Summer Precipitation Events Over the Western Slope of the Subtropical Andes

1074 MONTHLY WEATHER REVIEW VOLUME 142 Summer Precipitation Events over the Western Slope of the Subtropical Andes MAXIMILIANO VIALE AND RENE´ GARREAUD Departamento de Geofı´sica, Facultad de Ciencias Fı´sicas y Matematicas, Universidad de Chile, Santiago, Chile (Manuscript received 14 August 2013, in final form 22 October 2013) ABSTRACT Summertime [December–February (DJF)] precipitation over the western slopes of the subtropical Andes (328–368S) 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 im- agery, 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. 1. Introduction extratropical disturbances during summertime can also produce precipitation over the mountains but their Summer precipitation events over major mountain frequency is greatly reduced in subtropical latitudes ranges are often of a convective nature. They are con- (Garreaud and Rutlland 1997). In these cases, precipi- trolled by synoptic-scale flow and thermally driven tation tends to be more stable and stratiform in nature. mountain-scale circulations. For instance, the moisture The Andes cordillera is a tall mountain range extend- source of convective storms over the central Andes (178– ing along the west coast of South America from 108Nto 238S) is located over the lowlands to the east and is 538S and exerting a strong influence on the regional cli- transported to the high terrain by the daytime plain-to- mate (e.g., Garreaud 2009). In this work we focus on mountain breeze, in which intensity and extent are precipitation occurring during austral summer months modulated by the synoptic-scale zonal flow aloft (e.g., [December–February (DJF)] over the western slope of Garreaud 1999; Falvey and Garreaud 2005). Thermally the subtropical Andes (328–368S). In this range of lati- driven mountain circulations (e.g., Whiteman 2000) have tudes the Andes is only ;200 km wide but its mean height also been suggested as a key factor controlling the small- exceeds 4000 m MSL (see Fig. 1b and 3c) thus acting as scale spatial (,150km) and temporal variation of cloud- a climatic wall between central Chile to the west and the iness and precipitation within the tropical central Andes Argentina’s lowland to the east (e.g., Prohaska 1976; (Giovannettone and Barros 2009), the Sierra Madres in Miller 1976; see also Fig. 1). Austral summer is the dry Mexico (Giovannettone and Barros 2008), and the Hima- season in central Chile, so we anticipate that summer layas (Barros et al. 2004). Passage of eastward-moving, precipitation over the western slope subtropical Andes accounts for less than 10% of the annual total. Conse- quently, summer storms there have received less atten- Corresponding author address: Maximiliano Viale, Departamento de Geofısica, Universidad de Chile, Blanco Encalada 2002, Santiago tion than their winter counterparts (Falvey and Garreaud ~ 8370449, Chile. 2007; Barrett et al. 2009; Viale and Nunez 2011; Garreaud E-mail: [email protected] 2013; Viale et al. 2013). DOI: 10.1175/MWR-D-13-00259.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 10/01/21 10:46 PM UTC MARCH 2014 V I A L E A N D G A R R E A U D 1075 FIG. 1. (a) Summer (DJF) mean 850-hPa map of geopotential height (contoured every 20 m), temperature (shaded), and wind vectors 2 (m s 1) obtained from CFSR data over the period 1998–2010. The box corresponds to the area of the plot in Fig. 2a. The radiosonde locations are indicated by circles and its respective ID (see Table 2). (b) Mean 328–368S cross-barrier plot of cloud frequency (reflectivity data between 227 and 25 dBZ) derived from CloudSat level-3 data for DJF over the 2007–10 period. During summer events, however, liquid precipitation Rutllant (1997). On the basis of low-resolution weather can occur as high as 4000 m MSL over the mountains, maps during 94 events recorded in a single mountain site quite farther above than the typical snow line in winter between 1970 and 1992, they found two significant weather (;2300 m MSL; Garreaud 2013). As a consequence, types differing in the intensity of the midlevel flow atop of summertime, convective storms1 have the potential to the Andes: a trough with strong westerlies and a weak trigger debris flows or landslides on the steep slopes of trough with weak westerly or easterly winds. In the present the Andes, producing serious damages on mountain