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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. [ABSTRACT FROM AUTHOR]

Warm and moist air masses, required to generate deep convection east of the Andes Cordillera, are generally the result of humidity transport by the so-called low-level jet (LLJ). In this paper, it is shown from detailed test cases that the eastern part of the continent and the adjacent Atlantic Ocean may constitute another source of moist, warm air, which could be of potential importance even in the presence of a southerly LLJ. The position of the anticyclones crossing South America from the Pacific Ocean to the Atlantic Ocean appears to be a key factor affecting the origin of moisture over the continent. In particular, the LLJ may weaken and even change its direction when the eastern side of the South Pacific anticyclone crosses the mountains; this wind reversal is generally associated with deep convection suppression. Thus, as a South Pacific anticyclone crosses the continent more to the east and its western side reaches the east coast of South America, deep convection can reappear east of the Andes, over the Mendoza region, although the LLJ is frequently suppressed. This is associated with a transport of warm and moist air from Uruguay, southeast Brazil, or even directly from the Atlantic Ocean. [ABSTRACT FROM AUTHOR]

The physical mechanisms and predictability associated with extreme daily rainfall in southeastern South America (SESA) are investigated for the December-February season in a two-part study. Through a k-mean analysis, this first paper identifies a robust set of daily circulation regimes that are used to link the frequency of rainfall extreme events with large-scale potential predictors at subseasonal-to-seasonal scales. This represents a basic set of daily circulation regimes related to the continental and oceanic phases of the South Atlantic convergence zone (SACZ) and wave train patterns superimposed on the Southern Hemisphere polar jet. Some of these recurrent synoptic circulation types are conducive to extreme rainfall events in the region through synoptic control of different mesoscale physical features and, at the same time, are influenced by climate phenomena that could be used as sources of potential predictability. Extremely high rainfall (as measured by the 95th and 99th percentiles) is associated with two of these weather types (WTs), which are characterized by moisture advection intrusions from lower latitudes and the Pacific Ocean; another three WTs, characterized by above-normal moisture advection toward lower latitudes or the Andes, are associated with dry days (days with no rain). The analysis permits the identification of several subseasonal-to-seasonal scale potential predictors that modulate the occurrence of circulation regimes conducive to extreme rainfall events in SESA. It is conjectured that a cross-time scale interaction between the different climate drivers improves the predictive skill of extreme precipitation in the region. [ABSTRACT FROM AUTHOR]

In this paper multiple atmospheric global circulation model (AGCM) integrations at T42 spectral truncation and prescribed sea surface temperature were used to drive regional spectral model (RSM) simulations at 80-km resolution for the austral summer season (January-February-March). Relative to the AGCM, the RSM improves the ensemble mean simulation of precipitation and the lower- and upper-level tropospheric circulation over both tropical and subtropical South America and the neighboring ocean basins. It is also seen that the RSM exacerbates the dry bias over the northern tip of South America and the Nordeste region, and perpetuates the erroneous split intertropical convergence zone (ITCZ) over both the Pacific and Atlantic Ocean basins from the AGCM. The RSM at 80-km horizontal resolution is able to reasonably resolve the Altiplano plateau. This led to an improvement in the mean precipitation over the plateau. The improved resolution orography in the RSM did not substantially change the predictability of the precipitation, surface fluxes, or upper- and lower-level winds in the vicinity of the Andes Mountains from the AGCM. In spite of identical convective and land surface parameterization schemes, the diagnostic quantities, such as precipitation and surface fluxes, show significant differences in the intramodel variability over oceans and certain parts of the Amazon River basin (ARB). However, the prognostic variables of the models exhibit relatively similar model noise structures and magnitude. This suggests that the model physics are in large part responsible for the divergence of the solutions in the two models. However, the surface temperature and fluxes from the land surface scheme of the model [Simplified Simple Biosphere scheme (SSiB)] display comparable intramodel variability, except over certain parts of ARB in the two models. This suggests a certain resilience of predictability in SSiB (over the chosen domain of study) to variations in horizontal... [ABSTRACT FROM AUTHOR]

