NSF ATM-0214285
P.I.: M. Vuille,  J. Quade (Univ. of Arizona) and J. Betancourt (USGS and Univ. of Arizona)

     Discrepancies in timing, magnitude and direction of climate change are common among diverse paleoclimate records in the subtropical Andes. These inconsistencies may be due to poor dating, varying temporal resolution, different response times and sensitivities to temperature and precipitation, disagreements about field evidence and climatic interpretation, or simply the sheer vastness and geographic complexity of climate across such a large and topographically complex region. Recent advances in understanding modes and mechanisms of modern and past climate variability suggest that tropical SST’s play a key role in forcing upper-air circulation and precipitation anomalies in both summer and winter. For example, summer convective rainfall on the Altiplano and Pacific slope is locally controlled by the amount of near-surface water vapor that is transported episodically west and up from continental lowlands to the east. This moisture transport across the subtropical Andes is regulated, not by the amount of moisture in the adjacent lowlands, but by easterly (wet) and westerly (dry) zonal flow anomalies. These anomalies are in turn regulated by tropical Pacific SST gradients (in general, more transport during La Niña, less during El Niño). Variations in the position and intensity of the southern westerlies and the winter storm track are modulated by equator-to-pole temperature and pressure gradients, also with strong teleconnections to ENSO. Wet winters in central Chile usually result from more frequent winter storms imbedded in an abnormally strong South Pacific jet stream that extends well east of normal during El Niño events. Tropical SST forcing of precipitation anomalies in both summer and winter probably operate on all time scales, markedly so on the western Altiplano and highlands of the Atacama Desert.
     Because much of paleoclimatic research is recent, and many of the proxies are unique, few records have yet to be replicated in the subtropical Andes. Multiple cores can usually be taken and compared from a single site, but there is only one Nevado Sajama and only one Lake Titicaca. Syntheses of central Andes paleoclimatology to some extent involve comparing apples and oranges with few chances to duplicate the same kind of record at multiple localities. During the past 3 years, we have been developing vegetation and ground-water histories from fossil rodent middens and wetland deposits, respectively, along a 1500-km transect on the Pacific slope of the central Andes. A strength of this paleoclimate transect is the ability to apply the same methodologies across broad elevational, latitudinal, climatic, vegetation and hydrological gradients. The rodent midden data allow us to estimate precipitation amounts and seasonality independent of temperature effects.
    We now propose to focus three activities on the steep and unique transition between summer and winter rainfall regimes that occurs along a 300-km stretch of highlands above the Atacama Desert, between 2500-3500m elevation and 23-26ºS latitude: 
     (1) Collect, analyze and date another 100 middens at another five sites [increase full-glacial coverage in the southern part of Salar de Atacama (~23-24ºS) and develop new chronologies from high and low elevations at two different latitudes south of 24ºS]. These chronologies will be used to establish the timing, magnitude and seasonality of precipitation increases during the last 40,000 years; 
     (2) map and date the shoreline and spillway stratigraphy of Salar de Punta Negra to evaluate overflow conditions that could explain filling of Salar de Atacama 100 km to the north. Today, Salar de Punta Negra gets half of its precipitation in winter and half in summer, while Salar de Atacama receives 90% in summer. A northerly shift in the southern westerlies could have filled Salar de Punta Negra and then overflowed into Salar de Atacama, explaining an important discrepancies in paleorecords from the region; 
     (3) analyze measured precipitation and high-resolution cloud cover data throughout the subtropical Andes to evaluate the idea that the Salar de Atacama / Salar Punta Negra region may receive its moisture from a partially different moisture source (Chaco region), and through different pathways than the majority of the NE- Altiplano.