South American Low-level Jet Research Articles

Enriched Analyses with Assimilation of SALLJEX Data

Abstract This paper aims at generating a set of enriched analyses by assimilating the data collected during the South American Low-Level Jet Experiment (SALLJEX) in southeastern South America during the summer season 2002/03. The analyses are generated with the Brazilian Regional Atmospheric Modeling System (BRAMS) using all available observations and achieving 20-km horizontal resolution and 3-h temporal resolution. This paper describes the methods used to generate and evaluate the analyses. The results indicate that the method is appropriate because data assimilation provides an improved fit between analyses and observations. In addition, enriched analyses show a better representation of mesoscale structure in case studies. This satisfactory evaluation allows one to use the set of analyses to perform case studies at higher resolution than is currently available, with the certainty that these analyses reproduce the atmospheric conditions better than those that do not assimilate data.

Influence of the Andes Mountains on South American moisture transport, convection, and precipitation

Mountain ranges are known to have a first-order control on mid-latitude climate, but previous studies have shown that the Andes have little effect on the large-scale circulation over South America. We use a limited-domain general circulation model (RegCM3) to evaluate the effect of the Andes on regional-scale atmospheric dynamics and precipitation. We present experiments in which Andean heights are specified at 250 m, and 25, 50, 75, and 100% of their modern values. Our experiments indicate that the Andes have a significant influence on moisture transport between the Amazon Basin and the central Andes, deep convective processes, and precipitation over much of South America through mechanical forcing of the South American low-level jet (LLJ) and topographic blocking of westerly flow from the Pacific Ocean. When the Andes are absent, the LLJ is absent and moisture transport over the central Andes is mainly northeastward. As a result, deep convection is suppressed and precipitation is low along the Andes. Above 50% of the modern elevation, a southward flowing LLJ develops along the eastern Andean flanks and transports moisture from the tropics to the subtropics. Moisture drawn from the Amazon Basin provides the latent energy required to drive convection and precipitation along the Andean front. Large northerly moisture flux and reduced low-level convergence over the Amazon Basin leads to a reduction in precipitation over much of the basin. Our model results are largely consistent with proxy evidence of Andean climate change, and have implications for the timing and rate of Andean surface uplift.

Observational evidences on the modulation of the South American Low Level Jet east of the Andes according the ENSO variability

Abstract. The differences on the phase and wavelength of the quasi-stationary waves over the South America generated by El Niño (EN) and La Niña (LN) events seem to affect the daily evolution of the South American Low Level Jet east of the Andes (SALLJ). For the austral summer period of 1977–2004 the SALLJ episodes detected according to Bonner criterion 1 show normal to above-normal frequency in EN years, and in LN years the episodes show normal to below-normal frequency. During EN and LN years the SALLJ episodes were associated with positive rainfall anomalies over the La Plata Basin, but more intense during LN years. During EN years the increase in the SALLJ cases were associated to intensification of the Subtropical Jet (SJ) around 30° S and positive Sea Level Pressure (SLP) anomalies over the western equatorial Atlantic and tropical South America, particularly over central Brazil. This favored the intensification of the northeasterly trade winds over the northern continent and it channeled by the Andes mountain to the La Plata Basin region where negative SLP are found. The SALLJ cases identified during the LN events were weaker and less frequent when compared to those for EN years. In this case the SJ was weaker than in EN years and the negative SLP anomalies over the tropical continent contributed to the inversion of the northeasterly trade winds. Also a southerly flow anomaly was generated by the geostrophic balance due to the anomalous blocking over southeast Pacific and the intense cyclonic transient over the southern tip of South America. As result the warm tropical air brought by the SALLJ encounters the cold extratropical air from the southerly winds over the La Plata basin. This configuration can increase the conditional instability over the La Plata basin and may explain the more intense positive rainfall anomalies in SALLJ cases during LN years than in EN years.

