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RenéD. Garreaud

Abstract

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 (∼7 g kg−1), 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 θ e 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 θ e 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.

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RenéD. Garreaud

Abstract

Synoptic-scale incursions of cold, midlatitude air that penetrate deep into the Tropics are frequently observed to the east of the Andes cordillera. These incursions are a distinctive year-round feature of the synoptic climatology of this part of South America and exhibit similar characteristics to cold surges observed in the lee of the Rocky Mountains and the Himalayan Plateau. While their large-scale structure has received some attention, details of their mesoscale structural evolution and underlying dynamics are largely unknown. This paper advances our understanding in these matters on the basis of a mesoscale numerical simulation and analysis of the available data during a typical case that occurred in May of 1993.

The large-scale environment in which the cold air incursion occurred was characterized by a developing midlatitude wave in the middle and upper troposphere, with a ridge immediately to the west of the Andes and a downstream trough over eastern South America. At the surface, a migratory cold anticyclone over the southern plains of the continent and a deepening cyclone centered over the southwestern Atlantic grew mainly due to upper-level vorticity advection. The surface anticyclone was also supported by midtropospheric subsidence on the poleward side of a jet entrance–confluent flow region over subtropical South America. The northern edge of the anticyclone followed an anticyclonic path along the lee side of the Andes, reaching tropical latitudes 2–3 days after its onset over southern Argentina. The concomitant cold air produced low-level (surface to ∼800 hPa) cooling on the order of 10°C over the subtropical part of the continent (as far north as 10°S). Based on the observations and model results, a three-stage evolution of the cold air incursion is suggested. The initial cooling to the south of 30°S and far from the Andes is mainly produced by the geostrophic southerly winds between the continental anticylone and the developing low off the coast of Argentina. As the surface pressure increases over southern Argentina, a large-scale meridional pressure gradient is established between the migratory anticyclone and the continental trough farther to the north. The blocking effect of the Andes leads to an ageostrophic, low-level southerly flow that advects cold air into the subtropics. Finally, as the cold air moves to the north of 18°S, the blocking effect of the Andes weakens (because the adjustment back to geostrophy is quite slow at these low latitudes) and the cold air spread out over the Tropics. In the last two stages of the incursion the strong pressure (temperature) gradient drives the northward accelaration of the low-level winds, while horizontal advection of cold air by southerly winds maintains the strong temperature gradient against the dissipative effects of the strong surface heat fluxes.

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RenéD. Garreaud

Abstract

Precipitation over the South American Altiplano (about 4000 m above sea level) is mostly concentrated during the austral summer (December–January–February) when mean easterly flow in the middle and upper troposphere favors the moisture transport from the interior of the continent toward the central Andes. Within the wet season, rainy days tend to cluster in rainy episodes of about a week long, interrupted by somewhat longer dry periods. Based on one-site, research-quality observations over the western Altiplano, it has been suggested that occurrence of deep, moist convection is largely controlled by the availability of water vapor in the local boundary layer.

In this work the author evaluates the representativeness of the observations in the western Altiplano in a regional context and investigates if the hypotheses derived from these data are generally applicable to the rest of the plateau. The study is based on surface synoptic data and atmospheric reanalysis. The relationship between moisture fluctuations on the Altiplano and the lowlands to the east of the central Andes is also addressed. It is found that intraseasonal moisture fluctuations tend to be coherent on the Altiplano and closely related to basinwide episodes of active/suppressed moist convection. On the other hand, near-surface moisture variability over the lowlands to the east of the central Andes is too small and noisy to explain the persistent, large-amplitude fluctuations in the Altiplano. Thus, moisture and rainfall variability over the Altiplano is strongly dependent on the intensity of the moisture transport over the eastern slope of the Andes rather than the precise low-level conditions on the central part of the continent.

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RenéD. Garreaud

Abstract

Extratropical precipitation is primarily produced by cold and warm fronts associated with surface cyclones and upper-level troughs. The growth of these midlatitude storms is partially controlled by the dry baroclinicity of the troposphere, which in turn can be roughly quantified by the intensity of the upper-level zonal flow. Orographic rainfall, an important component of the precipitation in several midlatitude regions, is also partially determined by the intensity of the cross-mountain midlevel winds. Thus, at monthly and longer time scales, variations of precipitation and zonal flow aloft (as well as wind shear) at a given location should exhibit some degree of coherence. In this work the local covariability of these variables is documented over intermonthly and interannual time scales, using global precipitation products and atmospheric reanalysis from 1979 to 2004. The spatial correspondence between the precipitation and two indices of synoptic activity in the extratropics is also documented.

The local correlation (r 0) between monthly anomalies of precipitation and upper-level (300 hPa) zonal flow varies in space, from moderately and even highly significant values (r 0 ∼ 0.3 to 0.7) over the midlatitude oceans to near zero over the interior of continental areas. Broadly similar results are found when considering the monthly variance of the high-pass-filtered meridional wind (an index of eddy activity) or the midlevel Eady growth rate. The local correlation map between precipitation and low-level (850 hPa) zonal flow is similar to its upper-level counterpart, but the correlations over open ocean are somewhat weaker, while orographic effects show up more clearly. The correlations are positive and large upstream of the major north–south-oriented mountain ranges, as strong westerlies promote upslope rain in addition to storm-related precipitation. In contrast, the correlation tends to be negative downstream of the ranges, as strong westerlies enhance the rain shadow effects over the lee side.

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RenéD. Garreaud

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.

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Mark Falvey
and
René Garreaud

Abstract

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.

