Julia Nogues-Paegle(1), Carlos R. Mechoso (2), Rong Fu(3), E. Hugo Berbery(4), Winston. C. Chao(5), Tsing-Chang Chen(6), Kerry Cook(7), Alvaro F. Diaz(8), David Enfield(9), Rosana Ferreira(4), Alice M. Grimm(10), Vernon Kousky(11), Brant Liebmann (12), Jose Marengo(13),  Kingtse Mo(11), J. David Neelin(2), Jan Paegle(1), Andrew W. Robertson(14), Anji Seth(14), Carolina S. Vera(15), Jiayu Zhou(16)
 

(1) Department of Meteorology, University of Utah, USA, (2) Department of Atmospheric Sciences, University of California, Los Angeles, USA, (3) Georgia Institute of Technology; Earth & Atmospheric Sciences, USA, (4) Department of Meteorology, University of Maryland, USA, (5) Laboratory for Atmospheres, NASA/Goddard Space Flight Center, USA, (6) Department of Geological and Atmospheric Sciences, Iowa State University, USA, (7) Department of Earth and Atmospheric Sciences, Cornell University, USA, (8) Instituto de Mecánica de Fluidos e Ingeniería Ambiental, Universidad de la Republica, Uruguay, (9) NOAA Atlantic Oceanographic Laboratory, USA, (10) Department of Physics, Federal University of Paraná, Brazil, (11) Climate Predication Center/NCEP/NWS/NOAA, USA, (12) NOAA-CIRES Climate Diagnostics Center, USA, (13) Centro de Previsao do Tempo e Estudos de Clima, CPTEC, Brazil, (14) International Research Institute for Climate Prediction, Lamont-Doherty Earth Observatory of Columbia University, USA, (15) CIMA/Departmento de Ciencias de la Atmósfera, University of Buenos Aires, Argentina, (16) Goddard Earth Sciences Technology Center, University of Maryland, USA.

Abstract
 

1. Introduction
 

2. South American monsoon system (SAMS)
 

3. Variability

Mesoscale structure and diurnal modulations

Intraseasonal variations

Interannual variability

Decadal and longer time-scale variability

Variability of river flows 21

4. Numerical Simulations and Predictability

AGCM simulations

Models with variable resolution

Simulations with ETA model

NCAR Regional Climate Model

Predictability of River Flows

5. Summary and future challenges

Bolivian High

SACZ

Low Level Jets

Continental Scale Precipitation

Modeling
 

Figure Captions
 

 

References

Abstract

A review and discussion of outstanding recent findings on the South American Monsoon System (SAMS) is presented. The South American monsoon system develops over a land mass characterized by a large area crossed by the equator, very high mountains to the west that effectively block air transport from the ocean in the west, and surface conditions range from rain forest to deserts. Plentiful moisture transport from the Atlantic maintains a precipitation maxima over central Brazil. In addition, the subtropical plains of South America benefit from moisture transport from tropical latitudes that results in abundant precipitation over northern and central Argentina and Uruguay. The combination of this heat source and the orography results in seasonal evolution of convection unique to this region. Furthermore, there is an important influence of mid-latitude systems organizing tropical precipitation.

The findings presented emphasize the system complexity, highlight the importance of the South American continent as the core of atmospheric linkages with the adjoining oceans, and pose new questions on Amazon-deforestation impact and on water and energy cycles over the two largest river basins (Amazon and La Plata).

A discussion on the many important questions that remain on the relative roles of orography, the Bolivian altiplano, and Brazilian highlands, the Andes mountains, tropical heat sources (both over South America and other continents) and anomalies in sea-surface temperatures on regulating circulation features over South America on the evolution of SAMS is also presented.

1. Introduction

A complex variety of regional and remote factors contribute to define the climate over South America. The continent itself has unique geographical features. South America extends from about 10N to 55S, thus it is crossed by the equator. There are high mountains with very steep slopes along the western side (the Andes Mountains).  Surface cover  varies from tropical forests over Amazonia to high altitude deserts over the Bolivian altiplano. Climate variability elsewhere has significant impacts on the climate over South America. Links between sea surface temperatures (SST) anomalies associated with El Niño/Southern Oscillation (ENSO) and rainfall as well as circulation anomalies over the continent have been demonstrated by several studies. In turn, climate variability over South America can influence atmospheric patterns in the surrounding oceans extending to the Atlantic and South Pacific subtropical highs.

The climate of South America, therefore, is a most exciting topic of investigation. An important challenge to progress has been posed by the general paucity of surface-based observations over and around the continent, and by the inadequacy of available upper-level soundings to resolve key topographically-bound circulations, their diurnal evolution, and related horizontal and vertical fluxes of moisture, heat, and momentum. Nevertheless, significant progress towards a better understanding and increased accuracy of predictions has been achieved, particularly in the last decade. The availability of high-quality observational datasets produced by reanalysis at major operational centers has been a major contributor to progress. Studies based on a hierarchy of numerical models of the atmosphere have played a particularly important role in view of data sparcity. Models have helped to gain insight and test hypotheses on the effects on climate of a highly complex orography, variability in surface conditions, and multiplicity of phenomena with a wide range of space and time scales.

A key factor towards progress in the understanding and prediction of the South American climate in the last decade has been the establishment of special national and international research programs, which have both stimulated interest in the scientific issues and provided crucial support for research efforts. The Pan-American Climate Studies (PACS) program sponsored by the Office of Global Programs of the US National Oceanic and Atmospheric Administration (OGP/NOAA) sharpened the science focus, encouraged the establishment of international collaborations, and provided resources for pilot projects that are improving the monitoring of climate variability in the Americas. The Climate Variability (CLIVAR) component of the World Climate Reseach Programme (WCRP) established the Variability of American Monsoon Systems (VAMOS) panel (see http://www.clivar.org/organization/vamos/) , which aims to a better understanding, simulation and prediction of the American monsoon systems and their variability. VAMOS has been very successful in creating consensus on an international research program to investigate the warm season climate of the Americas, and on the development and implementation of international field programs. The recently organized US CLIVAR established the Climate Variability and Predictability/Pan-American (CLIVAR/PAN-AM) program, which is the US contribution to the international CLIVAR through VAMOS. National CLIVAR groups have also been organized in Argentina, Brazil, Chile and Uruguay. Another major WCRP component, the Global Energy and Water Cycle Experiment (GEWEX) has been involved through its Hydrometeorology Panel (GHP) in the Large-Scale Experiment in the Biosphere and Atmosphere of the Amazon Basin (LBA). The US National Science Foundation (NSF) and several national science agencies have established the Interamerican Institute for Global Change Research (IAI), which has encouraged and supported international research groups on aspects of the American climate.

The present paper reviews recent work focused on a better understanding of the South American monsoon system (SAMS hereafter) and warm season precipitation that has been primarily organized/supported by PACS, VAMOS and CLIVAR/PAN AM, although contributions by other programs may be substantial in some cases. Section 2 describes the main components of the SAMS, with a focus on the seasonal cycle of precipitation. Section 3 discusses the variability and predictability of the SAMS. Section 4 presents results from numerical models. Section 5 contains an overall summary with emphasis on future directions and challenges.

2. The South American monsoon system (SAMS)
Figure 1 superimposes the summer rainfall on other outstanding features of the season's atmospheric circulation over South America. (In this paper "seasons" are understood in the context of Southern Hemisphere.) The configuration shown in Fig. 1 is referred to as the South American monsoon system (SAMS, hereafter). The conventional definition of "monsoon" invokes a seasonal reversal of large-scale circulations driven by differential heating between continents and ocean. Zhou and Lau (1998) examine this definition in the context of the South American monsoon and conclude that a monsoon climate exists over South America. The upper levels are characterized by high pressure over the Bolivian plateau (Bolivian High) and low pressure over Northeast Brazil (Nordeste Trough). At low levels, easterly flow from the Atlantic Ocean is channeled southward by the Andes mountains into the Chaco Low. Another characteristic feature is the southeastward extension of cloudiness and precipitation from the southern Amazon towards southeast Brazil and the neighboring Atlantic Ocean. This northwest-southeast band of localized convection is referred to as the South Atlantic Convergence Zone (SACZ, hereafter), where there is a concentrated release of latent heat into the atmosphere.