Predictability of South American low‐level jet using QuikSCAT ocean surface wind

The applicability of NASA QuikSCAT ocean surface wind was tested for predicting South American low‐level jets (SALLJs) with a statistical model. Our previous study (Wang, H. and Fu, R., 2004, Influence of cross‐Andes flow on the South American low‐level jet. Journal of Climate, 17, pp. 1247–1262) has examined the dynamic process associated with austral winter SALLJs using the ECMWF Reanalyses (ERA) and identified the mechanism that controls the seasonal and synoptic variations of the SALLJ. It was found that the SALLJ is maintained by strong zonal pressure gradients, with a maximum near 850 hPa caused by deflection of upstream zonal flow crossing the Andes and lee cyclogenesis. The robustness of this mechanism was further examined in this study using the NCEP–NCAR Reanalysis 1 (NCEP‐R1) and NCEP–DOE Reanalysis 2 (NCEP‐R2). The northerly LLJs to the east of the Andes are strongest in ERA, with wind speeds well above 15 m s−1. In NCEP‐R1 and R2, typical wind speeds are about 12 and 10 m s−1, respectively. A statistical analysis of the three reanalysis datasets indicates that the SALLJ significantly correlates with the zonal wind of previous days over the South Pacific, particularly with the surface zonal wind. Based on this result, a statistical model introduced in Wang and Fu (2004) was employed in this study for forecasting the SALLJ using the QuikSCAT ocean surface wind as a predictor. The model was applied to June, July and August of 1999 to 2006 for up to 5 day forecasts of the SALLJ. Cross validations of the hindcasts indicate significant predictability of strong LLJ events with the QuikSCAT ocean surface wind data.

A high-resolution 43-year atmospheric hindcast for South America generated with the MPI regional model

An evaluation of the present-day climate in South America simulated by the MPI atmospheric limited area model, REMO, is made. The model dataset was generated by dynamical downscaling from the ECMWF-ERA40 reanalysis and compared to in-situ observations. The model is able to reproduce the low-level summer monsoon circulation but it has some deficiencies in representing the South American Low-Level Jet structure. At upper levels, summer circulation features like the Bolivian High and the associated subtropical jet are well simulated by the model. Sea-level pressure fields are in general well represented by REMO. The model exhibits reasonable skill in representing the general features of the mean seasonal cycle of precipitation. Nevertheless, there is a systematic overestimation of precipitation in both tropical and subtropical regions. Differences between observed and modeled temperature are smaller than 1.5°C over most of the continent, excepting during spring when those differences are quite large. Results also show that the dynamical downscaling performed using REMO introduces some enhancement of the global reanalysis especially in temperature at the tropical regions during the warm season and in precipitation in both the subtropics and extratropics. It is then concluded that REMO can be a useful tool for regional downscaling of global simulations of present and future climates.

How Does Soil Moisture Influence the Early Stages of the South American Monsoon?

Abstract This article discusses the feedbacks between soil moisture and precipitation during the early stages of the South American monsoon. The system achieves maximum precipitation over the southern Amazon basin and the Brazilian highlands during the austral summer. Monsoon changes are associated with the large-scale dynamics, but during its early stages, when the surface is not sufficiently wet, soil moisture anomalies may also modulate the development of precipitation. To investigate this, sensitivity experiments to initial soil moisture conditions were performed using month-long simulations with the regional mesoscale Eta model. Examination of the control simulations shows that they reproduce all major features and magnitudes of the South American circulation and precipitation patterns, particularly those of the monsoon. The surface sensible and latent heat fluxes, as well as precipitation, have a diurnal cycle whose phase is consistent with previous observational studies. The convective inhibition is smallest at the time of the precipitation maximum, but the convective available potential energy exhibits an unrealistic morning maximum that may result from an early boundary layer mixing. The sensitivity experiments show that precipitation is more responsive to reductions of soil moisture than to increases, suggesting that although the soil is not too wet, it is sufficiently humid to easily reach levels where soil moisture anomalies stop being effective in altering the evapotranspiration and other surface and boundary layer variables. Two mechanisms by which soil moisture has a positive feedback with precipitation are discussed. First, the reduction of initial soil moisture leads to a smaller latent heat flux and a larger sensible heat flux, and both contribute to a larger Bowen ratio. The smaller evapotranspiration and increased sensible heat flux lead to a drier and warmer boundary layer, which in turn reduces the atmospheric instability. Second, the deeper (and drier) boundary layer is related to a stronger and higher South American low-level jet (SALLJ). However, because of the lesser moisture content, the SALLJ carries less moisture to the monsoon region, as evidenced by the reduced moisture fluxes and their convergence. The two mechanisms—reduced convective instability and reduced moisture flux convergence—act concurrently to diminish the core monsoon precipitation.

