René D. Garreaud

Toward a Unified View of the American Monsoon Systems

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.

The climate of the Altiplano: observed current conditions and mechanisms of past changes

Abstract The large-scale controls on the climate of the South American Altiplano are investigated using local observations, reanalysis data and general circulation model experiments. The objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial–interglacial. Our results suggest that, on all timescales, the climatic conditions on the Altiplano are closely related to the upper-air circulation, with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions. Different factors influence the upper-air circulation on the different timescales. Intraseasonal variability is a reflection of the position and intensity of the Bolivian High, which is modulated by Rossby waves emanating from the midlatitude South Pacific. The annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere, rather than direct insolation forcing over the Altiplano or moisture changes in the source area. Interannual variability is primarily related to changes in the mean zonal flow over the Altiplano, reflecting changes in meridional baroclinicity between tropical and subtropical latitudes, which in turn is a response to sea-surface temperature changes in the tropical Pacific. Orbitally forced changes in the land–sea contrast drive continental-scale circulation changes, which significantly alter the zonal flow over the Altiplano. On glacial–interglacial timescales, the contrast in heating between northern and southern hemispheres during the glacial leads to upper-air easterly anomalies throughout the tropics. On modern timescales the marked submonthly, seasonal and interannual changes of moisture over the Altiplano cannot be accounted for by moisture changes in the humid tropical lowlands. However, the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands, which counteracts the effect of the upper-level circulation, resulting in little overall change in precipitation. This observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the Altiplano on a wide range of timescales.

Interannual (ENSO) and Interdecadal (ENSO-like) Variability in the Southern Hemisphere Tropospheric Circulation*

Abstract Recent work has identified variability in the Pacific Ocean SST with a structure qualitatively similar to ENSO, but at lower frequencies than ENSO. Zhang et al. have documented the atmospheric circulation anomalies in the Tropics and Northern Hemisphere that are associated with decadal ENSO-like variability and compared these anomalies to those associated with the (interannual) ENSO cycle. Here the authors extend the study of Zhang et al. to the Southern Hemisphere using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data for 1958–96. Consistent with previous studies, the Southern Hemisphere circulation anomalies associated with ENSO display a teleconnection pattern from the central tropical Pacific into the far southeastern Pacific Ocean. Comparatively larger circulation anomalies are found in the Southern Hemisphere associated with the decadal ENSO-like variability, though aloft the structure of the anomalies emphasizes the polar vortex wit...

Interannual rainfall variability over the South American Altiplano

Summertime (December‐February) precipitation is virtually the only water resource over the South American Altiplano, a semiarid, high-level plateau entrenched in the central Andes. On the interannual timescale, Altiplano rainfall exhibits pronounced fluctuations between drought and very wet conditions, with subsequent impacts on agriculture and hydrology. In this work, the large-scale patterns of convective cloudiness and circulation associated with interannual variability of the summer rainfall over this region are investigated using a regression analysis between relevant atmospheric fields (NCEP‐NCAR reanalysis, outgoing longwave radiation) and an index of convection over the Altiplano. It is found that the seasonal-mean, large-scale zonal flow over the central Andes is directly related with the number of days with easterly flow within the season, that, in turn, favor the occurrence of summertime deep convection on the Altiplano by transporting moist air from the interior of the continent. Consequently, interannual variability of the seasonal-mean zonal wind explains nearly half of the variance of summertime convection over the Altiplano through an easterly/wet‐westerly/dry pattern. The circulation anomalies are in geostrophic balance with changes in the meridional baroclinicity at the southern border of the tropical belt. Thus, a previously

Cold Air Incursions over Subtropical South America: Mean Structure and Dynamics

Abstract Synoptic-scale incursions of midlatitude air moving into subtropical South America (to the east of the Andes Cordillera) are observed to occur year-round with a periodicity of about 1–2 weeks. During wintertime, they have a profound impact upon the low-level temperature field, and extreme episodes produce freezing conditions from central Argentina to southern Brazil and Bolivia. Warm season episodes produce less dramatic variations of temperature, but they organize deep convection in the form of synoptic-scale bands of convective cloudiness along the leading edge of the cool air. On the basis of 17 yr of NCEP–NCAR reanalysis and outgoing longwave radiation fields, the mean, synoptic-scale structure, and evolution of these incursions is documented, using a simple compositing technique. The underlying physical mechanisms responsible for the occurrence of these incursions are also investigated by diagnosing the leading dynamic and thermodynamic forcing of their development.

The Diurnal March of Convective Cloudiness over the Americas

Abstract Based on nine years (1983–91) of infrared data from geostationary satellites (the B3 ISCCP product), several features of the diurnal march of the frequency of convective cloudiness over the tropical and subtropical Americas are documented with 3-h temporal resolution and 0.5° × 0.5° latitude–longitude spatial resolution. The frequency of convective cloudiness in each grid box is defined in terms of the fraction of temporal samples that exhibit cloud-top temperatures colder than 235 K. The effect of varying the threshold is explored and selected results are compared with rainfall estimates based on microwave (SSM/I) imagery. Convective cloudiness over most land areas exhibits a coherent diurnal march with relatively clear mornings, a rapid afternoon buildup, and a more gradual nighttime decay. The highest, coldest convective clouds peak a few hours earlier than those with lower tops. Morning to noontime maxima tend to be prevalent over offshore waters that experience significant convection such as...