A pictorial view of the evolution of convection leading to the establishment and demise of SAMS can be obtained from Fig. 2, which shows outgoing longwave radiation (OLR) during selected pentads along the calendar year. The development phase of SAMS during spring (September-November) is characterized by a rapid southward shift of the region of intense convection from northwestern South America to the southern Amazon Basin and Brazilian highlands (Planalto). The migration of convection from the Isthmus of Panama to the central Amazon during spring can occur in as little as one month. The initial migration of rainfall is lead by southward reversal of the cross-equatorial in the northwest corner of the South American continent (Wang and Fu 2002). As precipitation increases south of the equator, the Bolivian High becomes established near 15 S, 65 W. To the east of the Bolivian High, an upper-level cold trough forms off the east coast of Brazil. These features are most intense during the height of the southern summer (e.g., Kousky and Ropelewski, 1997). The decay phase of SAMS begins in late summer and continues through fall as convection gradually retreats northward toward the equator (Fig. 2, bottom panels). The development and decay phases of SAMS are both modulated by incursions of drier and cooler air from the mid-latitudes over the interior of subtropical South America (Garreaud, 1999; Vera and Vigliarolo, 2000). The mechanisms that control outstanding features in the evolution of the patterns shown in Fig. 1 are not well understood at the present time (see Marengo et al., 2001). For instance, the dynamical and physical processes that participate in the shift of patterns shown in the panels of Fig. 1 have received relatively little detailed study.

A spectral analyses of streamfunction and meridional wind at 200 mb has suggested that the Bolivian High-Nordeste Low system is basically a short wave (zonal wavenumbers 2-6) regime embedded in and modulated by the ultra-long wave regime of the tropical Southern Hemisphere (Chen et al., 1999). This short-wave regime exhibits monsoon-like characteristics, i.e., a vertical phase reversal in the midtroposphere, and a quarter-wave shift between velocity potential and streamfunction. The presence of divergence centers of the short-wave regime over eastern Brazil and eastern Africa suggests that the Bolivian high formation is linked to condensational heating over the Amazon, while the Nordeste low is also affected by condensational heating over Africa. On the other hand, the divergence center of the long-wave regime is over the western tropical Pacific. A simple diagnostic calculation that involves the equations for velocity potential maintenance (which links velocity potential and diabatic heating) and streamfunction budget (which couples streamfunction and velocity potential) supports the argument that the Bolivian High-Nordeste Low system is generated by local forcing over South America and remote forcing from Africa.

The strength and location of the SACZ are modulated by several processes. Cold fronts associated with synoptic systems that migrate from higher latitudes into northern Argentina and southern Brazil are often accompanied by enhanced deep convection over the western and southern Amazon, which affects the location of the SACZ and increases the southward flux of moisture from lower latitudes. Wave trains that originate in the central Pacific also modulate precipitation over the continent. In turn, organized convection over the continent affects the Pacific and tropical Atlantic through compensatory descending motions.

In December, the SACZ is in an eastward location. This is associated with drier, southeasterly flow over eastern Bolivia and lower precipitation in the Altiplano, and with higher precipitation over much of Brazil. In Janurary, the SACZ is farther west, which allows a strong supply of moist, unstable air to the Altiplano from the northwest along the eastern flank of the Andes. In February, precipitation decreases as this northwesterly flux of moisture is reduced. On synoptic time scales, precipitation over the Bolivian Altiplano is associated with three types of events (Garreaud, 2000). First, low pressure systems develop over northern Argentina (Chaco lows) or farther south, with a westward shift of the SACZ and generally dry conditions in the Amazon basin. Those systems advect warm, moist, unstable air along the eastern flank of the Andes, which triggers convection on the Altiplano that extends eastward into the SACZ. Second, cold-core lows develop over southeastern South America accompanied by an intensification of the SACZ and an upper-level trough over southern Brazil. The Bolivian high is shifted to the southwest and there is anomalous northerly, low-level flow along the flank of the Andes. Third, the SACZ, extends westward in association with an anomalous flux of moisture from the eastern Amazon basin and the South Atlantic toward the central Andes. The Bolivian High strengthens and shifts to the south, and much of the Amazon basin is dry. On the Altiplano, precipitation is higher in January than in either December or February because there are more frequent, albeit not necessarily more intense, rain-producing events.

A closer look into the evolution of precipitation in the Brazilian Amazon is given by Fig. 3 (Marengo at al., 2001). This shows calendar season values for the period 1976-1994, as obtained by averaging data from 430 stations onto a 2.5 longitude by 2.5 latitude grid. Data from the other countries that are partly in the Amazon were not available. To a first approximation, the annual cycle of precipitation follows the sun. The largest values are in the southeast during summer, which is also the wettest three-month period overall, and in the north and northwest during winter while the southeast is quite dry (Fig. 3c). The larger values during the transition seasons are relatively centered in latitude within the basin (Figs. 3b and 3d), which is consistent with the equatorial crossing of the sun. Nevertheless, rainfall during fall is heavier and more evenly distributed in longitude than in spring.

Figure 3 also shows striking east-west contrasts, as well as substantial differences, between rainfall patterns in the Brazilian Amazon during spring and fall. South of the equator, therefore, the wet and dry seasons are very distinct (e.g., Kousky, 1980, 1988; Horel et al., 1989; Rao and Hada, 1990; Rao et al., 1996; Kousky and Ropelewski, 1997; Marengo et al., 2001). The southeast is quite wet during summer and extremely dry during winter, while the northwest shows little seasonal variation. The highest three-month-season total rainfall near the mouth of the Amazon River exceeds 1200 mm, which appears to result from nighttime convergence of the easterly trades with the land breeze (see also Rao and Hada, 1990; Figueroa and Nobre, 1990; Marengo and Nobre, 2001). Nevertheless, there are local maxima exceeding 1000 mm in the northwest, southeast, and in a small patch of the eastern Amazon near the equator. The pattern of annual precipitation (not shown) is more similar to that of the three-month minimum than to the three-month maximum. This suggests that annual precipitation is not a good indicator of the locations with heaviest rainfall rates. For example, annual precipitation in the northwest exceeds by more than 50% that in the southeast, yet values corresponding to maximum precipitation are about equal.

Much of the energy required for convection over the Amazon is provided by static instability processes (Fu et al. 1998). In this way, a relatively small increase of shear instability can have large impact on the development of mesoscale convection. A better understanding of static instability changes is important to explain how large-scale atmospheric dynamic processes affect rainfall, and to infer the relative importance between land surface processes and atmospheric effects. Fu et al. (1998) found by analyzing multi-year radiosonde profiles that the seasonal onset of convection in the equatorial Amazon is largely in response to

moistening of the planetary boundary layer (PBL) and lowering of temperature at its top. A close examination of the transition from dry to wet season using ECMWF reanalyses (Li and Fu 2002) suggest that the increase of land surface fluxes, especially latent heat flux, is a key driver to initiate the transition from the dry to wet season, whereas subsequent increase of large-scale moisture transport  greatly accelerates this process.  The initial increase of land surface fluxes elevate the low-level equivalent potential temperature, conducive to convection, and may drive the reversal of cross-equatorial flow in western Amazon.  Furthermore, the resulting increase of moisture convergence in the Amazon basin and stretching of the air column  drives upper air divergence spinning up the Bolivian High until the surface fluxes no longer increase.

3. Variability

a) Mesoscale structure and diurnal modulations.