The Impact of High-Resolution SALLJEX Data on Global NCEP Analyses

Abstract A data assimilation study was performed to assess the impact of observations from the South American Low-Level Jet Experiment (SALLJEX) on analyses in the region east of the Andes Mountains from western Brazil to central Argentina. The Climate Data Assimilation Systems (CDAS)-1 and -2 and the Global Data Assimilation System (GDAS) were run with and without the additional SALLJEX rawinsondes and pilot balloon observations. The experiments for each data assimilation system revealed similar features, with a stronger low-level flow east of the Andes when SALLJEX data were included. GDAS had the strongest low-level jet (LLJ) when compared with observations. In the experiments that used additional rawinsonde and pilot balloon data, the LLJ was displaced westward in comparison to the analyses run without the SALLJEX data. The vertical structure of the meridional wind in the analyses was much closer to observed rawinsonde profiles in the experiments that included SALLJEX data than in the control experiments, and the results show that, although there are more pilot balloon observations than rawinsonde observations in the SALLJEX dataset, most of the improvements in the analyses can be obtained by only including rawinsonde observations. This was especially true for GDAS. The results of this study can serve as a benchmark for similar data impact studies using higher-resolution data assimilation systems.

Dry spots and wet spots in the Andean hotspot

AbstractAim To explain the relationship between topography, prevailing winds and precipitation in order to identify regions with contrasting precipitation regimes and then compare floristic similarity among regions in the context of climate change.Location Eastern slope of the tropical Andes, South America.Methods We used information sources in the public domain to identify the relationship between geology, topography, prevailing wind patterns and precipitation. Areas with contrasting precipitation regimes were identified and compared for their floristic similarity.Results We identify spatially separate super‐humid, humid and relatively dry regions on the eastern slope of the Andes and show how they are formed by the interaction of prevailing winds, diurnally varying atmospheric circulations and the local topography of the Andes. One key aspect related to the formation of these climatically distinct regions is the South American low‐level jet (SALLJ), a relatively steady wind gyre that flows pole‐ward along the eastern slopes of the Andes and is part of the gyre associated with the Atlantic trade winds that cross the Amazon Basin. The strongest winds of the SALLJ occur near the ‘elbow of the Andes’ at 18° S. Super‐humid regions with mean annual precipitation greater than 3500 mm, are associated with a ‘favourable’ combination of topography, wind‐flow orientation and local air circulation that favours ascent at certain hours of the day. Much drier regions, with mean annual precipitation less than 1500 mm, are associated with ‘unfavourable’ topographic orientation with respect to the mean winds and areas of reduced cloudiness produced by local breezes that moderate the cloudiness. We show the distribution of satellite‐estimated frequency of cloudiness and offer hypotheses to explain the occurrence of these patterns and to explain regions of anomalously low precipitation in Bolivia and northern Peru. Floristic analysis shows that overall similarity among all circumscribed regions of this study is low; however, similarity among super‐humid and humid regions is greater when compared with similarity among dry regions. Spatially separate areas with humid and super‐humid precipitation regimes show similarity gradients that are correlated with latitude (proximity) and precipitation.Main conclusions The distribution of precipitation on the eastern slope of the Andes is not simply correlated with latitude, as is often assumed, but is the result of the interplay between wind and topography. Understanding the phenomena responsible for producing the observed precipitation patterns is important for mapping and modelling biodiversity, as well as for interpreting both past and future climate scenarios and the impact of climate change on biodiversity. Super‐humid and dry regions have topographic characteristics that contribute to local climatic stability and may represent ancestral refugia for biodiversity; these regions are a conservation priority due to their unique climatic characteristics and the biodiversity associated with those characteristics.