This paper deals with the relationship between the interannual variability of sea surface temperature (SST) and its associated atmospheric circulation and rainfall variability over southeastern South America (SSA), namely the subtropical region east of the Andes between 20° and 4°S, during the austral spring. Rainfall in SSA and SST interannual variability is studied using canonical correlation analysis. The first two modes show the wellknown warm-wet and cold-dry pattern between the equatorial SST and rainfall over most of this part of the world. However, SST in the equatorial regions does not modulate rainfall variability among El Niño (EN) years or among La Niña (LN) years. On the other hand, it does modulate this variability between EN and LN cases as a whole and among neutral cases indicating that the SSA rainfall response to equatorial Pacific SST is not linear over the observed SST range, having no dependence on the extremes of this range. In contrast, among EN events, SST in the subtropical south-central Pacific (SSCP) modulates the seasonal rainfall over most of SSA. Also, when all the years are considered, this SST has a correlation with precipitation of magnitude similar to those corresponding to the SST in EN regions. Consistent with this, the circulation field has enhanced cyclonic (anticyclonic) advection over subtropical SSA when SST in the SSCP is cold (warm). SSTs in EN-3 or EN-3.4 regions and in the SSCP are negatively correlated, but their correlation is practically zero when only EN cases or only neutral cases are considered, and very small in LN cases. This allows a stratification analysis composing cases according to different SSTs in EN-3 and EN-3.4 regions with almost constant SST in the SSCP and similarly according to different SST in the SSCP with approximately constant SST in EN regions. The composite difference fields with constant equatorial SST and with constant SST in the SSCP have a wave train in the Pacific... [ABSTRACT FROM AUTHOR]

Current climate models project that Antarctic sea ice will decrease by the end of the twenty-first century. Previous studies have suggested that Antarctic sea ice changes have impacts on atmospheric circulation and the mean state of the Southern Hemisphere. However, little is known about whether Antarctic sea ice loss may have a tangible impact on climate extremes over the southern continents and whether ocean–atmosphere coupling plays an important role in changes of climate extremes over the southern continents. In this study, we conduct a set of fully coupled and atmosphere-only model experiments forced by present and future Antarctic sea ice cover. It is found that the projected Antarctic sea ice loss by the end of the twenty-first century leads to an increase in the frequency and duration of warm extremes (especially warm nights) over the southern continents and a decrease in cold extremes over most regions. The frequency and duration of wet extremes are projected to increase over South America and Antarctica, whereas changes in dry days and the longest dry spell vary with regions. Further Antarctic sea ice loss under a quadrupling of CO2 leads to similar but larger changes. Comparison between the coupled and atmosphere-only model experiments suggests that ocean dynamics and their interactions with the atmosphere induced by Antarctic sea ice loss play a key role in driving the identified changes in temperature and precipitation extremes over southern continents. By comparing with global warming experiments, we find that Antarctic sea ice loss may affect temperature and precipitation extremes for some regions under greenhouse warming, especially Antarctica. [ABSTRACT FROM AUTHOR]

Transient incursions of midlatitude air to the east of the Andes Mountains into subtropical and tropical latitudes are a distinctive feature of the synoptic climatology over South America. The mean synoptic-scale structure of these incursions is documented in this paper on the basis of a compositing analysis of NCEP--NCAR reanalyzed meteorological fields and satellite-based outgoing longwave radiation measurements. Although these incursions are a year-round phenomenon, with relatively modest seasonal changes in their structure, this analysis is focused on the austral summer, when they have their largest impact in the precipitation field. The summertime incursions move equatorward at a mean speed of 10 m s 21 and retain their identities over intervals of about 5 days. The upper-level circulation is characterized by a midlatitude trough--ridge couplet that provides the quasigeostrophic forcing of the system. Before the onset of the incursions of midlatitude air, the approaching upper-level trough tends to deepen the semipermanent area of low pressure over the central plains of the continent (25°--10°S). The strong southward advection of warm, humid air creates favorable conditions for the development of deep convection over central and southern Argentina. During the next two days, a migratory cold anticyclone, steered by the ridge aloft, moves onto the southern plains of South America (40°-- 35°S), establishing a large-scale, southward-directed pressure gradient. Due to the blocking effect of the Andes, the induced low-level circulation assumes the form of an agesotrophic southerly flow that produces the equa-torward advection of the cold air. In this stage, enhanced convection occurs in a well-defined band of the intensified low-level convergence at the leading edge of the cold surge. This synoptic-scale banded structure in the convective cloudiness also emerges as the dominant mode of the day-to-day variability of the deep... [ABSTRACT FROM AUTHOR]