Convection has a strong diurnal cycle over tropical and sub-tropical South America. This cycle is influenced by a number of effects including the obvious low-level stabilization and destabilization that occur with surface radiational cooling at night and heating during the day. Such a characterization does not apply to regions where convection most commonly occurs at night, such as the central plains of the US during summer. A number of studies have suggested that the diurnal oscillations of the SALLJ and PBL convergence, induced by the sloping terrain, are responsible for modulations of the low level moisture supply on those time scales. The response amplifies near critical latitudes, where the frequency of the forcing is approximately equal to the Coriolis parameter. The theory also suggests a phase shift of approximately 10 hours from 20 to 40° latitude for diurnally forced oscillations (Paegle and McLawhorn, 1983). The critical latitude for diurnally forced motions in the PBL response is somewhat variable around 30° for a number of reasons (Smith and Mahrt, 1981). However the large phase change in the vicinity of 30° should be observable. This is difficult to resolve with data sets over the US, because data are available mainly poleward of 30° and the distribution of mountains, plains and seas change abruptly at this latitude. On the other hand, South America has extensive mountain ranges and broad valleys from about 50° S to the equator.

Recent analyses of wind soundings during summer have revealed intriguing aspects of the SALLJ in central South America that are not easily reconciled by theoretical analyses that have been successfully applied in the North American framework (Douglas et al., 1999). The principal discrepancies are a deep late afternoon, rather than a shallow early morning wind maximum at Santa Cruz, Bolivia (Douglas et al. 1999), and substantially enhanced nocturnal cloudiness and rainfall at this foothill site. A number of mechanisms influence the diurnal cycle of moisture budgets over South America. These mechanisms include diurnal oscillations of the SALLJ, convective instability, and land and sea-breezes (Berbery and Collini, 2000), as well as solenoidal circulations generated by elevated plateau effects.

Warm-season precipitation processes over North and South America are strongly modulated by low-level jets. The structure of these  jets is important since this wind system transports moisture rapidly over large distances, e.g., east of the Rockies and Andes Mountains. This moisture often condenses and precipitates in the region of low-level convergence situated downstream of the jet core. Here, explosive mesoscale convective complexes (MCCs) occur preferentially at night (Velasco and Fritsch, 1987).

b) Intraseasonal variations

An outstanding feature of the warm season precipitation over much of eastern and southern Brazil is the high variability on time scales from a few days out to a few weeks. This variability has been related to changes in the position and intensity of the SACZ (e.g., Casarin and Kousky, 1986; Kousky and Cavalcanti, 1988; Nogues-Paegle and Mo, 1997; Nogues-Paegle et al., 2000; Silva and Kousky, 2000). Analysis of persistent wet and dry conditions over tropical and subtropical eastern South America during the austral summer reveals a dipole pattern of rainfall anomalies, with one center over southeastern Brazil in the vicinity of the SACZ and another center over southern Brazil, Uruguay and northeastern Argentina (e.g., Casarin and Kousky, 1986: Kousky and Cavalcanti, 1988; Kousky and Kayano, 1994; Nogues-Paegle and Mo, 1997; Nogues-Paegle et al., 2000; Silva and Kousky, 2000). At times, this seesaw pattern appears to be a regional component of a larger scale system, with the southward extension and strengthening of the SACZ found in association with enhanced tropical convection over the central and eastern Pacific and dry conditions over the western Pacific and the maritime continent (Casarin, and Kousky 1986; Nogues-Paegle et al., 2000). Convection is simultaneously suppressed in the region of the South Pacific convergence zone (SPCZ), over the Gulf of Mexico, and in the ITCZ over the North Atlantic. In the opposite phase, there is a strong influx of moisture from the tropics into central Argentina and southern Brazil, which is enhanced by the SALLJ.

Analysis of OLR and velocity potential over tropical and subtropical South America shows distinct peaks in the intraseasonal band at 20-25 day and 30-60 day (e.g: Liebmann et al., 1999; Li and Le Treut, 1999). The 20-25 day peak has been linked to the remote forcing over southwest Australia, which originates a wave train propagating southeastward, rounding the southern tip of South America and turning toward the northeast. This is one of the components of the Pacific-South American teleconnection pattern (PSA, Mo and Nogues-Paegle, 2001). It has been suggested that this teleconnection pattern impacts the SACZ, which results in a regional seesaw pattern of alternating dry and wet conditions (Nogues-Paegle and Mo, 1997; Nogues-Paegle et al., 2000; and others). The 30-60 day peak has been linked by most studies to the Madden-Julian oscillation (MJO). Another study based on a rotated EOF analysis applied to data provided by 516 stations in locations ranging from Patagonia to Northeast Brazil during the period 1965-1990 also finds a dipole-type structure in rainfall variability in the 30-70 day band (Grimm et al., 2000a; Ferraz and Grimm, 2000). There is also evidence that the relatively fast onset of convection over southeastern tropical South America is associated with MJO activity (Vera and Nobre, 1999).

c) Interannual Variability

The interannual variability of rainfall has a pronounced regional dependence. Precipitation over northern South America, central Chile, southern Brazil, eastern Argentina and Uruguay are strongly modulated by ENSO events (Pisciottano et al. 1994, Grimm et al. 1998, and Grimm et al. 2000). Connections to the Pacific are established through Rossby-wave trains and through vertical Walker- type circulations. Variability of the SACZ has been also linked to that in Atlantic SSTs. Furthermore, diabatic heating associated with the SAMS may in turn exert an important influence on the boreal winter subtropical jet over eastern North America through regional Hadley circulations (Nogues-Paegle et al., 1998). Recent modeling studies have suggested that interannual variations in the winter NAO index during boreal winter  may be attributable to variations in tropical heating associated with SAMS (Watanabe and Kimoto, 1999; Robertson et al., 2000; Cassou and Terray, 2000).

The extreme phases of the ENSO cycle have a significant impact on the overall strength of the SAMS and the rainfall pattern over tropical South America (e.g., Aceituno, 1988; Kousky and Kayano, 1994). When SSTs in the tropical Pacific are warmer than average (El Niño conditions) the rising motion over tropical South America is weaker than normal (Fig. 4a) and rainfall over the eastern Amazon and Northeast Brazil is below normal (Rao et al., 1986). When SSTs in the tropical Pacific are below average (La Niña conditions) the rising motion is stronger than average (Fig. 4b) and greater-than-average rainfall is observed in the above regions. El Niño years commonly exhibit two tropospheric anomalous warming centers straddling the equator over the central-eastern equatorial Pacific stretching northeastward and southeastward to the Gulf of Mexico and South America, respectively (Rasmusson and Mo, 1993; Lau and Zhou, 1999). A stronger (weaker) than normal subtropical jet stream over central Chile and north-central Argentina during El Niño (La Niña) episodes is accompanied by wetter (drier) than normal conditions over southern Brazil, Northeast Argentina and Uruguay, and stronger (weaker) than normal low-level southward flux of moisture over subtropical South America east of the Andes. Furthermore, anomalous mass distribution induced by El Niño is consistent with significant pressure increase over northwestern Africa and the subtropical South Atlantic and decrease from the subtropical South Pacific to the Gran Chaco of South America, reinforcing the summertime regional pressure gradient and resulting in a stronger than normal flux of tropical moisture toward higher latitudes over South America. During warm ENSO events land-sea temperature contrasts can result in differences of up to 2^o C between the northern South American landmass and the eastern tropical Pacific off the coast. This has been associated with migration of deep convection westward across the landmass from the south-central Amazon to the Peruvian coast (Pulwarty and Diaz, 1993). Deep convection over this region during the austral fall may not only result from warmer SSTs but also as part of the seasonal migration from the Amazon basin itself. A corresponding eastward displacement of the centroid of deep convection took place during the strong La Niña event of 1988-89. Further studies of the relationships between the demise of the SAM and the strength of this east-west land-sea temperature contrast and its modulation by cloudiness (see Yu and Mechoso, 1999) during ENSO, are needed.