Mesoscale Convective Systems over Southeastern South America and Their Relationship with the South American Low-Level Jet

Abstract Prior studies have shown that the low-level jet is a recurrent characteristic of the environment during the initiation and mature stages of mesoscale convective systems (MCSs) over the Great Plains of the United States. The South American low-level jet (SALLJ) over southeastern South America (SESA) has an analogous role, advecting heat and moisture from the Amazon basin southward into the central plains of southeastern South America, generating ideal environmental conditions for convection initiation and growth into MCSs. This research has two purposes. One is to describe the characteristics of a 3-yr MCS sample in South America, south of the equator, and its related geographical distribution of convection frequency. The other is to advance the knowledge of the evolution of favorable environmental conditions for the development of large MCSs, specifically those that mature under SALLJ conditions. High horizontal and temporal resolution satellite images are used to detect MCSs in the area for the period 1 September 2000–31 May 2003. Operational 1° horizontal resolution fields from NCEP are used to examine the environment associated with the systems and for the same period. Differences between tropical and subtropical MCSs in terms of size, diurnal cycle, and duration are found. Tropical MCSs are smaller, shorter in duration, and are characterized by a diurnal cycle mainly controlled by diurnal radiative heating. Subtropical MCSs show a preference for a nocturnal phase at maturity over Argentina, which contrasts with a tendency for a daytime peak over Uruguay and southern Brazil. In all seasons, at least one subtropical MCS developed in 41% of the SALLJ days, whereas in the days with no SALLJ conditions this percentage dropped to 12%. This result shows the importance of the synoptic conditions provided by the SALLJ for the development of MCSs and motivates the study of the atmospheric large-scale structure that evolves in close coexistence between SALLJ and subtropical organized convection at the mature stage. The large-scale environment associated with large long-lived MCSs during SALLJ events over SESA evolves under thermodynamic and dynamic forcings that are well captured by the compositing analysis. Essential features are low-level convergence generated by an anomalous all-day-long strong low-level jet prior to the development of the system, overlapped by high-level divergence associated with the anticyclonic flank of the entrance of an upper-level jet streak. This provides the dynamical forcing for convection initiation in an increasingly convectively unstable atmosphere driven by an intense and persistent horizontal advection of heat and moisture at low levels. These processes act during at least one diurnal cycle, enabling gradual building of optimal conditions for the formation of the largest organized convection in the subtropical area. The frequency of convection culminates in a geographically concentrated nocturnal maximum over northeast Argentina on the following day (MCS–SALLJ day). The northeastward displacement and later dissipation of subtropical convection are affected by a northward advance of a baroclinic zone, which is related to horizontal cold advection and divergence of moisture flux at low levels, both contributing to the stabilization of the atmosphere.

Synergism between the Low-Level Jet and Organized Convection at Its Exit Region

Abstract Previous studies suggest that the enhanced meridional extent of some South American low-level jet events (known as Chaco jets) is a consequence of a positive feedback between the low-level wind and strong convection that is usually observed at their exit region. To assess how this interaction takes place, a Chaco low-level jet event observed between 18 and 19 December 2002 (i.e., during the South America Low-Level Jet Experiment) and the associated mesoscale convective system that evolved at its exit region have been selected to perform numerical experiments where diabatic heating effects associated with phase changes can be quantified. This case study has also been used to analyze the diurnal oscillations related to planetary boundary layer (PBL) mechanisms in order to describe whether the observed evolution of the low-level wind can be explained either by PBL-related forcing or by the interaction with convection. The sensitivity experiments confirm that there is a positive feedback at low levels between convection and the northerly wind flow that becomes accelerated and also aids in the identification of a strong coupling between organized convection and the upper-level circulation, resulting in an increase of the upper-level jet strength downstream of the simulated precipitation area. A conceptual model of how these systems (i.e., convection, low- and upper-level jets) mutually interact is proposed, which differs from coupling mechanisms documented for the Great Plains low-level jet.