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RenéD. Garreaud
and
José Rutllant

Abstract

Subsynoptic, warm core low pressure areas are frequently observed along the west coast of subtropical South America during austral winter. These so-called coastal lows (CLs) tend to develop as an upper-air, midlatitude ridge is approaching the subtropical Andes and, therefore, while pressure is increasing aloft and farther to the south. These CLs have a profound impact in the coastal weather associated with a rapid transition from clear skies and stronger than average equatorward low-level flow to overcast conditions and relaxed equatorward (or even poleward) flow. Weather conditions inland mostly reflect the associated changes in the strength and height of the base of the subsidence inversion.

In this work, a mesoscale simulation of a typical CL episode is performed using a numerical weather prediction model [the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)] Comparison with observations reveals that the model simulation properly captures the large-scale pattern as well as many of the mesoscale features that characterize the CL. The model results were then used to diagnose the CL. It is found that the coastal troughing is largely due to the marked adiabatic warming of the lower troposphere (including a significant strengthening of the temperature inversion). The large-scale subsidence ahead of the incoming upper-air ridge axis is enhanced as the low-level easterly flow is constrained by the western slope of the subtropical Andes. The low-level wind off the subtropical coast is close to geostrophic balance and it is fed by air parcels that 1–2 days before had been located in the middle troposphere over the Pacific Ocean. The easterly flow is set up as the alongshore pressure gradient becomes poleward oriented because of the extratropical ridging. This gradient is further enhanced as the CL develops at subtropical latitudes. As soon as the ridge axis crosses, the low-level easterly flow vanishes and a shallow, narrow tongue of northwesterlies and stratocumulus clouds propagates poleward from northern Chile. Shortly thereafter, the trapped wind reversal merges with the incoming synoptic-scale, tropospheric deep-cyclonic circulation.

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RenéD. Garreaud
and
Ricardo Muñoz

Abstract

The extensive and persistent deck of stratocumulus (Sc) over the subtropical southeast Pacific (SSEP) plays an important role in the regional and global climate. As in other subtropical regions, the Sc form at the top of a marine boundary layer (MBL), capped by the subsidence inversion. A distinctive feature of this subtropical deck is its pronounced dawn-to-afternoon decrease in cloud amount and liquid water path, partially associated with a regular and marked descent of the inversion base and the warming of the lower troposphere. Furthermore, coastal observations in this area reveal a diurnal cycle in air temperature encompassing up to 5 km MSL.

In this work, 15-day regional numerical simulations using the fifth-generation PSU–NCAR Mesoscale Model (MM5) in November (austral spring), May (late fall), and January (summer) 2001 were used to document the mean diurnal cycle in circulation and low-level cloudiness over the SSEP. The simulated amplitude, depth, and phase of the diurnal cycle in air temperature, wind, and cloudiness at the northern coast of Chile and over open ocean compare quite favorably with their observational counterparts.

Large-scale subsidence prevails over the SSEP on a daily average. Between 1 and 5 km, however, the vertical velocity exhibits a marked diurnal cycle, largely produced by a band of upward motion propagating from the southern coast of Peru into the SSEP during late afternoon and night. Such an “upsidence wave” was found in the three simulations. The upsidence wave produces a significant cooling, leading to a consistent diurnal cycle in air temperature in low- and midlevels over the SSEP. The impact of the vertical velocity cycle on the MBL was further studied using a 1D version of the MM5 with higher resolution. The deepening of the MBL during the upsidence period induces a more turbulent MBL and more entrainment. The warming and drying of the MBL result in a greater dissipation of the cloud layer in the afternoon, increasing the amplitude of the diurnal cycle in Sc cloud amount with respect to the cycle forced by absorption of solar radiation only.

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RenéD. Garreaud
and
Patricio Aceituno

Abstract

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 documented relationship between El Niño–Southern Oscillation (ENSO) phenomenon and interannual rainfall variability over the Altiplano is explained by the generalized warming (cooling) of the tropical troposphere during the negative (positive) phase of ENSO and the associated strengthening (weakening) of the westerlies over the central Andes.

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Maximiliano Viale
and
René Garreaud

Abstract

Summertime [December–February (DJF)] precipitation over the western slopes of the subtropical Andes (32°–36°S) accounts for less than 10% of the annual accumulation, but it mostly occurs as rain and may trigger landslides leading to serious damages. Based on 13 year of reanalysis, in situ observations, and satellite imagery, a synoptic climatology and physical diagnosis reveal two main weather types lead to distinct precipitation systems. The most frequent type (~80% of the cases) occurs when a short-wave midlevel trough with weak winds and thermally driven mountain winds favor the development of convective precipitation during the daytime. The trough progresses northwest of a long-lasting warm ridge, which produces low-level easterly airflow that enhances its buoyancy as it moves over the arid land of western Argentina toward the Andes. The weak winds aloft facilitate the penetration of the moist easterly flow into the Andes. Midlevel flow coming from the west side of the Andes is decoupled from the low-level maritime air by a temperature inversion, and thus provides little moisture to support precipitation. The less frequent type (~20% of the cases) occurs when a deep midlevel trough and strong westerly flow produces stratiform precipitation. This type has a baroclinic nature akin to winter storms, except that they are rare in summer and there is no evidence of a frontal passage at low levels. The lifting and cooling ahead of the trough erode the typical temperature inversion over the Pacific coast, and thus allows upslope transport of low-level marine air by the strong westerlies forming a precipitating cloud cap on the western slope of the Andes.

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