More than 67% of the interannual variance in monthly mean precipitation on the Altiplano is associated with a  northward displaced Bolivian High, conducive to a cyclonic anomaly on the Altiplano, and a region of cool, dry, southerly flow at low levels east of the Altiplano (Lenters and Cook 1999). This pattern occurred during the 1986/7 ENSO event, which brought dry conditions to the Altiplano region during January and, especially, February of 1987. The onset of this dry period was associated with the equatorward penetration of a cold front, which brought dry, stable conditions that inhibited convection on the Altiplano for weeks. Thus, the interannual variability signal in this case was related to a single synoptic event which is, at times associated with ENSO warm conditions.

Linkages between the Pacific Ocean and rainfall anomalies over South America are often interpreted as the large-scale response to the easterly displaced convection.  This response can be described by the leading EOFs of height or streamfunction anomalies, which are referred to as the Pacific-South American modes (PSA1 and PSA2) since they arch over the South Pacific and South America. PSA1 and 2 both have zonal wavenumber 3-type hemispheric patterns in mid to high latitudes, and a well-defined wave train with large amplitude in the Pacific-South American sector (Mo, 2000; Mo and Nogues-Paegle, 2001). The PSA1 and PSA2 patterns exhibit characteristics of Rossby wave response to perturbations in the South Pacific suggesting that their ubiquitous presence on various time scales may be an inherent feature of atmospheric variability. The PSA 1 pattern appears to be related to ENSO (Mo and Nogues-Paegle 2001), with associated summer rainfall deficits over northeastern South America, and enhanced precipitation over southeastern South America (as described above). PSA 2 appears to be associated with the quasi-biennial component of ENSO, which has a period of 22-28 months. The strongest connection between PSA 2 and the Tropics is during spring. The associated rainfall pattern over South America shows a dipole pattern with out-of-phase anomalies between the SACZ and the subtropical plains around 35° S.

During summer, a large-scale anomalous upper-tropospheric stationary eddy is found to be associated with interannual variations in the intensity of the SACZ in the NCEP/NCAR Reanalysis data (Robertson and Mechoso, 2000). An anomalous cyclonic eddy accompanies an intensified SACZ with anomalous descent to the southwest, and weakened low-level jet east of the Andes. The anticyclonic case is opposite. The wave appears to be a natural mode of atmospheric variability of an essentially extra-tropical character: the upper-tropospheric vorticity balance is characteristically extratropical, and the vertical structure is equivalent barotropic. This signature is similar to that found by Mo and Nogues-Paegle (2001) fourth PSA 2 mode.

Kalnay and Halem (1981) identified a similar (though less localized) stationary Rossby wave in the lee of the Andes in 1979 from the First GARP Global Experiment (FGGE). Subsequent GCM experiments by Kalnay et al. (1986) indicated that this wave could exist independently of the Andes, but that tropical heating over either the Pacific or Atlantic sector could generate it. On sub-monthly timescales, Liebmann et al. (1999) found that intensified SACZ episodes are accompanied by a trough to the southwest that is produced by a transient Rossby wavetrain from higher southern latitudes. The interannual SACZ/trough structure may be a rectification of these intraseasonal events, i.e. the product of random sampling of different numbers of submonthly events in any given summer.

Over the southwest Atlantic, SST anomalies are found to accompany interannual intensifications of the SACZ, with negative anomalies north of about 40 ° S, and positive ones to the south. The latter coincide with decreased westerly winds, and are thus consistent in sign with the effect of reduced evaporation. The cold anomalies to the north partially underlie the cold atmospheric trough associated with the intensified SACZ; they thus also tend to be consistent with atmospheric forcing, both thermodynamically and through anomalous Ekman pumping (Kalnay et al. 1986). On the other hand, it is plausible that these latter negative SST anomalies may reinforce the overlying atmospheric trough and intensify the monsoonal circulation by strengthening the land-sea temperature contrast. Coupled ocean-atmosphere interaction may be involved. Consistent with Mo and Nogues-Paegle (2001), a similar SACZ/eddy structure is found in austral spring, at which time it is strongly teleconnected with ENSO through a PSA wavetrain. During austral summer (January-March) the teleconnection with ENSO disappears, re-emerging weakly in the fall (April-June). Grimm (2002) has shown that the seasonal description of ENSO impact on South America through seasonal averages miss  sub-seasonal abrupt changes. Such intra-seasonal changes may suggest a dominant role of regional processes over remote influences during the South American monsoon. 




A clear simultaneous relationship between SST anomalies in the southwestern Atlantic and precipitation in Uruguay (of the warm-wet or cold-dry types) during DJF is shown by Diaz (2000). At least two relevant questions arise: 1) are the SST anomalies the cause of the precipitation anomalies, or are they both driven by the atmosphere?, and 2) what is the the link between these anomalies and the SACZ?

d) Decadal and longer time-scale variability

A number of studies have reported the existence of decadal and longer time-scales variability in South American rainfall. Most studies have related the change of rainfall to regional and global SST variations. Increased cyclonic activity around Newfoundland has been associated with enhanced rainfall in Northeast Brazil (Namias, 1972). This implies a relationship with the North Atlantic Oscillation (NAO), which varies on the decadal time scale and is strongest in austral summer. Wagner (1996) used surface and subsurface ship observations and demonstrated a trend of interhemispheric SST gradient in association with a substantial SST increase in the South Atlantic SST centered at 20 °-30°S in austral summer season. He found a positive correlation between this SST trend and the precipitation anomalies over Northeast Brazil and attributed that to the southward displacement of the lower tropospheric wind confluence zone. This result was confirmed by Marengo et al. (1998), who showed a slow increase of rainfall in northeast Brazil in historical record, and by Hastenrath (2000) who showed concordant upper level changes in vertical motion and divergent circulations in the NCEP/NCAR reanalysis. Long-term variations have also been found in historic hydroclimatological records of tropical South America. A negative trend in two regions of western and central Amazonia and a positive trend in eastern Amazonia over the period from 1960s to 1980s was demonstrated by Dias de Paiva and Clarke (1995).

Using CPC Merged Analysis of Precipitation (CMAP) data, Zhou and Lau (2000) showed that the second and the third principal modes of low-passed South American summer rainfall are related to decadal and longer-term variability, respectively. The decadal variation presents a pattern of meridional shift of the ITCZ on both the eastern Pacific and western Atlantic Ocean, such a shift is closely related to the decadal change of the cross-equatorial SST gradient (Nobre and Shukla 1996; Rao et al. 1999) with modification of ENSO variations. The long-term trend shows increase of rainfall over the west coast of Ecuador-Colombia and tropical eastern Brazil and decrease over the surrounding areas and the equatorial North Atlantic Ocean.

The climate change that occurred in the Pacific around the late 1970s and continued since then has usually been associated with the Pacific decadal oscillation (PDO, Mantua et al. 1997). Prior to the mid-1970s (since at least the early 1950s) the eastern Pacific had been relatively cool and the central North Pacific warm. After the mid-1970s, the eastern Pacific, both north and south of the equator, has been warm and the central North Pacific cool. Enfield and Mestas-Nuñez (1999) show composite maps of anomalous circulations based on two components of the NINO3 SST index (5S-5N, 90W-150W): a filtered component (1.5-8 year band) representing canonical global SST variability associated with ENSO, and a residual component containing primarily the low-frequency SST change of recent decades. A global complex EOF mode of SST anomalies constructed using an updated version of the Kaplan et al. (1998) SST dataset (1870-1998), reproduces the known canonical aspects of ENSO, including phase propagation of SST within and between ocean basins. These results are surprising and significant in at least two respects: the effects of warming in the filtered components are generally opposite, and they are particularly strong in the South American sector.