work we aim to understand the physical processes leading mining sites (e.g., Golder Associates 2009), interna- to summer precipitation events on the western slope of the tional highways, and other facilities at the Andean subtropical Andes, an aspect not addressed by Garreaud foothills (e.g., Sepulveda and Padilla 2008). There is also and Rutllant (1997). To this end, we reexamine the at- evidence of stratiform rain events during summer— tending synoptic conditions using state-of-the-art reanalysis more akin to winter storms—associated with strong data and radiosonde observations during 13 summers midlevel westerlies affecting climbers of the many high between 1998 and 2010, describe the cloudiness pattern peaks in this Andean region; these events have resulted and local conditions using high-resolution satellite im- in fatalities in the worst cases (C. Bravo 2013, personal agery and an expanded surface observation network, communication). and perform a trajectory analysis to determine the water The basic climatology and synoptic environment vapor source of these storms. during summer precipitation events over the western slope The paper is organized as follows. In section 2 we of the subtropical Andes was addressed by Garreaud and present the observational data and explain the cluster analysis used to identify the main weather types associated with precipitation over the west side of the sub- 1 Some of these events go unrecorded (or undersampled) given tropical Andes. Section 3 provides a climatological their isolated, convective nature and the sparse station network at overview of precipitation and cloudiness over the sub- high elevation. For instance, light precipitation (less than tropical Andes. In section 4 we describe the synoptic and 2 10 mm day 1) was recorded in only two mountain stations in cen- regional conditions associated with summer precipita- tral Chile during a couple of events occurred when writing this tion events, while in section 5 we address the moisture manuscript (21 January and 8 February 2013). These events, however, produced a sudden increase in the flow and sedimentary sources and the physical mechanisms responsible for load of several Andean rivers causing a 2-day shutdown of the summer precipitation. The results are summarized in drinking water supply for the Santiago metropolitan area. section 6. Unauthenticated | Downloaded 10/01/21 10:46 PM UTC 1076 MONTHLY WEATHER REVIEW VOLUME 142 FIG. 2. (a) Summertime (DJF) mean precipitation (mm), (b) frequency of rainy days (%), and (c) percentage of the annual total (mm) for the 2005–10 period. The mountain stations are plotted by black circles with a white cross in its center in (a), while in (b) and (c) the mountain, low Chilean, and low Argentinean stations are plotted with square, gray, and black filled circles, respectively. The mountain stations are defined by an altitude higher than 1300 m and by a distance lesser than 50 km of the Argentine–Chile frontier (i.e., the line water divide which approximately represent the crest of the Andes). 2. Data and methods There are four radiosonde stations surrounding the subtropical Andes (shown in Fig. 1, details in Table 2): a. Surface and radiosonde data twoclosetothefootoftheAndesat338S[Santo Figure 2a shows the location of 153 surface stations Domingo (Chile) and Mendoza (Argentina) and two with daily precipitation records [at 1200 UTC (0900 farther to the east on the Argentinean plains (Santa LT)] used in this study, superimposed on a topographic Rosa and Cordoba)]. Their data and metadata were ob- map of the subtropical Andes (328–378S) and the adja- tained from the Integrated Global Radiosonde Archive cent lowlands. Most of these stations are located at low (IGRA), a global upper-air dataset from the National elevations in central Chile (114 stations; see details on Climatic Data Center (NCDC; Durre et al. 2006). For the Table 1) and western Argentina (32 stations, Table 1). composite analysis of vertical profiles, radiosonde data Only seven stations are located on the Andes Mountain, from the 1200 UTC launches were homogenized in 11 with elevations higher than 1300 m MSL and less than standard vertical levels (from 1000 to 200 hPa by per- 50 km from the Andean crest, which we refer to here- forming lineal vertical interpolation using data from after as mountain stations.

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