Seasonal climate forecasts have socioeconomic value, and the quality of the forecasts is important to various societal applications. Here we evaluate seasonal forecasts of three climate variables, vapor pressure deficit (VPD), temperature, and precipitation, from operational dynamical models over the major cropland areas of South America; analyze their predictability from global and local circulation patterns, such as El Niño–Southern Oscillation (ENSO); and attribute the source of prediction errors. We show that the European Centre for Medium-Range Weather Forecasts (ECMWF) model has the highest quality among the models evaluated. Forecasts of VPD and temperature have better agreement with observations (average Pearson correlation of 0.65 and 0.70, respectively, among all months for 1-month-lead predictions from the ECMWF) than those of precipitation (0.40). Forecasts degrade with increasing lead times, and the degradation is due to the following reasons: 1) the failure of capturing local circulation patterns and capturing the linkages between the patterns and local climate; and 2) the overestimation of ENSO’s influence on regions not affected by ENSO. For regions affected by ENSO, forecasts of the three climate variables as well as their extremes are well predicted up to 6 months ahead, providing valuable lead time for risk preparedness and management. The results provide useful information for further development of dynamical models and for those who use seasonal climate forecasts for planning and management. SIGNIFICANCE STATEMENT: Seasonal climate forecasts have socioeconomic value, and the quality of the forecasts is important to their applications. This study evaluated the quality of monthly forecasts of three important climate variables that are critical to agricultural management, risk assessment, and natural hazards warning. The findings provide useful information for those who use seasonal climate forecasts for planning and management. This study also analyzed the predictability of the climate variables and the attribution of prediction errors and thus provides insights for understanding models’ varying performance and for future improvement of seasonal climate forecasts from dynamical models. [ABSTRACT FROM AUTHOR]

El Niño–Southern Oscillation (ENSO) has a huge influence on Antarctic climate variability via Rossby wave trains. In this study, the asymmetry of the ENSO teleconnection in the Southern Hemisphere, together with the mechanisms involved, is systematically investigated. In four reanalysis datasets, the composite atmospheric circulation anomaly in austral winter over the Amundsen Sea during La Niña is situated more to the west than during El Niño. This asymmetric feature is reproduced by ECHAM5.3.2 forced with both composite and idealized symmetric sea surface temperature anomalies. Utilizing a linear baroclinic model, we find that ENSO-triggered circulation anomalies in the subtropics can readily extract kinetic energy from the climatological mean flow and develop efficiently at the exit of the subtropical jet stream (STJ). The discrepancy in the location of the STJ between El Niño and La Niña causes asymmetric circulation responses by affecting the energy conversion. During El Niño years, anomalous tropical convective precipitation increases the meridional temperature gradient, which in turn leads to the strengthening of the STJ and the eastward movement of the jet core and jet exit in the Pacific. With the movement of the STJ exit, the wave train tends to develop over the eastern region. The opposite is the case during La Niña when the westward shift of the jet exit favors the development of the wave train in the western region. Our findings expand the current understanding regarding ENSO teleconnection. [ABSTRACT FROM AUTHOR]

Future projections of precipitation change over tropical land are often enhanced by vegetation responses to CO2 forcing in Earth system models. Projected decreases in rainfall over the Amazon basin and increases over the Maritime Continent are both stronger when plant physiological changes are modeled than if these changes are neglected, but the reasons for this amplification remain unclear. The responses of vegetation to increasing CO2 levels are complex and uncertain, including possible decreases in stomatal conductance and increases in leaf area index due to CO2 fertilization. Our results from an idealized atmospheric general circulation model show that the amplification of rainfall changes occurs even when we use a simplified vegetation parameterization based solely on CO2-driven decreases in stomatal conductance, indicating that this mechanism plays a key role in complex model projections. Based on simulations with rectangular continents we find that reducing terrestrial evaporation to zero with increasing CO2 notably leads to enhanced rainfall over a narrow island. Strong heating and ascent over the island trigger moisture advection from the surrounding ocean. In contrast, over larger continents rainfall depends on continental evaporation. Simulations with two rectangular continents representing South America and Africa reveal that the stronger decrease in rainfall over the Amazon basin seen in Earth system models is due to a combination of local and remote effects, which are fundamentally connected to South America's size and its location with respect to Africa. The response of tropical rainfall to changes in evapotranspiration is thus connected to size and configuration of the continents. [ABSTRACT FROM AUTHOR]