The Nature of a Heat Wave in Eastern Argentina Occurring during SALLJEX

Abstract This note describes the physical processes associated with the occurrence of a heat wave over central Argentina during the austral summer of 2002/03, during which the South American Low-Level Jet Experiment (SALLJEX) was carried out. The SALLJEX heat wave that lasted between 25 January and 2 February 2003 was punctuated by extreme conditions during its last 3 days, with the highest temperature recorded over the last 35 yr at several stations of the region. It was found that not only the activity of synoptic-scale waves, but also the intraseasonal oscillation variability, had a strong impact on the temperature evolution during this summer. During the weeks previous to the heat wave development, an intensified South Atlantic convergence zone (SACZ) dominated the atmospheric conditions over tropical South America. Temperatures started to increase in the subtropics due to the subsidence and diabatic warming associated with the SACZ, as depicted by SALLJEX upper-air observations. An extratropical anticyclone that evolved along southern South America further intensified subsidence conditions. By the end of January the warming processes associated with SACZ activity weakened, while horizontal temperature advection began to dominate over central Argentina due to the intensification of the South American low-level jet. This mechanism led to temperature extremes by 2 February with temperature anomalies at least two standard deviations larger than the climatological mean values. Intense solar heating favored by strong subsidence was responsible for the heat wave until 31 January, after which horizontal temperature advection was the primary process associated with the temperature peak.

Investigation of seasonal prediction of the South American regional climate using the nested model system

The National Centers for Environmental Prediction (NCEP) regional Eta model was nested in the NCEP Atmosphere General Circulation Model (AGCM) to provide an assessment of the dynamic downscaling method in seasonal climate simulations over South America (SA). This study examined 15 seasons, summer and winter, of significant El Niño–Southern Oscillation years. Results showed that the nested model improved the simulation of seasonal mean lower and upper level circulations in both warm and cold seasons. The summer mean meridional wind along 20°S in the nested model simulation exhibited two jet cores, one located just east of the Andes associated with the South American low level jet (SALLJ) and a second, shallower jet located over eastern Brazil. Conversely, winter cross sections showed only the SALLJ, with higher wind speeds. These features are consistent with observation. AGCM results did not show any noticeable jet structure in either season. The nested model also produced better simulations of seasonal mean rainfall and 2‐m temperature. Summer (winter) precipitation root‐mean‐square error averaged over the entire continent was reduced from 2.09 (2.97) mm d−1 in the AGCM to 1.37 (2.33) mm d−1 in the nested model. The improvement persisted during the entire simulation period over selected areas of the continent. Overall, the Eta model reduced the rainfall positive bias over northern SA and negative bias over the subtropics in the AGCM simulation because of better representation of moisture transport by the SALLJ. However, the simulation of the Intertropical Convergence Zone by the nested model was weak because of weak moisture flux convergence. Its possible causes are discussed in the paper.

Toward a Unified View of the American Monsoon Systems

Abstract An important goal of the Climate Variability and Predictability (CLIVAR) research on the American monsoon systems is to determine the sources and limits of predictability of warm season precipitation, with emphasis on weekly to interannual time scales. This paper reviews recent progress in the understanding of the American monsoon systems and identifies some of the future challenges that remain to improve warm season climate prediction. Much of the recent progress is derived from complementary international programs in North and South America, namely, the North American Monsoon Experiment (NAME) and the Monsoon Experiment South America (MESA), with the following common objectives: 1) to understand the key components of the American monsoon systems and their variability, 2) to determine the role of these systems in the global water cycle, 3) to improve observational datasets, and 4) to improve simulation and monthly-to-seasonal prediction of the monsoons and regional water resources. Among the recent observational advances highlighted in this paper are new insights into moisture transport processes, description of the structure and variability of the South American low-level jet, and resolution of the diurnal cycle of precipitation in the core monsoon regions. NAME and MESA are also driving major efforts in model development and hydrologic applications. Incorporated into the postfield phases of these projects are assessments of atmosphere–land surface interactions and model-based climate predictability experiments. As CLIVAR research on American monsoon systems evolves, a unified view of the climatic processes modulating continental warm season precipitation is beginning to emerge.

