaries

In this work we document and analyze the hydrological annual cycles characterized by a rapid transition between low and high flows in the Amazonas River (Peruvian Amazon) and we show how these events, which may impact vulnerable riverside residents, are related to regional climate variability. Our analysis is based on comprehensive discharge, rainfall and average suspended sediment data sets. Particular attention is paid to the 2010–11 hydrological year, when an unprecedented abrupt transition from the extreme September 2010 drought (8300 m3 s−1) to one of the four highest discharges in April 2011 (49 500 m3 s−1) was recorded at Tamshiyacu (Amazonas River). This unusual transition is also observed in average suspended sediments. Years with a rapid increase in discharge are characterized by negative sea surface temperature anomalies in the central equatorial Pacific during austral summer, corresponding to a La Niña-like mode. It originates a geopotential height wave train over the subtropical South Pacific and southeastern South America, with a negative anomaly along the southern Amazon and the southeastern South Atlantic convergence zone region. As a consequence, the monsoon flux is retained over the Amazon and a strong convergence of humidity occurs in the Peruvian Amazon basin, favoring high rainfall and discharge. These features are also reported during the 2010–11 austral summer, when an intense La Niña event characterized the equatorial Pacific.

Aqua‐planet simulations of the formation of the South Atlantic convergence zone

AbstractThe impact of Amazon Basin convection and cold fronts on the formation and maintenance of the South Atlantic convergence zone (SACZ) is studied using aqua‐planet simulations with a general circulation model. In the model, a circular patch of warm sea‐surface temperature (SST) is used to mimic the effect of the Amazon Basin on South American monsoon convection. The aqua‐planet simulations were designed to study the effect of the strength and latitude of Amazon Basin convection on the formation of the SACZ. The simulations indicate that the strength of the SACZ increases as the Amazon convection intensifies and is moved away from the equator. Of the two controls studied here, the latitude of the Amazon convection exerts the strongest effect on the strength of the SACZ. An analysis of the synoptic‐scale variability in the simulations shows the importance of frontal systems in the formation of the aqua‐planet SACZ. Composite time series of frontal systems that occurred in the simulations show that a robust SACZ occurs when fronts penetrate into the subtropics and become stationary there as they cross eastward of the longitude of the Amazon Basin. Moisture convergence associated with these frontal systems produces rainfall not along the model SACZ region and along a large portion of the northern model Amazon Basin. Simulations in which the warm SST patch was too weak or too close to the equator did not produce frontal systems that extended into the tropics and became stationary, and did not form a SACZ. In the model, the SACZ forms as Amazon Basin convection strengthens and migrates far enough southward to allow frontal systems to penetrate into the tropics and stall over South America. This result is in agreement with observations that the SACZ tends to form after the onset of the monsoon season in the Amazon Basin. Copyright © 2012 Royal Meteorological Society

Summer precipitation variability over Southeastern South America in a global warming scenario

December–January–February (DJF) rainfall variability in southeastern South America (SESA) is studied in 18 coupled general circulation models from the WCRP/CMIP3 dataset, for present climate and the SRES-A1B climate change scenario. The analysis is made in terms of properties of the first leading pattern of rainfall variability in the region, characterized by a dipole-like structure with centers of action in the SESA and South Atlantic Convergence Zone (SACZ) regions. The study was performed to address two issues: how rainfall variability in SESA would change in a future climate and how much of that change explains the projected increasing trends in the summer mean rainfall in SESA identified in previous works. Positive (negative) dipole events were identified as those DJF seasons with above (below) normal rainfall in SESA and below (above) normal rainfall in the SACZ region. Results obtained from the multi-model ensemble confirm that future rainfall variability in SESA has a strong projection on the changes of seasonal dipole pattern activity, associated with an increase of the frequency of the positive phase. In addition, the frequency increase of positive dipole phase in the twenty first century seems to be associated with an increase of both frequency and intensity of positive SST anomalies in the equatorial Pacific, and with a Rossby wave train-like anomaly pattern linking that ocean basin to South America, which regionally induces favorable conditions for moisture transport convergence and rainfall increase in SESA.

Regional climate simulations over South America: sensitivity to model physics and to the treatment of lateral boundary conditions using the MM5 model

In this study the capability of the MM5 model in simulating the main mode of intraseasonal variability during the warm season over South America is evaluated through a series of sensitivity experiments. Several 3-month simulations nested into ERA40 reanalysis were carried out using different cumulus schemes and planetary boundary layer schemes in an attempt to define the optimal combination of physical parameterizations for simulating alternating wet and dry conditions over La Plata Basin (LPB) and the South Atlantic Convergence Zone regions, respectively. The results were compared with different observational datasets and model evaluation was performed taking into account the spatial distribution of monthly precipitation and daily statistics of precipitation over the target regions. Though every experiment was able to capture the contrasting behavior of the precipitation during the simulated period, precipitation was largely underestimated particularly over the LPB region, mainly due to a misrepresentation in the moisture flux convergence. Experiments using grid nudging of the winds above the planetary boundary layer showed a better performance compared with those in which no constrains were imposed to the regional circulation within the model domain. Overall, no single experiment was found to perform the best over the entire domain and during the two contrasting months. The experiment that outperforms depends on the area of interest, being the simulation using the Grell (Kain–Fritsch) cumulus scheme in combination with the MRF planetary boundary layer scheme more adequate for subtropical (tropical) latitudes. The ensemble of the sensitivity experiments showed a better performance compared with any individual experiment.