Rainfall over tropical South America is known to be modulated by convectively coupled Kelvin waves (CCKWs). In this work, the origin and dynamical features of South American Kelvin waves are revisited using satellite-observed brightness temperature, radiosonde, and reanalysis datasets. Two main types of CCKWs over the Amazon are considered: Kelvin waves with a Pacific precursor, and Kelvin waves with a precursor originating over South America. Amazonian CCKWs associated with a preexisting Kelvin convection in the eastern Pacific account for about 35% of the total events. The cases with South American precursors are associated with either pressure surges in the western Amazon from extratropical wave train activity, responsible for 40% of the total events, or "in situ" convection that locally excites CCKWs, accounting for the remaining 25%. The analysis also suggests that CCKWs associated with different precursors are sensitive to Pacific sea surface temperature. Kelvin wave events with a Pacific precursor are more common during ENSO warm events, while Kelvin waves with extratropical South American precursors show stronger activity during La Niña events. This study also explores other triggering mechanisms of CCKWs over the Amazon. These mechanisms are associated with 1) extratropical Rossby wave trains not necessarily of extratropical South American origin; 2) CCKWs initiated in response to the presence of the southern and/or double intertropical convergence zone (ITCZ) in the eastern Pacific Ocean; and 3) possible interaction between CCKWs and other equatorial waves in the Amazon. [ABSTRACT FROM AUTHOR]

La Plata basin is one of the most important agricultural and hydropower-producing regions in the world. Extreme climate events such as cold and heat waves and frost events have a significant socioeconomic impact. This work analyzes the influence of the surface circulation in southern South America on daily maximum temperature TMAX and daily minimum temperature TMIN in southern La Plata basin. A Z test for the comparison of mean values and a Kolmogorov-Smirnov test for the comparison of distributions of TMAX and TMIN associated with each circulation pattern were performed. Specific daily surface circulation types are found to contribute to TMAX and TMIN anomalies and to have a predominant occurrence in the development of the extreme temperature events in the region. The TMAX spatial response to the regional low-level circulation is more homogenous and extended than is the response of TMIN. [ABSTRACT FROM AUTHOR]

The South American monsoon system (SAMS) is the most important climatic feature in South America and is characterized by pronounced seasonality in precipitation during the austral summer. This study compares several statistical properties of daily gridded precipitation from different data (1998-2008): 1) Physical Sciences Division (PSD), Earth System Research Laboratory [1.0° and 2.5° latitude (lat)/longitude (lon)]; 2) Global Precipitation Climatology Project (GPCP; 1° lat/lon); 3) Climate Prediction Center (CPC) unified gauge (CPC-uni) (0.5° lat/lon); 4) NCEP Climate Forecast System Reanalysis (CFSR) (0.5° lat/lon); 5) NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis (0.5° lat/0.3° lon); and 6) Tropical Rainfall Measuring Mission (TRMM) 3B42 V6 data (0.25° lat/lon). The same statistical analyses are applied to data in 1) a common 2.5° lat/lon grid and 2) in the original resolutions of the datasets. All datasets consistently represent the large-scale patterns of the SAMS. The onset, demise, and duration of SAMS are consistent among PSD, GPCP, CPC-uni, and TRMM datasets, whereas CFSR and MERRA seem to have problems in capturing the correct timing of SAMS. Spectral analyses show that intraseasonal variance is somewhat similar in the six datasets. Moreover, differences in spatial patterns of mean precipitation are small among PSD, GPCP, CPC-uni, and TRMM data, while some discrepancies are found in CFSR and MERRA relative to the other datasets. Fitting of gamma frequency distributions to daily precipitation shows differences in the parameters that characterize the shape, scale, and tails of the frequency distributions. This suggests that significant uncertainties exist in the characterization of extreme precipitation, an issue that is highly important in the context of climate variability and change in South America. [ABSTRACT FROM AUTHOR]