Large-Scale Control on the Patagonian Climate

AbstractPatagonia, located in southern South America, is a vast and remote region holding a rich variety of past environmental records but a small number of meteorological stations. Precipitation over this region is mostly produced by disturbances embedded in the westerly flow and is strongly modified by the austral Andes. Uplift on the windward side leads to hyperhumid conditions along the Pacific coast and the western slope of the Andes; in contrast, downslope subsidence dries the eastern plains leading to arid, highly evaporative conditions.The authors investigate the dependence of Patagonia’s local climate (precipitation and near-surface air temperature) year-to-year variability on large-scale circulation anomalies using results from a 30-yr-long high-resolution numerical simulation. Variations of the low-level zonal wind account for a large fraction of the rainfall variability at synoptic and interannual time scales. Zonal wind also controls the amplitude of the air temperature annual cycle by changi...

Multiscale Analysis of the Summertime Precipitation over the Central Andes

Precipitation over the central Andes in South America exhibits a marked annual march, with most of the rainfall concentrated during the austral summer season (December‐February), when the atmospheric circulation favors the uplifting of moist air from the lowlands to the east of the mountain range. Within its rainy season, the central Andes experiences week-long rainy and dry episodes. The large-scale and local conditions during these episodes are investigated using satellite imagery, reanalyzed atmospheric fields, and in situ data. Despite the deep layer of conditional instability prevalent during most summertime afternoons, deep convection can occur only on those days in which the mixing ratio within the local boundary layer exceeds some threshold (; 7gk g 21), yielding saturation of near-surface air parcels rising more than 600 m above ground. Convective cloudiness anomalies over the central Andes extend southeastward and tend to be concurrent with anomalies of opposite sign over the eastern part of the continent. Rainy (dry) episodes are also associated with anticyclonic (cyclonic) anomalies centered over subtropical South America that extend through the depth of the troposphere, accompanied by easterly (westerly) wind anomalies over the central Andes. These anomalies are presumably forced by planetary waves originating in the Southern Hemisphere extratropics. To gain insight into the regional processes linking the large-scale and local conditions, The Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model Version 5.2 was used to simulate contrasting rainy and dry episodes. The most marked and relevant differences are the strength and extent of diurnally varying flow over the eastern slope of the Andes. During the rainy simulation, strong easterly winds reach the upper part of the slope by midmorning, initiating an intrusion of warm and moist air (high ue air originating in the eastern lowlands) into the central Andes. In the dry case, the moisture transport from the east is restricted to the eastern slope of the Andes, and the central Andes is flooded by low ue air from the western foothills that cannot support deep convection even in the presence of localized updrafts. The momentum balance based on the model output indicates that turbulent momentum mixing from aloft (determined by the large-scale anomalies of the upper-level flow) into the convective boundary layer is the leading term causing the differences in the daytime upslope flow (and hence moisture transport) over the upper part of the eastern side of the Andes between rainy and dry simulations.

Summertime Incursions of Midlatitude Air into Subtropical and Tropical South America

Abstract 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−1 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 tend...

Wintertime Precipitation Episodes in Central Chile: Associated Meteorological Conditions and Orographic Influences

Central Chile (32°–35°S) is a mountainous and densely populated strip of land between the South American Pacific coast and the main divide of the Andes, 5000 m in height. In this study, wintertime precipitation episodes in central Chile are characterized using precipitation gauge, river discharge, radiosonde, and Special Sensor Microwave Imager (SSM/I) passive microwave radiometer observations over a 10-yr period (1993–2002). Precipitation episodes that typically occur as cold frontal rainstorms move over the region from west to east, within which the cross-mountain flow is blocked at lower levels. The influence of the Andes on the climatological precipitation pattern extends several hundred kilometers upstream of the coast. Over the mainland, the wintertime precipitation is most strongly related to the height of the mean topography surrounding the rain gauge sites, rather than the actual altitudes of the instruments, although higher-elevation locations are not well sampled by available rainfall observations. Between the coast and foothills of the Andes, the precipitation pattern is relatively uniform despite the complex coastal topography. On the western face of the Andes climatological enhancement factors of between 1 and 3 are inferred. Regression analysis against radiosonde data at a coastal site reveals that the precipitation is strongly related to the zonal (cross mountain) moisture flux. The strongest relationship is found when the moisture flux is multiplied by the relative humidity. This variable explains 50% of the variance in daily area average precipitation in central Chile and up to 60% of the variance in the daily precipitation recorded at individual stations. The factors contributing to events of heavy precipitation enhancement in the Andes were examined. Events of heavy, but isolated, precipitation in the Andes tend to occur in the warmer, prefrontal sector of approaching storms and are associated with unusually high moisture fluxes near to and above the crest of the mountain range. Strongly frontal episodes, characterized by widespread rainfall throughout central Chile, lead to variable, but on average rather weak, enhancement in the Andes.