Modeled Chaco low-level jets and related precipitation patterns during the 1997–1998 warm season

Chaco jet events (CJEs) are a subset of South American low-level jet events to the east of the Andes, characterized by enhanced poleward penetration and by a strong impact on precipitation over southeastern South America. The present study uses the Eta model short range weather forecasts produced operationally in the Brazilian Center for Weather Forecasts and Climate Studies (Centro de Previsao de Tempo e Estudos Climaticos, CPTEC) to characterize the CJEs and the related precipitation during the 1997–1998 warm season.

Inter-El Niño variability and its impact on the South American low-level jet east of the Andes during austral summer − two case studies

Abstract. The impact of the maximum convection location over eastern and central Equatorial Pacific over the intensity and positioning of the South American Low-Level Jet east of the Andes (SALLJ) during the austral summer was investigated. The Bonner criteria 1 was applied to the NCEP-NCAR circulation fields during the El Niño of 1997/1998 and 2002/2003 to identify the SALLJ episodes. The composites of the atmospheric circulation over the South America during El Niño events showed that the SALLJ can be influenced by small displacements of the quasi-stationary Rossby waves position. During the strong El Niño event of 1997/1998 the SALLJ is maintained by the eastern trade winds. A low-level anomalous anticyclonic circulation over the central part of Brazil enhanced the wind in the nucleus of the jet and displaced its axis to the Northern Argentina and South of Brazil. However, the northern trade winds seem to maintain the SALLJ during the weak El Niño of 2002/2003. The jet was weaker and displaced more southeastward of Brazil than during the strong event.

Impact of New Solar Radiation Parameterization in the Eta Model on the Simulation of Summer Climate over South America

Abstract The regional Eta workstation (WS) model with horizontal resolution of 40 km has been integrated over South America for January 2003. The NCEP–DOE Reanalysis II was used for initial and lateral boundary conditions. The comparison of the model-simulated and satellite-derived values of monthly mean incident solar radiation at the surface demonstrates that the former values are larger by 20%–30% over the entire region. To improve the surface flux representation in the model, a new solar radiation scheme has been implemented in it. An offline comparison of the original and the new radiation schemes with the detailed line-by-line method demonstrates a higher accuracy for the new scheme. With the new scheme, the model-simulated incident solar radiation at the surface is in a better agreement with the satellite-derived data. Nevertheless, a noticeable systematic difference of 10%–20% still remains, probably because of the incorrect description of cloud parameters in the model. The lower incident solar radiation in the new version of the model causes a decrease of near-surface air temperature by 0.1°–1°C and a decrease of precipitation rate by up to 20%–30% over most of the continent. The increase in the simulated incident solar radiation and temperature is found in the region of the South Atlantic convergence zone, which is responsible for the enhanced cloudiness and precipitation in the central and southeastern parts of Brazil during summer. The model results are compared with observational data of meteorological stations, the Global Precipitation Climatology Project (GPCP), and the South American Low-Level Jet Experiment (SALLJEX) and are discussed.

The South American Low-Level Jet Experiment

Abstract Moisture is transported in South America westward from the tropical Atlantic Ocean to the Amazon basin, and then southward toward the extratropics. A regional intensification of this circulation to the east of the Andes Mountains is called the South American low-level jet (SALLJ), with the strongest winds found over eastern Bolivia. SALLJ is present all year and channels moisture to the La Plata basin, which is analogous to the better-known Amazon basin in terms of its biological and habitat diversity, and far exceeds the latter in its economic importance to southern and central South America in terms of hydroelectricity and food production. The relatively small SALLJ spatial scale (compared with the density of the available sounding network) has a limited understanding of and modeling capability for any variations in the SALLJ intensity and structure as well as its possible relationship to downstream rainfall. The SALLJ Experiment (SALLJEX), aimed at describing many aspects of SALLJ, was carried...