The existence of a significant simultaneous correlation between bimonthly mean precipitation anomalies over southeastern South America (SESA) and either the first or the second (depending on season) leading mode of interannual variability of upper-level wind over South America (SA) is demonstrated during all seasons except winter. The pattern associated with these modes of variability is similar during all seasons and consists of a continental-scale vortex centered over the eastern coast of subtropical SA. The vortex has a quasi-barotropic structure during all seasons, and its variability modifies moisture transport from the South American low-level jet and the western tropical Atlantic to SESA thus creating precipitation anomalies in this region. During spring (October–November) and summer (January–February) the circulation creates a second center of precipitation anomalies over the South Atlantic convergence zone that are of opposite sign to those over SESA, while during fall (April–May) precipitation anomalies are primarily confined to SESA. On the basis of the correlation between upper-level winds and precipitation, an empirical method to produce long-range forecasts of bimonthly mean precipitation over SESA is developed. Method tests in hindcast mode for the period 1959–2001 show a potential for reliable predictions during the southern spring, summer, and fall. The method is further tested in an experimental mode by using Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) wind hindcasts. Forecasts obtained in this way are skillful during spring only, with highest skill during El Niño–Southern Oscillation years. During summer and fall, the DEMETER forecasts of wind anomalies limit the method’s ability to make reliable real predictions. [ABSTRACT FROM AUTHOR]

The mechanisms associated with the intraseasonal variability of precipitation over South America during the spring season are investigated with emphasis on the influence of a quasi-stationary anomalous circulation over the southeastern South Pacific Ocean (SEP). A spectral analysis performed to the bandpass-filtered time series of daily precipitation anomalies for the La Plata Basin (LPB) and the South Atlantic convergence zone (SACZ) regions revealed several statistically relevant peaks corresponding to periods of roughly 23 days and 14–16 days—with the lower (higher) frequency peaks more prevalent for the SACZ (LPB). The large-scale circulation patterns preconditioning precipitation variability over both regions were explored by means of a regression analysis performed on the daily 500-hPa geopotential anomaly field provided by the NCEP–NCAR reanalysis dataset. The most prominent feature of the regression fields is the presence of a quasi-stationary anomalous anticyclonic (cyclonic) circulation over the southeastern South Pacific Ocean associated with positive rainfall anomalies over the LPB (SACZ) and, emanating from that high (low), an external Rossby wave propagating northeastward toward the South American continent. The synoptic-scale activity, quantified in terms of a frontal activity index, showed a strong influence on precipitation over the LPB and to a lesser extent over the SACZ. Moreover, the frontal activity is actually modulated by the anomalous high circulation over the SEP region. The behavior of this anomalous circulation may be supported by a positive feedback mechanism that can enhance the response of the high anomaly itself, which in turns reinforces the Rossby wave train propagating toward the South American continent. [ABSTRACT FROM AUTHOR]

The energetics and behavior of midtropospheric troughs over the Southern Hemisphere and their relationship with South America surface cyclogenesis were studied during the winters of 1999–2003. All surface cyclogenesis situations over Uruguay and adjacent areas associated with 500-hPa troughs were analyzed. The atmospheric circulation associated with type-B and type-C cyclones form the basis for two composites: composite B (with 25 cases) and composite C (with 13 cases). The results showed that the midtropospheric troughs were more intense in composite C than in composite B before the surface cyclogenesis and that the opposite occurred during the surface cyclogenesis. The baroclinic conversion was dominant in both composites. In composite B, the ageostrophic flux convergence (AFC) contributed positively to the intensification of the surface cyclone since it imported energy into the area before the cyclogenesis started. But in composite C, the AFC served as a sink because it exported energy. Based on these results, it can be concluded that (i) the trough was crucial for the cyclogenesis; (ii) the variables in the mid- and upper levels did not differ significantly from one composite to another; (iii) the northerly heat and moisture flow acted as a preconditioning for the cyclogenesis, mainly for composite C; (iv) the baroclinic conversion dominated the energetics; and (v) the AFC had only a secondary role, contributing negatively to the development of the cyclone in composite C and positively to the initial development in composite B. [ABSTRACT FROM AUTHOR]

The variation of air temperature at 2 m above the earth’s surface in South America (SA) between 1948 and 2007 is investigated primarily using the NCEP–NCAR reanalysis. In December–February (austral summer), the majority of SA has a mean temperature between 21° and 24°C during 1948–75, and for 1976–2007 the mean temperature is above 24°C. In June–August (austral winter), warmer temperatures are observed in the tropical region in the recent period. The results indicate that Northeast Brazil (NEB) and central Brazil are warmer in the more recent period. In the last seven years (2001–07) compared to the earlier periods, greater warming is noted in the tropical SA region, mainly in NEB and over the North Atlantic Ocean, and cooling is observed in part of the subtropical SA region. Supporting evidence for the warming in Brazil is given through analyses of station data and observational data. The results presented here indicate that the climate change over SA is likely not predominantly a result of variations in El Niño–Southern Oscillation (the most important coupled ocean–atmosphere phenomenon to produce climate variability over SA). Instead, the climate changes likely occur as a response to other natural variability of the climate and/or may be a result of human activity. However, even without ascertaining the specific causes, the most important finding in this work is to demonstrate that a change in the temperature patterns of SA occurred between 1948 and 2007. [ABSTRACT FROM AUTHOR]