Precipitation diurnal cycle and summer climatology assessment over South America: An evaluation of Regional Climate Model version 3 simulations

Regional Climate Model version 3 (RegCM3) simulations of 17 summers (1988–2004) over part of South America south of 5°S were evaluated to identify model systematic errors. Model results were compared to different rainfall data sets (Climate Research Unit (CRU), Climate Prediction Center (CPC), Global Precipitation Climatology Project (GPCP), and National Centers for Environmental Prediction (NCEP) reanalysis), including the five summers mean (1998–2002) precipitation diurnal cycle observed by the Tropical Rainfall Measuring Mission (TRMM)‐Precipitation Radar (PR). In spite of regional differences, the RegCM3 simulates the main observed aspects of summer climatology associated with the precipitation (northwest‐southeast band of South Atlantic Convergence Zone (SACZ)) and air temperature (warmer air in the central part of the continent and colder in eastern Brazil and the Andes Mountains). At a regional scale, the main RegCM3 failures are the underestimation of the precipitation in the northern branch of the SACZ and some unrealistic intense precipitation around the Andes Mountains. However, the RegCM3 seasonal precipitation is closer to the fine‐scale analyses (CPC, CRU, and TRMM‐PR) than is the NCEP reanalysis, which presents an incorrect north‐south orientation of SACZ and an overestimation of its intensity. The precipitation diurnal cycle observed by TRMM‐PR shows pronounced contrasts between Tropics and Extratropics and land and ocean, where most of these features are simulated by RegCM3. The major similarities between the simulation and observation, especially the diurnal cycle phase, are found over the continental tropical and subtropical SACZ regions, which present afternoon maximum (1500–1800 UTC) and morning minimum (0900–1200 UTC). More specifically, over the core of SACZ, the phase and amplitude of the simulated precipitation diurnal cycle are very close to the TRMM‐PR observations. Although there are amplitude differences, the RegCM3 simulates the observed nighttime rainfall in the eastern Andes Mountains, over the Atlantic Ocean, and also over northern Argentina. The main simulation deficiencies are found in the Atlantic Ocean and near the Andes Mountains. Over the Atlantic Ocean the convective scheme is not triggered; thus the rainfall arises from the grid‐scale scheme and therefore differs from the TRMM‐PR. Near the Andes, intense (nighttime and daytime) simulated precipitation could be a response of an incorrect circulation and topographic uplift. Finally, it is important to note that unlike most reported bias of global models, RegCM3 does not trigger the moist convection just after sunrise over the southern part of the Amazon.

Inter ENSO variability and its influence over the South American monsoon system

Abstract. Previous studies have discussed the interannual variability of a meridional seesaw of dry and wet conditions over South America (SA) associated to the modulation of the South Atlantic Convergence Zone (SACZ). However, they did not explore if the variability inter ENSO (El Niño Southern Oscillation) can be related to the phase changes of this dipole. To answer this question, an observational work was carried out to explore the atmospheric and Sea Surface Temperature (SST) conditions related to the same ENSO signal and to opposite dipole phases. Rotated Empirical Orthogonal Function (REOF) analysis was applied over normalized Chen precipitation seasonal anomalies in order to find the dipole mode in the Austral Summer (December to February). The fourth rotated mode, explaining 6.6% of the total variance, consists of positive loading over the SACZ region and negative loading over northern Argentina. Extreme events were selected and enhanced activity of SACZ during the Summer season (SACZ+) was identified in nine years: five during La Niña events (LN) and two in El Niño episodes (EN). On the other hand, inhibited manifestations of this system (SACZ-) were identified in seven years: four in EN and two during LN. Power spectrum analysis indicated that the interannual variability of the precipitation dipole seems to be related to the low frequency and to the quasi-biennial part of ENSO variability. The ENSO events with the same signal can present opposite phases for the dipole. The results suggest that the displacement of the convection over Indonesia and western Pacific can play an important role to modulate the seesaw pattern.

Characteristics of summertime daily rainfall variability over South America and the South Atlantic Convergence Zone

¶This paper presents an objective analysis of the structure of daily rainfall variability over the South American/South Atlantic region (15°–60° W and 0°–40° S) during individual austral summer months of November to March. From EOF analysis of satellite derived daily rainfall we find that the leading mode of variability is represented by a highly coherent meridional dipole structure, organised into 2 extensive bands, oriented northwest to southeast across the continent and Atlantic Ocean. We argue that this dipole structure represents variability in the meridional position of the South Atlantic Convergence Zone (SACZ). During early and later summer, in the positive (negative) phase of the dipole, enhanced (suppressed) rainfall over eastern tropical Brazil links with that over the subtropical and extra-tropical Atlantic and is associated with suppressed (enhanced) rainfall over the sub-tropical plains and adjacent Atlantic Ocean. This structure is indicative of interaction between the tropical, subtropical and temperate zones. Composite fields from NCEP reanalysis products (associated with the major positive and negative events) show that in early and late summer the position of the SACZ is associated with variability in: (a) the midlatitude wave structure, (b) the position of the continental low, and (c) the zonal position of the South Atlantic Subtropical High. Harmonic analysis of the 200 hPa geopotential anomaly structure in the midlatitudes indicates that reversals in the rainfall dipole structure are associated primarily with variability in zonal wave 4. There is evidence of a wave train extending throughout the midlatitudes from the western Pacific into the SACZ region. During positive (negative) events the largest anomalous moisture advection occurs within westerlies (easterlies) primarily from Amazonia (the South Atlantic). In both phases a convergent poleward flow results along the leading edge of the low-level trough extending from the tropics into temperate latitudes. High summer events differ from those in early and late summer in that the rainfall dipole is primarily associated with variability in the phase of zonal wave 3, and that tropical-temperate link is not clearly evident in positive events.