The global ENSO mode accounts for about 3/4 of the total SST anomaly variability, while the residual accounts for the rest. Interestingly, about 40-50% of the amplitudes of the record-setting 1982-83 and 1997-98 El Niño events is accounted for by the residual variability. Related to this, the ranking of the canonical ENSO events changes significantly with respect to the NINO3 index based on data (e.g., 1972-73 is equal to or stronger than 1982-83 and 1997-98). The indices of the canonical ENSO and residual (non-ENSO) variabilities in the NINO3 region can be used to obtain the associated boreal winter (December-January-February) global composite maps of the tropospheric direct circulation using the NCAR/NCEP reanalysis, 1950-99. The resulting maps of velocity potential and irrotational flow at 850 and 200 hPa, and vertical velocity at 500 hPa show a consistent picture of the 3-D direct circulation for the normal DJF season as well as for the ENSO component and the residual change component. The anomalous boreal winter circulations, composited on the interannual and residual components of the NINO3 variability, are quite different, from both climatology and from each other (Fig. 5) They imply nearly opposite departures from the normal circulation, based on comparable warming phases within the NINO3 region: The zonal Walker circulations at low latitudes are virtually opposite, having decadal subsidence near the dateline in place of the ENSO-related uplift (convection), and decadal uplift over northern South America, in place of ENSO-related subsidence. A similar trend was detected by Chen et al. (2000) who suggest that such changes might alleviate the effects of Amazonian deforestation.

e) Variability of river flows

The interdecadal variations of SST anomalies in the Nino 3 region are in phase with the interdecadal variations of streamflow anomalies of rivers in southeastern and northwestern South America, although these two regions show opposite responses to ENSO (Genta et al., 1998). In addition, a near-cyclic 15-17-year component in the SACZ was identified in the NCEP Reanalysis data. The southwest Atlantic SSTs and flows of rivers in the Plata Basin exhibit very similar oscillatory components (Robertson and Mechoso, 2000). On interdecadal time scales, SST anomalies associated with the SACZ tend to be monopolar and located north of 30 S. When the SACZ is intensified, the Paraná and Paraguay Rivers tend to swell while the Uruguay and Negro Rivers to the south tend to ebb; this north-south contrast in streamflow anomalies is most marked on the interdecadal time scale. An 8-9-year component has also identified in the Paraná and Paraguay rivers that is correlated with a similar component in the North Atlantic Oscillation, with the latter appearing to force the former through changes in the northeast trade winds (Robertson and Mechoso, 1998; Nogues-Paegle et al., 2000). Statistical analysis of rainfall data from over 40 gauges (Müller at al., 1998) in the upper Paraná basin showed evidence of increased rainfall after 1970, with the increases associated with increased frequency of rainfall events. The mean annual increase ranged from 8% to 17%. In the upper Paraguay basin, a comparison by logistic regression of rainfall pattern occurrence, indicated that dry spells were more persistent during the period 1960-70, when river flows were much lower than in the periods before and after; whilst on days when rain fell, it was of lower intensity (Collischonn et al , 2001).

4. Numerical Simulations and Predictability

a) AGCM simulations

A comparison of simulations of 1997/98 El Niño impact on SAMS by AGCMs (Zhou and Lau 2000) shows that the models have more predictive skill over tropical South America, where anomalies are governed by the Walker cell shift that is directly induced by the central-eastern Pacific warming. The studies find less skill over the subtropics, where anomalies are mostly caused by anomalous SAMS with large uncertainties due to poorly resolved orographic relief and surface conditions as well as different solutions by various models of the intrinsic dynamics of the system.

Modeling is an ideal tool to better understand the individual roles played by the shape and location of continents, topography, and SST distributions in the magnitude and geographical distribution of South American precipitation. However, the narrowness of the Andes' topography has slowed down progress toward capturing a reasonable simulation of climatological precipitation over South America by global models. The relatively coarse resolution of those models has difficulties with the height of the Andes. In addition, the popular spectral numerical techniques produce spectral noise (Gibbs phenomena) near the Andes (Lenters et al., 1995). The development of techniques to filter topography in numerical models in the early 1990's enabled much improved simulations of South American precipitation fields in global models. Using Lindberg and Broccoli's (1996) filtered topography in their AGCM, Lenters and Cook (1995) identified the individual roles of these surface features in determining South American precipitation climatology in summer. Three experiments were performed. The experiment referred to as SST includes realistic January SSTs and global topography. A zonally uniform SST prescription is used in the mountain and no-mountain experiments. Selected results from these experiments are shown in Fig. 6.

AGCM simulations suggest that the presence of the South American continent alone, without topography or longitudinal structure in the SST field, establishes the precipitation maxima in the Amazon, SACZ, and northern Andes (Fig. 6b). Precipitation in these regions is associated with low-level wind convergence maxima that are directly related to the interaction of the continental thermal low with the South Atlantic high and the northeasterly trades. The SACZ is more complicated, and precipitation in that region is also enhanced by southeastward moisture advection and moisture flux convergence by transient eddies. Topography introduces orographic precipitation maxima on the eastern flank of the central Andes and the western flank of the southern Andes (Fig. 6c). Strong precipitation at higher elevations in the central Andes is not orographic in the model, but it is associated with strong convergence and convection over the elevated surface. The presence of topography makes the SACZ more distinct, and strengthens and repositions the Amazonian precipitation maximum. While the presence of longitudinal structure in SSTs is not fundamental to the existence of any of the South America precipitation maxima, precipitation in the Amazon and SACZ is very sensitive to SST structure and also to land surface conditions, especially in northeast Brazil. Comparison of Figs. 6a and b, for example, demonstrates the importance of SSTs in positioning the Amazonian precipitation.

AGCM studies have also indicated that that the Bolivian High is primarily a response to condensational heating over the Amazon, and not to dynamical or thermal effects of the Andes mountains (Lenters and Cook, 1997). Gandu and Silva Dias (1998) pointed out that western tropical Pacific heating can affect the intensity and location of the Nordeste low, and Lenters and Cook (1997) noted that heating over Africa is crucial for closing the low. The role of tropical Pacific and Atlantic SSTs in shaping the seasonal patterns of the Amazon rainfall and its importance relative to land influence have also been tested explicitly by AGCMs. Fu et al. (2001) suggest that oceanic influences can be as important as those of land in determining the precipitation over the eastern Amazon during the equinox seasons. The seasonality of the land surface dominates that of the precipitation in the western Amazon throughout the year and that in the eastern Amazon during solstices. These influences are carried out through a direct thermal circulation and propagating of stationary Rossby waves. The direct thermal circulation originates either from the Atlantic ITCZ or from the eastern Pacific ITCZ, and induces subsidence as well as reduced precipitation over eastern Amazon. Rossby waves are forced by latent heating in the equatorial central Pacific, in a PSA-like mode into subtropical South America, strengthening the SACZ and suppressing convection over the equatorial eastern Amazon.

Experiments with numerical models suggest that east-west contrasts in tropical rainfall are associated with the seasonal cycle of SSTs, primarily in the Atlantic Ocean, which itself lags that of the ITCZ (Fu et al, 2001). The mechanism of influence is through the links between the seasonal cycles of SSTs and Atlantic ITCZ with its associated subsidence. In this regard, fall rather than summer is the rainfall season in Northeast Brazil as warmest SSTs in the adjacent Atlantic are consistent with a southernmost ITCZ and weakening of associated subsidence. The seasonal cycle of SSTs in the Pacific Ocean also contribute to the late rainy season in Northeast Brazil through its influence on the Pacific ITCZ, which itself is linked to the SACZ and the Nordeste Low.