The temporal stability of the southern annular mode (SAM) impacts on Southern Hemisphere climate during austral spring is analyzed. Results show changes in the typical hemispheric circulation pattern associated with SAM, particularly over South America and Australia, between the 1960s–70s and 1980s–90s. In the first decades, the SAM positive phase is associated with an anomalous anticyclonic circulation developed in the southwestern subtropical Atlantic that enhances moisture advection and promotes precipitation increase over southeastern South America (SESA). On the other hand, during the last decades the anticyclonic anomaly induced by the SAM’s positive phase covers most of southern South America and the adjacent Atlantic, producing weakened moisture convergence and decreased precipitation over SESA as well as positive temperature anomaly advection over southern South America. Some stations in the Australia–New Zealand sector and Africa exhibit significant correlations between the SAM and precipitation anomalies in both or one of the subperiods, but they do not characterize a consistent area in which the SAM signal can be certainly determined. Significant changes of SAM influence on temperature anomalies on multidecadal time scales are observed elsewhere. Particularly over the Australia–New Zealand sector, significant positive correlations during the first decades become insignificant or even negative in the later period, whereas changes of opposite sign occur in the Antarctic Peninsula between both subperiods. [ABSTRACT FROM AUTHOR]

A synthesis of several approaches to quantifying land–atmosphere interactions is presented. These approaches use data from observations or atmospheric reanalyses applied to atmospheric tracer models and stand-alone land surface schemes. None of these approaches relies on the results of general circulation model simulations. A high degree of correlation is found among these independent approaches, and constructed here is a composite assessment of global land–atmosphere feedback strength as a function of season. The composite combines the characteristics of persistence of soil moisture anomalies, strong soil moisture regulation of evaporation rates, and reinforcement of water cycle anomalies through recycling. The regions and seasons that have a strong composite signal predominate in both summer and winter monsoon regions in the period after the rainy season wanes. However, there are exceptions to this pattern, most notably over the Great Plains of North America and the Pampas/Pantanal of South America, where there are signs of land–atmosphere feedback throughout most of the year. Soil moisture memory in many of these regions is long enough to suggest that real-time monitoring and accurate initialization of the land surface in forecast models could lead to improvements in medium-range weather to subseasonal climate forecasts. [ABSTRACT FROM AUTHOR]

Rainfall in South America as simulated by a 24-ensemble member of the ECHAM 4.5 atmospheric general circulation model is compared and contrasted with observations (in areas in which data are available) for the period 1976–2001. Emphasis is placed on determining the onset and end of the rainy season, from which its length and rain rate are determined. It is shown that over large parts of the domain the onset and ending dates are well simulated by the model, with biases of less than 10 days. There is a tendency for model onset to occur early and ending to occur late, resulting in a simulated rainy season that is on average too long in many areas. The model wet season rain rate also tends to be larger than observed. To estimate the relative importance of errors in wet season length and rain rate in determining biases in the annual total, adjusted totals are computed by substituting both the observed climatological wet season length and rate for those of the model. Problems in the rain rate generally are more important than problems in the length. The wet season length and rain rate also contribute substantially to interannual variations in the annual total. These quantities are almost independent, and it is argued that they are each associated with different mechanisms. The observed onset dates almost always lie within the range of onset of the ensemble members, even in the areas with a large model onset bias. In some areas, though, the model does not perform well. In southern Brazil the model ensemble average onset always occurs in summer, whereas the observations show that winter is often the wettest period. Individual members, however, do occasionally show a winter rainfall peak. In southern Northeast Brazil the model has a more distinct rainy season than is observed. In the northwest Amazon the model annual cycle is shifted relative to that observed, resulting in a model bias. No interannual relationship between model and observed onset dates is expected unless onset in the model and observations has a mutual relationship with SST anomalies. In part of the near-equatorial Amazon, there does exist an interannual relationship between onset dates. Previous studies have shown that in this area there is a relationship between SST anomalies and variations in seasonal total rainfall. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A methodology is proposed in which a few prognostic variables of a regional climate model (RCM) are strongly constrained at certain wavelengths to what is prescribed from the bias-corrected atmospheric general circulation model (AGCM; driver model) integrations. The goal of this strategy is to reduce the systematic errors in a RCM that mainly arise from two sources: the lateral boundary conditions and the RCM errors. Bias correction (which essentially corrects the climatology) of the forcing from the driving model addresses the former source while constraining the solution of the RCM beyond certain relatively large wavelengths in the regional domain [also termed as scale-selective bias correction (SSBC)] addresses the latter source of systematic errors in RCM. This methodology is applied to experiments over the South American monsoon region. It is found that the combination of bias correction and SSBC on the nested variables of divergence, vorticity, and the log of surface pressure of an RCM yields a major improvement in the simulation of the regional climate variability over South America from interannual to intraseasonal time scales. The basis for such a strategy is derived from a systematic empirical approach that involved over 100 regional seasonal climate integrations. [ABSTRACT FROM AUTHOR]

