VOLUME 12 JOURNAL OF HYDROMETEOROLOGY AUGUST 2011 Climatology of Winter Orographic Precipitation over the Subtropical Central Andes and Associated Synoptic and Regional Characteristics MAXIMILIANO VIALE Programa Regional de Meteorolog´ıa, Instituto Argentino de Nivolog´ıa, Glaciolog´ıa y Ciencias Ambientales (IANIGLA), CCT–CONICET, Mendoza, Argentina MARIO N. NUN˜ EZ Centro de Investigaciones del Mar y la Atmo´ sfera, CONICET-UBA, and Departamento de Ciencias de la Atmo´ sfera y Oce´anos, Universidad de Buenos Aires, Buenos Aires, Argentina (Manuscript received 25 February 2010, in final form 16 November 2010) ABSTRACT Winter orographic precipitation over the Andes between 308 and 378S is examined using precipitation gauges in the mountains and adjacent lowlands. Because of the limited number of precipitation gauges, this paper focuses on the large-scale variation in cross-barrier precipitation and does not take into account the fine ridge–valley scale. The maximum amount of precipitation was observed on the windward slope of the mountain range below the crest, which was twice that observed on the low-windward side between 32.58 and 348S. Toward the east of the crest, precipitation amounts drop sharply, generating a strong cross-barrier gradient. The rain shadow effect is greater in the north (328–34.58S) than in the south (358–36.58S) of the low-lee side, which is probably due to more baroclinic activity in southernmost latitudes and a southward decrease in the height of the Andes enabling more spillover precipitation. The effect of the Andes on winter precipitation is so marked that it modifies the precipitation regimes in the adjacent windward and leeward lowlands north of 358S. Based on the fact that ;75% of the wintertime precipitation accumulated in the fourth quartile, through four or five heavy events on average, the synoptic-scale patterns of the heavy (into fourth quartile) orographic precipitation events were identified. Heavy events are strongly related to strong water vapor transport from the Pacific Ocean in the pre-cold-front environment of extratropical cyclones, which would have the form of atmospheric rivers as depicted in the reanalysis and rawinsonde data. The composite fields revealed a marked difference between two subgroups of heavy precipitation events. The extreme (100th–95th percentiles) events are associated with deeper cyclones than those for intense (95th–75th percentiles) events. These deeper cy- clones lead to much stronger plumes of water vapor content and cross-barrier moisture flux against the high Andes, resulting in heavier orographic precipitation for extreme events. In addition, regional airflow charac- teristics suggest that the low-level flow is typically blocked and diverted poleward in the form of an along-barrier jet. On the lee side, downslope flow dominates during heavy events, producing prominent rain shadow effects as denoted by the domain of downslope winds extending to low-leeward side (i.e., zonda wind). 1. Introduction (Trenberth 1991; Hoskins and Hodges 2005), when cyclones moving eastward produce strong cross-barrier The role of the Andes in supplying water through flow that results in upslope precipitation on the wind- orographic precipitation is of vital importance for ad- ward slope and rain shadow effect on leeward slopes. jacent lowlands in Chile and western Argentina. In These orographic effects accentuate in mountain ranges winter, the subtropical central Andes (SCA; 308–378S) oriented perpendicular to the prevalent horizontal is mostly affected by the northern flank of storm tracks flow and, hence, have great influence on the climate in adjacent areas, producing strong windward–leeward gradients in vegetation and water availability [e.g., Corresponding author address: Maximiliano Viale, Instituto New Zealand Alps (Griffiths and McSaveney 1983; Argentino de Nivologı´a, Glaciologı´a y Ciencias Ambientales, Av. Adria´n Ruiz Leal s/n, Parque Gral. San Martı´n, CC 330, 5500 Wratt et al. 2000), the Cascades in Oregon (Smith Mendoza, Argentina. et al. 2005), and the southern Andes (Smith and Evans E-mail: [email protected] 2007)]. DOI: 10.1175/2010JHM1284.1 Ó 2011 American Meteorological Society 481 Unauthenticated | Downloaded 10/07/21 08:21 PM UTC 482 JOURNAL OF HYDROMETEOROLOGY VOLUME 12 Orographic effects on the horizontal flow result in vapor transport concentrates over an extensive and nar- a very complex distribution of precipitation across the row region of high water vapor content associated with the mountain range. A dense observation network is nec- low-level jet in the broader warm and pre-frontal zone of essary to depict the spatial precipitation pattern, which the polar front (i.e., the ‘‘warm conveyor belt’’; Browning is a real limitation in the South American Andes. For 1990). This long and narrow corridor of water vapor above example, Falvey and Garreaud (2007, hereafter FG07) the ocean accounts for essentially the total meridional found an orographic precipitation enhancement close transport at middle latitudes, so it has been named to 2–3 in SCA using mostly river