Other AGCM experiments conclude that the Asian and Australian summer monsoon circulations are largely independent of the two continental masses as well as the martime continent (Chao, 2000); Chao and Chen, 2001a and b). This suggests that in these areas land-sea contrast is not a necessary condition for the monsoon to be established. On the other hand, there is a large change in African and South American summer monsoon circulations, when the corresponding continent is replaced by ocean. Their interpretation for the origin of monsoon is that the summer monsoon is the result of the ITCZ's (intertropical convergence zones) peak being substantially (more than 10 degrees) away from the equator. The origin of the ITCZ's and their locations in latitude have been interpreted by Chao (2000). The longitudinal location of the ITCZ's is determined by the distribution of surface conditions. ITCZ's favor locations of high SST as in the western Pacific and Indian Ocean, or tropical landmass, due to land-sea contrast, as in tropical Africa and South America. Thus, the role of landmass in the origin of the monsoon, when important, can be replaced by an ocean of sufficiently high SST.

Using a coupled intermediate atmosphere model with simple land and ocean mixed layer models, Chou et al. (2000) and Chou and Neelin (2000) examined the interaction between land and ocean, as well as land surface hydrological feedback in determining the spatial pattern of the monsoon rainfall. Figure 7 shows in schematic form mechanisms that they suggest are relevant to the large-scale aspects of summer monsoon systems, as applied to South America. Divergence of ocean heat transports reduces heat input into the atmosphere. In surrounding ocean regions and thus tends to favor continental precipitation, as does upper ocean heat storage in the summer season. Transport of low moisture static energy air from ocean to land during summer (and export of high moist static energy air from the continent) is referred to as the "ventilation mechanism". Numerical experiments suggest this is an important factor in determining the poleward extension of the monsoon rainfall over the continent. A Rossby wave circulation forced by the monsoon convection tends to disfavor convection in the western part of the continent, as postulated by Rodwell and Hoskins (1996). This circulation also acts to promote convection in the eastern part of the continent, so there is a strong interaction between the convection and the Rossby wave circulation. This is referred as "interactive Rodwell-Hoskins mechanism". Soil hydrology feedbacks would tend to disfavor land convection but are overruled by the mechanisms that favor land convection zones relative to surrounding oceans. The land hydrology feedbacks provide a secondary factor in tending to reduce the poleward extension of convection over land.

b) Models with variable resolution.

The coarse resolution of constant horizontal grid models precludes adequate detail in local precipitation patterns. This problem is alleviated with variable grid models that allow resolution of meso-scale features while retaining globally interacting influences. Wang et al. (1999) demonstrate that the latter approach mimics the Great Plains nocturnal precipitation maximum. They also demonstrate the importance of two-way interaction between the highly resolved regional domain and the remainder of the global atmosphere to obtain reasonable precipitation simulations. Figure 8 illustrates one possible grid-generation mechanism in a global, variable resolution approach and Fig. 9 displays resulting vertical wind structure. A deep northerly jet core over Bolivia, Paraguay and northern Argentina, and shallow coastal and offshore wind systems are evident.

A stretched coordinate model developed at GSFC/NASA has also been applied to study the role of interactions between regional (e.g., land use and vegetation cover) and large-scale processes in determining the seasonal to interannual variability of the atmosphere in the Amazon region (Ferreira et al., 2000). The NASA/GSFC NSIPP-1 climate model is presently being run using a variable resolution horizontal grid. The NSIPP1-climate GCM uses the MOSAIC land surface model. The MOSAIC land surface model (Koster and Suarez, 1996) includes the controls exerted by vegetation on the surface energy and water budgets, as well as a representation of the sub-grid variability in land cover. Sea surface temperatures are prescribed from observations. All of the variable resolution horizontal grids in use by this model have a uniform resolution region of interest of 20 degrees by 20 degrees over the Amazon region and increasingly coarser resolution away from the region of interest. In the region of interest, horizontal resolutions as fine as a quarter of a degree have been used successfully. The coarsest horizontal resolution used is 4 degrees and it occurs in the antipodal point with respect to the Amazon region. Simulations with a grid that has resolution varying from a quarter of a degree in the region of interest to 4 degrees in the antipodal point can be performed 20 times faster than the corresponding uniform grid simulations at a quarter degree global resolution. Uniform grid GCM ensembles show marked differences in predictability around South America in this model (Ferreira et al. 2000). Averages for nine member ensembles indicates that there is not as much intra-ensemble variability over the Amazon as there is over the SACZ region. Inspection of individual ensemble member reveals that the variability over the SACZ region in the model is due to changes in orientation and intensity of the SACZ. The stronger intra-ensemble variability (or noise) makes rainfall in the SACZ more unpredictable than rainfall in the Amazon basin. The stretched coordinate model run in ensemble mode would help resolve the issue of whether this lack of predictability is due to poor resolution of highly variable surface features or to the influence of more unpredictable mid-latitude synoptic scale systems.

c) Simulations with the ETA model

The regional forcings that may affect the long term variability are of mesoscale nature. Similarly, one of the most remarkable aspects of the LLJs is their mesoscale nature. While their extent is typically of the order of 2000 km, their width is only about 500 km. Moreover, they have a large diurnal cycle that affects the downstream convergence of moisture flux and precipitation. Mesoscale model forecasts discussed in Berbery and Collini (2000) have shown that the warm season LLJ east of the Andes is strongest during nighttime; however, as indicated earlier, no observations are available to verify that this is the case. On the other hand, given that the model simulations realistically reproduce the nighttime maximum of precipitation at the exit region of the LLJ in the subtropics, it is conceivable that other modeled aspects, including the nighttime maximum in the LLJ intensity and its related convergence, are all part of a consistent physical evolution.

Despite many common features, the North and South American LLJs have also significant differences (Nogués-Paegle and Berbery 2000); probably the most intriguing is, as suggested by global analyses, the existence of the South American LLJ even during the cold season, when the typical thermal forcing mechanisms are weaker. Next, the characteristics of the cold season LLJ as estimated from regional short term forecasts with the Eta model are documented.

The Eta model is a mesoscale regional model used at the National Centers for Environmental Prediction (NCEP) for their operational short term forecasts over the U.S. For the present study, NCEP's current model version was adapted to South America, with a grid spacing of 80 km and 38 vertical levels (see domain in Fig. 1 of Berbery and Collini, 2000). Unlike NCEP's operational version that runs from its own data assimilation system, here the initial conditions for the atmosphere and land are taken from NCEP's global data assimilation system, and thus are not optimally compatible with the Eta model physics, but it is assumed that after 12 hs of integration all adjustments have already occurred; the boundary conditions are taken from the global model forecasts.

The methodology follows that described in Berbery et al. (1996) and Berbery and Collini (2000) by which routine short term (12-36 hs) forecasts are averaged to produce a "climatology." In the present case, the results to be discussed here cover 45 days from mid-July to the end of August of 2000. Although not shown here, the model reproduces all precipitation centers observed in a climatology using satellite estimates of precipitation (based on Xie and Arkin's (1997) dataset).

The time-averaged vertically-integrated moisture flux field presented in Fig. 10 reveals large westward moisture flux near the tropics, while near the Andes there is a southeastward transport of water vapor toward northern Argentina-southern Brazil. Precipitation up to 3 mm day^-1 over this region (contours and shades in Fig. 10) is associated with the moisture flux convergence (not shown) downstream of the moisture flux maximum. This winter precipitation appears to have as a triggering mechanism the recurrent passage of cold fronts progressing northeastward east of the Andes from subpolar latitudes, and of upper-level troughs propagating eastward at subtropical latitudes, both of which may develop as cyclogeneses (Necco, 1982a,b). Several authors have documented that the eastern part of South America and adjacent Atlantic Ocean, between 20ºS and 35ºS is a preferred region of cyclogenesis (e.g. Necco 1982a,b). Vera et al. (2001) showed that the second leading mode of austral winter variability on synoptic-time scales over that region is associated with the propagation of upper-level troughs at around 30°S from the Pacific Ocean into the continent. Those systems then intensify at lower levels in the vicinity of the South American coast, and the enhanced moisture transports from tropical latitudes along the eastern portion of the low-level system favors precipitation occurrence. The precipitation produced by those systems accounts for more than 60% of the mean austral winter accumulated precipitation over central and eastern Argentina.