The impact of South American orography on subtropical stratocumulus clouds off the Peruvian coast is investigated in the context of an atmospheric general circulation model. It is found that stratocumulus incidence is significantly reduced when South American orography is removed. Key to this behavior is a decrease in lower tropospheric stability (LTS) that allows for more frequent stratocumulus destruction through the model’s cloud-top entrainment instability mechanism. The role of orography in enhancing Peruvian stratocumulus is as follows. Within the PBL, orography deflects the midlatitude westerly winds equatorward in association with cold air advection and blocking of the low-level flow from the continent. Above the PBL, the steep and high South American orography deflects a significant portion of the midlatitude westerlies equatorward. This flow sinks along the equatorward sloping isentropes, thus promoting subsidence. Both processes increase LTS over the stratocumulus region. In further AGCM experiments, the sensitivity of Peruvian stratocumulus to the use of unsmoothed orographic boundary conditions is assessed. The results show no significant differences to the control simulation, which uses smoothed orography. This suggests that, in the context of GCMs, a representation of South American orography more detailed than is generally used has little potential for improving the performance of coupled ocean–atmosphere models in the eastern tropical Pacific. [ABSTRACT FROM AUTHOR]

A regional climate model driven by reanalyzed atmospheric forcing is used to investigate 1) the large-scale circulation anomalies that were driven by sea surface temperatures (SSTs), which resulted in extreme rainfall anomalies during January-May 1983 (dry) and 1985 (wet) in tropical South America; 2) the effects of vegetation and soil moisture in the interior Amazon basin on regional circulations, moisture transport, and rainfall; and 3) the sensitivity of regional model results to domain size. Seasonal integrations demonstrated that by prescribing observed SSTs and applying reanalyses-derived forcing along the boundaries of the control domain, the regional climate model (RegCM) was able to simulate the dramatically different large-scale circulations in the two years, as well as the resulting rainfall differences. Thus, the large-scale forcing apparently has a first-order effect on the region. The regional model shows reduced rainfall in the western Amazon compared with observed estimates that are associated with weak low-level moisture transport from the Atlantic. The sensitivity experiments to surface forcing in the Amazon, employing a large (10.8 × 10[SUP7]km[SUP2]) and a small (5.7 × 10[SUP7]km[SUP2]) domain, show that both simulation and sensitivity are a function of domain size in the Tropics. However, the spatial scales and hence the domains required are larger in the Tropics than in the midlatitudes. The perturbations employed in this study influence the large-scale tropical circulation. This feedback is damped by the lateral boundary conditions of the control (smaller) domain. [ABSTRACT FROM AUTHOR]