discharge estimates. ‘‘atmospheric river’’ (Zhu and Newell 1998). Recent Between 408 and 488S in the southern Andes, Smith and composite studies of atmospheric rivers have demon- Evans (2007) reported the highest drying ratio1 values strated their crucial role in modulating heavy orographic ever found in a mountain range using stable isotope data rainfall, snowpack variability, and flooding in western from stream water. Over other more-sampled mountain North America (Ralph et al. 2004, 2005a, 2006; Neiman ranges, the maximum precipitation was identified on the et al. 2008). Given the significant contribution of at- windward slope of the high Cascade or Sierra Nevada mospheric rivers to the extreme precipitation in the ranges (Colle and Mass 2000; Smith et al. 2005; Leung western United States, the results of the Hydrometeo- and Qian 2003) and/or over the crest of the low Oregon rology Testbed (HMT) project-West field programs, coastal mountains (Colle and Mass 2000) or low New conducted by the National Oceanic and Atmospheric Zealand Alps (Sinclair et al. 1997). Using one of the Administration (NOAA) since 2005 (Ralph et al. 2005b, densest rain gauge networks on a mountain range, Frei 2010), may be applicable to other north–south mountain and Scha¨r (1998) identified finescale spatial variabil- ranges such as the Andes. ity as the most prominent characteristic in precipitation Because of sparse distribution of surface stations in fields over the European Alps, with precipitation en- the Andes of South America, studies of winter oro- hancement on the upslope peaks and shielding in inner graphic precipitation are limited. Recent modeling valleys. At the small ridge–valley scale, strong precip- case studies have documented that local airflow char- itation gradients were also documented over the Olympic acteristics and precipitation patterns result from the Peninsula in Washington; these remained relatively con- interaction between the synoptic-scale flow and the stant on time scales ranging from annual to single event, topography of SCA (Barrett et al. 2009; Viale and thus suggesting the dominant role of the topography in Norte 2009, hereafter VN09). A climatological approach determining the spatial precipitation pattern (Anders of winter precipitation and their associated synoptic et al. 2007; Minder et al. 2008). By contrast, the Andes are conditions have been addressed by FG07, but is limited a data-poor region. Therefore, we focus on variations in to the low-windward side and windward slope of the the cross-mountain direction of precipitation over the subtropical Andes. Our study extends the climatolog- (still poorly understood) broad scale of ;50 km, dis- ical approach of winter orographic precipitation to the tinguishing between robust cross-barrier zones of the low- leeward slope and low-lee side of SCA, making use of windward side, windward slope, immediate leeward slope, the less sparse network of precipitation gauges avail- and low-leeward side. able over the mountains and both adjacent low sides The orographic precipitation pattern depends on syn- between 308 and 378S. We also explore the synoptic optic forcing, including air mass stability, moisture con- and regional air mass features up- and downstream of tent, and direction and strength of wind, which in turn the barrier that accompanied heavy orographic pre- interacts with the topography. Junker et al. (2008) and cipitation events, including the possible linkage with Pandey et al. (1999) showed that deeper cyclones lo- landfalling atmospheric rivers on the western coast of cated off the western U.S. coast lead to stronger winds South America. and moisture fluxes against the Sierra Nevada, result- The remainder of this article is structured as fol- ing in heavier precipitation. Based on data collected lows: the data and topographic features, as well as along and off the California coast during the California the precipitation-event dataset and their composite Landfalling Jets Experiment (CALJET) and the Pacific methodology, are described in the next section. In Landfalling Jets Experiment (PACJET) (Ralph et al. section 3 we examine the spatial, seasonal, and daily 1999), Ralph et al. (2004, 2005a) documented that water distribution of winter precipitation over the mountain range and their surroundings. The synoptic and regional conditions during the heavy precipitation events and 1 Drying ratio is the ratio between the water vapor flux removed their links with atmospheric rivers are analyzed in sec- as precipitation on the mountain and the incoming water vapor flux tion 4. Our main results are discussed and summarized in against the mountain. section 5. Unauthenticated | Downloaded 10/07/21 08:21 PM UTC AUGUST 2011 V I A L E A N D N U N˜ EZ 483 FIG. 1. Orographic region investigated in this study (elevations in m; above 500 m are shaded) and the stations used (filled and empty circles represent stations with daily and monthly precipitation data, respectively). The stations enclosed by the vertical rectangle with dashed white lines correspond to high-mountain locations (i.e., alt .