Figure 11 presents vertical sections across the maximum moisture flux east of the Andes (line AB in Fig. 10). The wind perpendicular to line AB (Fig. 11a) shows a maximum east of the Andes at about 700 hPa that is detached from the upper levels, suggesting a typical low-level jet structure. At the lower levels and particularly over the Pacific Ocean the flow tends to be in the opposite direction, following the predominant circulation of the subtropical anticyclone. The core of the jet is somewhat higher than that during summer (Berbery and Collini 2000), and more remarkably depicts a weak diurnal cycle that cannot be associated with the summer mechanisms of diurnal oscillations which typically account for changes in wind magnitude and direction at the top of the boundary layer (see, e.g., Bonner and Paegle 1970; review by Stensrud 1996). In this case, the difference between nighttime and daytime winds (Fig. 11b) suggests a decrease of the wind intensity at the lower levels and an increased intensity on the upper portion of the jet, probably representing a vertical shift of the jet's core. As its summer counterpart, this winter LLJ transports much of the moisture from the tropics to the subtropics (Fig. 11c); because of the stratification of moisture, the maximum moisture flux occurs at a somewhat lower level than the wind, between 725 and 825 hPa.

d) NCAR Regional Climate Model

Giorgi et al (1993) examined the variations in strength of the SALLJ and associated moisture transports in two extreme seasons (1983 and 1985) using the NCAR regional climate model (NCAR RegCM) intitialized and driven at the boundaries by NCEP-NCAR Reanalyses fields (Kalnay et al., 1996). The RegCM is initialized on January 1 of the two years and then forced only at the boundaries at six hour intervals for the five month integrations. Moisture is transported by the diurnally evolving SALLJ from the Amazon basin towards higher latitudes, into northern Argentina, Paraguay and Southern Brazil, where precipitation has a maximum during nighttime (Berbery and Collini, 2000; Berri and Inzunza, 1993; Virji, 1981).

The large-scale setting in 1983 was dominated by a strong El Niño event, which was associated with a zonal shift in the Walker Circulation and increased anti-cyclonic flow in the Atlantic. Strong southeast trades helped to restrict the southward migration of the ITCZ and led to very dry conditions in Northeast Brazil. The Reanalyses fields show a strong low-level northwesterly flow from the Amazon basin to southern Brazil during this season, where rainfall was above normal. In 1985 a moderate cold event was in progress in the equatorial Pacific, while cooler than average temperatures prevailed in the tropical Atlantic, north of the equator and warmer than average temperatures to the south. The Reanalyses fields also show increased northeasterly trades, a strong southward migration of the ITCZ and abundant rainfall in Northeast Brazil and the Amazon. The Bolivian high was well developed and the low-level northwesterly flow from the Amazon towards the southeast was reduced.

The RegCM captures the main circulation features described above in both years, and simulates the substantial increase in rainfall in the northeast in 1985 compared with 1983. Figure 12 shows the vertically integrated meridional moisture transport (contours) and total field (vectors) for FMAM 1983 from (a) Reanalyses driven RegCM and (b) Reanalyses, and for FMAM 1985 from (c) Reanalyses driven RegCM and (d) Reanalyses. In the model, the LLJ and associated moisture transport takes a more northerly path and extends farther south into northern Argentina than in the reanalyses, which shows a more northwesterly transport with an exit into southern Brazil. This difference between the model and reanalyses is evident in both years and may result from the enhanced resolution (60 Km) of the RegCM. A reduction in the strength of the southward moisture transport is seen in both the reanalyses and the RegCM simulation in 1985 when compared with 1983. A possible mechanism for this change in the low level jet is as follows. The jet, to a first approximation, is controlled by the east-west pressure gradient between the Atlantic and the Andes. In 1983 the South Atlantic high was strong due to increased subsidence resulting from a shifted Walker Circulation. In 1985 cold conditions resulted in a stronger North Atlantic high and northeast trade winds, and a weaker south Atlantic high. Thus, the low-level northerly flow is strengthened in 1983 by the increased high pressure to the east.

e) Predictability of River Flows

The slow cyclic components in the Plata Basin provide a basis for empirical stream flow prediction. Predictability of the Paraná river has been investigated by extracting near-cyclic 25, 9 and 17-year components in summer-season stream flows at Corrientes, over the period 1904-1997, and fitting autoregressive models to each (Robertson et al., 2001). The 9-year component yields successful tercile categorical hindcasts up to 3 years in advance, with below-average flows being more predictable than above-average flows. A prediction including data up to austral summer 1999 suggests increased probability of below-average flows until 2005.

Care is needed, however, with long-term predictions based on river flow. Although runoff integrates effects of climate change over a drainage basin, it is also affected by land-use change, and it is known (Bruijnzeel, 1996; Sahin and Hall, 1996) that deforestation - which has been widespread in some parts of the Plata Basin - often results in runoff increases. Moreover, annual runoff is not measured directly, but is estimated by means of a calibration curve ("rating curve") from which river discharge is estimated, given daily observations of water level in the river (e.g., Mosley and McKerchar, 1993). Because of sediment deposition and/or erosion in river channels as a consequence of deforestation, the rating curve may change with time, and requires constant scrutiny and, if necessary, adjustment. Even without complications arising from land-use change, the uncertainty in the annual flow in the river Paraná at Corrientes has been estimated as roughly equal to the annual flow in the River Thames (Clarke et al., 2000). For the Amazon at Óbidos, the uncertainty in annual runoff is about equal to the annual flow in the Rhine.

5. Summary and future challenges

a. Bolivian High

General features of the South American Monsoon system (SAMS) are described in the context of the mean seasonal cycle. The evolution of the Bolivian anticyclone, which is a major feature of the upper-level circulation during the austral summer, is directly related to the evolving pattern of precipitation. As the Bolivian anticyclone intensifies, upper-level cyclonic flow intensifies over the eastern tropical Pacific and central tropical Atlantic. These oceanic troughs are cold core systems characterized by an absence of deep convection. The southward advance of precipitation is rather rapid during late August and September and may be aided by vigorous synoptic systems at higher latitudes in the Southern Hemisphere. Further investigation of these tropical/intertropical interactions would improve our understanding of the monsoon variability as well, and help us place the South American monsoon system in a global context.

Many previous theoretical and diagnostic studies have dynamically linked the Bolivian High formation to condensational heating over the Amazon (Silva Dias et al. 1983; and others). During El Niño years, there is a tendency to severe drought from Northeast Brazil to central Amazon, and significantly less release of convective heating in the troposphere over that region. Accordingly, a weaker than average Bolivian High would be expected. Upper-level highs actually increase during strong El Niño episodes throughout the global tropics. Consequently, there is a need to examine in more detail the thermal versus dynamical impact on the interannual variability of upper-level circulation features, such as the Bolivian High over South America.

b. SACZ

Accompanying the development of the SAMS, a zone of enhanced cloudiness and precipitation extends from southeastern South America southeastward over the adjacent Atlantic Ocean. This South Atlantic Convergence Zone is a prominent feature during late spring and early summer. Intraseasonal variability is strongest over eastern and southern Brazil, related to changes in the position and intensity of the SACZ.

On interannual time scales and longer, the SACZ is associated with an upper-tropospheric eddy circulation that is almost isolated during peak summer, but is strongly teleconnected with ENSO during austral spring. A coherent interdecadal component can be identified that is also present in river flows of the Plata Basin and SW Atlantic SSTs. This interdecadal component is also correlated with SST anomalies over the Pacific that resemble the Pacific decadal oscillation which has a similar time scale, which suggests a global-scale mode (Chao et al. 2000). Further analysis of Plata-basin river flows also identifies a near-decadal component, unrelated to the SACZ, which appears to be associated with the boreal winter NAO circulation. These slow near-cyclic river flow variations suggest useful predictability several years in advance. The causes for the amplitude modulation of the near-decadal oscillations of river flow in the Plata Basin during the last century remain to be determined.