A regional climate model driven at the lateral boundaries by ensemble integrations of a general circulation model (GCM) is used to investigate 1) the large-scale circulation anomalies associated with tropical sea surface temperatures (SSTs) that lead to extreme rainfall anomalies during January-May of 1983 (dry) and 1985 (wet) in tropical South America, and 2) the sensitivity of the nested model results to the choice of domain. The nested model is composed of the Regional Climate Model (RegCM) and the Community Climate Model version 3 (CCM3), both developed at the National Center for Atmospheric Research. In Part I of this study, the RegCM's ability to simulate circulation and rainfall observed in the two extreme seasons was demonstrated when driven at the lateral boundaries by reanalyzed forcing. Seasonal integrations with the RegCM driven by GCM ensemble-derived lateral boundary forcing demonstrate that the nested model responds well to the SST forcing, by capturing the major features of the circulation and rainfall differences between the two years. The GCM-driven model also improves upon the monthly evolution of rainfall compared with that from the GCM. However, the nested model rainfall simulations for the two seasons are degraded compared with those from the reanalyses-driven RegCM integrations. The poor location of the Atlantic inter-tropical convergence zone (ITCZ) in the GCM leads to excess rainfall in Nordeste in the nested model. An expanded domain was tested, wherein the RegCM was permitted more internal freedom to respond to SST and regional orographic forcing. Results show that the RegCM is able to improve the location of the ITCZ, and the seasonal evolution of rainfall in Nordeste, the Amazon region, and the southeastern region of Brazil. However, it remains that the limiting factor in the skill of the nested modeling system is the quality of the lateral boundary forcing provided by the global model. [ABSTRACT FROM AUTHOR]

A comparison of the submonthly variability of atmospheric circulation and organization of convection in South America during January-February-March of 1998 (JFM98) and January-February-March of 1999 (JFM99) is presented. According to the National Centers for Environmental Prediction reanalysis, the South American low-level jet (SALLJ) was about twice as strong during JFM of the 1998 E1 Niño episode than during JFM of the 1999 La Niña episode. The difference in SALLJ strength between these two years translated into stronger transport of moist tropical air into the subtropics during JFM98 than during JFM99. An objective tracking technique was used to identify large, long-lived convective cloud systems in infrared imagery. The stronger SALLJ was accompanied by larger and more numerous long-lived convective cloud systems and nearly twice as much rainfall in subtropical South America (parts of southern Brazil, Uruguay, and Argentina) during JFM98 than during JFM99. The difference between JFM98 and JFM99 SALLJ strength in Bolivia is in part explained by submonthly variability associated with the South Atlantic convergence zone (SACZ). Periods when the SACZ is present are marked by southerly or weak northerly winds in Bolivia. The South Atlantic convergence zone was more prominent during JFM99 than during JFM98 contributing to a weaker SALLJ during JFM99. Large, long-lived convective cloud systems in subtropical South America tended to occur during times when the SACZ was absent and the SALLJ was strong over Bolivia. Interannual variability associated with the E1 Niño-Southern Oscillation also contributed to the observed interannual variability of the SALLJ in Bolivia. In the tropical portions of South America nearly 6 times more large, long-lived convective cloud systems were observed during JFM99 than during JFM98. This was accompanied by more plentiful precipitation in portions of the Amazon basin and in the Bolivian Altiplano during... [ABSTRACT FROM AUTHOR]

The El Niño-Southern Oscillation (ENSO) has an established impact on precipitation in Uruguay during austral spring (October-December). This impact is absent in peak summer (January-February), and returns weakly in fallwinter (March-July). Interannual and intraseasonal variability of the atmospheric circulation over South America and the South Pacific is investigated using the NCEP-NCAR reanalysis data for these seasons. The leading empirical orthogonal function (EOF) of seasonally averaged 200-hPa winds over South America is found to be associated with ENSO through a pronounced Walker cell component in all seasons. However, during spring, this pattern acquires an extratropical teleconnection that links the circulation over southeastern South America (SESA) with ENSO. This extratropical teleconnection disappears in summer, when the circulation over SESA is dominated by variability in the South Atlantic convergence zone. In fall, extratropical South America again becomes affected by a wavelike pattern that extends over the South Pacific, but it is uncorrelated with ENSO. On intraseasonal timescales, a cluster analysis of daily geopotential height fields over the South Pacific sector reveals three wave train-like circulation regimes with similar structures in all seasons. During the transition seasons (but not summer), the frequencies of occurrence of two of these regimes are found to be significantly different from normal in years when the interannual wavelike EOFs are pronounced. Interannual anomaly patterns are constructed from the intraseasonal regimes according to these changes in their frequency of occurrence, and shown to resemble quite closely the interannual EOFs over the South Pacific sector. These results provide evidence that the interannual teleconnection patterns seen over the South Pacific in austral spring and fall-winter are due to changes in the frequency of occurrence and amplitude of intraseasonal circulation regimes. The Rossby wave... [ABSTRACT FROM AUTHOR]