Moisture variability over the South American Altiplano during the South American Low Level Jet Experiment (SALLJEX) observing season

During the austral summer, precipitation over the high‐altitude plateau of the South American Altiplano exhibits a marked intraseasonal variability which has been associated with alternating moist and dry conditions observed at surface stations near the Altiplano western cordillera. In this study the characteristics of humid (wet) and dry (dry) episodes observed at a station on the western rim of the Altiplano during the summer of 2002–2003 are examined using observational data from the South American Low Level Jet Experiment (SALLJEX) and satellite‐based estimates of precipitable water (PW) from Global Positioning System, Moderate Resolution Imaging Spectrometer, and Passive Microwave Radiometer instruments. We find that on wet days, mean moist conditions (PW greater than 10 mm) prevail across the entire plateau. This moisture is apparently brought to the Altiplano by sustained westward advection of air at similar altitude above the continental basin and propagates across the Altiplano to moisten the atmosphere over the Pacific Ocean a considerable distance (>1000 km) west of the plateau. In contrast, during dry events, significant drying of the boundary layer is observed mainly along the western mountain range and over the southern Altiplano. There is no evidence of strong midlevel advection of dry air across the northern Altiplano, and moisture remains constant and high over the northern basin and along the Altiplano eastern ridge. Consequently, precipitation and convective cloudiness show the strongest relation to moisture along the western cordillera of the Altiplano. Significant diurnal variability of moisture is found to occur most strongly on the outer slopes of the Altiplano. There is little indication that daytime upslope flow on the eastern slope of the Altiplano plays a major role in the transport of moisture to the Altiplano during wet episodes.

Subseasonal Variations of Rainfall in South America in the Vicinity of the Low-Level Jet East of the Andes and Comparison to Those in the South Atlantic Convergence Zone

Abstract Regional and large-scale circulation anomalies associated with variations in rainfall downstream of the South American low-level jet are identified and compared to those in the South Atlantic convergence zone (SACZ). Composites of precipitation associated with strong jets reveal an approximate doubling of the quantities one would expect from climatology, with an evolution of the rainfall pattern from south to north. The occurrence of extreme precipitation events follows a similar pattern. Meridional cross sections of composite wind reveal a distinct low-level jet near 20°S and a baroclinic development farther south that appears to force the jet. Geopotential height, temperature, and large-scale wind composites suggest that this developing disturbance is tied to a wave train that originates in the midlatitude Pacific and turns equatorward as it crosses the Andes Mountains. Similar composites based on SACZ rainfall reveal similar features, but of opposite sign, suggesting that the phase of the wave...

Climatology of the Low-Level Jet East of the Andes as Derived from the NCEP–NCAR Reanalyses: Characteristics and Temporal Variability

Abstract A climatology of the South American low-level jet east of the Andes (SALLJ) is developed using the 1950– 2000 circulation and moisture fields from the NCEP–NCAR reanalyses and available upper-air observations made in Bolivia and Paraguay since 1998. Upper- and low-level circulation fields were derived for seasonal means and SALLJ composites during the warm and cold seasons. The Bonner criterion 1 was applied for sites in central Bolivia and downstream near northern Paraguay, to determine the spatial and temporal characteristics of the SALLJ. On the circulation characteristics, SALLJ composites during the warm season show the enhanced low-level meridional moisture transport coming from equatorial South America as well as an upper-level wave train emanating from the west Pacific propagating toward South America. The intensification of the warm season SALLJ follows the establishment of an upper-level ridge over southern Brazil and a trough over most of Argentina. The circulation anomalies at upper a...