The SACZ location is an important factor that determines features of the continental-scale circulation and moisture fluxes into the South American monsoon regions. In turn, the intensity and positioning of the SACZ is sensitive to continental precipitation features as well as global-scale circulation anomalies. A better, more fundamental, understanding of the SACZ and its connections to the global circulation is needed to advance our understanding and ability to predict the South American monsoon system.

c. Low Level Jets

Observations and model studies have shown that during the warm season a poleward LLJ structure develops over the eastern slopes of the Rockies, but during winter no such circulation feature is observed. Contrary to this behavior, global analyses an regional model short term forecasts reveal that the circulation east of the Andes has a LLJ structure throughout the year. Results presented here suggest that the winter LLJ is the same order of magnitude as the summer one, although it is located at a somewhat higher altitude and displays a much weaker diurnal cycle. The LLJ in winter seems important to feed moisture from the tropics to facilitate the precipitation processes ahead of frontal systems typical for the mid and subtropical latitudes of South America. Questions remain on the relative importance of orography, convection, elevated heat source and low-level stability in the maintenance and diurnal phasing of this LLJ. More observations are required to resolve this issue due to the extensive data gaps at the core of the jet in a region that encompasses Bolivia, Paraguay, western Brazil and northern Argentina.

d. Continental Scale Precipitation

The SAMS generally starts weakening during March as solar heating decreases over subtropical South America. Recent studies suggest that a warm Pacific and cold Atlantic Oceans result in a delayed onset and early withdrawal of monsoonal rains over the Amazon basin. Many aspects on the ENSO impact on the onset and withdrawal of the SAMS and the relationships between interannual and intraseasonal variability of SAMS remain to be quantified.

Tropospheric response to equatorial Pacific ENSO anomalies needs to be considered in the context of the response to other remote forcings and time-scales. For example, ENSO teleconnections are decadally reinforced over northern and eastern South America with La Nina events plus background warming, both processes conducive to enhanced convection over this region.

The degree to which the late 20th century change in background climate can be attributed to natural decadal variability, anthropogenic climate change, or both needs to be ascertained. That both are occurring simultaneously has become increasingly evident (Andronova and Schlesinger, 2000). Many attempts currently underway to understand and predict the regional climate impacts of interannual variations in SST (notably ENSO) cannot be done without regard to what is happening to the background climate. Confusion between scientists can easily occur depending upon choices that are made for defining climatologies vis-a-vis the phase of the background variability. Our community is now challenged to explain these relationships and assure that future prediction models properly account for them.

e. Modeling

Models have difficulties with the steep Andean topography. The Andes are poorly resolved by global models, in which resolution is constrained by the validity of physical parameterizations as well as by practical considerations of computing resources and time, as well as by our knowledge of the climate system, and the Andes are poorly resolved in global models. Techniques for filtering such steep topography have been developed over the last decade, and this has improved our ability to model the South American monsoon system. These filtering techniques tend to preserve the volume of the topography, not its height, because linear dynamical theory (and its comparison with observations) suggests that this is the most important factor for properly simulating the response of the large-scale circulation to mountains. Global models produce much improved simulations of South America precipitation with this filtered topography, but there are still significant problems to overcome. For example, precipitation in the central Andes is overestimated because artificially low elevations cause the low-level air to be too moist. The model may produce a good simulation of the wind convergence in this region, but if the converging air is too moist the precipitation will be too large. Simply raising the filtered topography in the model closer to the true elevation of the Andes, so that the low-level moisture fields are accurate, might result on a degraded large-scale circulation and, therefore poor precipitation simulations.

This problem is ameliorated by modeling at the higher resolutions possible with regional climate models. Some results suggest that decreasing the grid size produces better precipitation results over the Andes (Berbery and Collini, 2000). Nevertheless, even with a resolution of 30- km, Andean peaks that reach to 4 km or more are not resolved and the elevation of the surface is below 3 km everywhere (Cook et al., 2000). Our understanding of the South America monsoon climate would be advanced by the development of techniques to accurately represent the effects of high, steep topography in models.

Much work is needed to understand dynamical downscaling methods and their capabilities for improving (or degrading) skill in climate forecasts. The relative utility of locally nested and variable resolution global approaches to predictability and climate simulations remains to be determined. Essential to this process is improved monitoring and diagnosis of these regional circulations at frequencies and horizontal scales which will provide useful validation data for regional climate models.
 

 


FIGURE CAPTIONS

FIG. 1. Schematic illustration of the summer monsoon system over South America. Shading represents topography.
 

 

FIG. 2. Average (1979-1995) outgoing longwave radiation (OLR) for selected pentads.
 

 

FIG. 3. Climatological total precipitation in mm for a) December-February, b) March-May, c) June-August, and d) September-November.
 

 

FIG. 4 Pressure-longitude section of the anomalous relative humidity and divergent circulation (5 °N-5°S) for December 1997-February 1998 (top) and December 1988-February 1989 (bottom). The divergent circulation is represented by vectors of combined pressure vertical velocity and divergent zonal wind. Shading and contours denote anomalous relative humidity (%). Anomalies are departures from the 1979-1995 base period monthly means.
 

 

FIG. 5 200 hPa velocity potential (contours) and irrotational flow (arrows).

Upper: the 50-year boreal winter mean (December-January-February). Middle: composite mean departures, for positive minus negative phases of the global ENSO mode. Lower: composite mean departures, for 1978-99 minus 1950-77, of the non-ENSO residual in the NINO3 region. Regions of high-level outflow (low-level confluence) are indicated by diverging arrows and solid contours. Hadley flows (or their departures) are indicated by regions of meridionally oriented arrows north of the equator.
 

 

FIG. 6. Precipitation rate for January as (a) observed (from Legates and Willmott 19990); and modeled in the (b) SST, (c) mountain, and (d) no-mountain experiments. Contour is 3 mm/day.
 

 

FIG. 7. Schematic of mechanisms relevant to large-scale aspects of the South American summer monsoon, following Chou and Neelin (2001), including the "interactive Rodwell-Hoskins mechanism" and the "ventilation mechanism".
 

 

FIG. 8. a) Uniform grid on customary spherical coordinate. b) Uniform grid after rotation of pole to South America. c) Utah variable resolution model grid after pole rotation and subsequently telescoped grid near pole. d) Resultant orography.
 

 

FIG. 9. Six day averaged simulation of January horizontal wind by the Utah variable resolution model at a) 2,500 m, b) 1,500 m, c) 400 m, and d) 100 m above surface.
 

 

FIG. 10: Vertically integrated moisture flux (arrows) and precipitation (contours and shades) for austral winter 2000. Units are kg (m s)-1 and mm day-1 respectively. AB represents a line perpendicular to the largest moisture flux east of the Andes.
 

 

FIG. 11: (a) Cross section of wind perpendicular to line AB in Fig. 9. Contour interval is 2 m s-1; (b) diurnal amplitude of the wind across line AB. Contour interval is 1 m s-1. Nighttime wind is taken from forecasts at 0600 UTC (approximately 0200 LST) and daytime wind from the 1800 UTC forecast (about 1400 LST); (c) same as (a) for the moisture flux. Contour interval is 15 (g /kg) ( m/s)
 

 

FIG. 12. Vertically integrated meridional moisture transport (shaded , countours from -150 to 150, with 30 kg / (m s ) ) and total field (vectors) for FMAM 1983 from Reanalyses driven RegCM (upper left) and Reanalyses (upper right), and for FMAM 1985 from Reanalyses driven RegCM (lower left) and Reanalyses (lower right)

1. Figure 1
2. Figure 2
3. Figure 3
4. Figure 4
5. Figure 5
6. Figure 6
7. Figure 7
8. Figure 8
9. Figure 9
10. Figure 10
11. Figure 11
12. Figure 12

 
 
 
 
 
 
 
 

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