climate and lake level history of the northern altiplano bolivia as recorded in holocene sediments of the rio desaguadero abstract strata exposed in terraces and modern cutbanks along the rio desaguadero contain a variety of lithofacies that were deposited in four distinct facies associations these facies associations document a history of aggradation and downcutting that is linked to holocene climate change on the altiplano braided stream meandering stream deltaic and shoreline and lacustrine sediments preserved in multi level terraces in the northern rio desaguadero valley record two high water intervals one between 4500 and 3900 yr bp and another between 2000 and 2200 yr bp these wet periods were interrupted by three periods of fluvial downcutting centered at approximately 4000 yr bp 3600 yr bp and after 2000 yr bp braided river sediments preserved in a single terrace level in the southern rio desaguadero valley record a history of nearly continuous fluvial sedimentation from at least 7000 yr bp until approximately 3200 yr bp that was followed by a single episode post 3210 yr bp of downcutting and lateral migration the deposition and subsequent fluvial downcutting of the northern strata was controlled by changes in effective moisture that can be correlated to holocene water level fluctuations of lake titicaca the deposition and dissection of braided stream sediments to the south are more likely controlled by a combination of base level change and sediment input from the rio mauri

sedimentology and stable isotopes from a lacustrine to palustrine limestone deposited in an arid setting climatic and tectonic factors miocene pliocene opache formation atacama desert chile abstract field relations petrography gis and geochemistry of a late miocene pliocene limestone the opache formation calama basin atacama desert chile shows that depositional facies diagenetic histories and isotopes differ from eastern to western parts of the basin we recognize two facies lacustrine characterized by ostracod diatom mudstone and wackestone and palustrine characterized by both shallow water and pedogenic features gis analysis of the basin topography reveals an intrabasinal low in the eastern area and steeper hydrological gradients to the west with the calama constriction restricting flow between east and west petrography and geochemical results confirm that the calama constriction influenced depositional parameters ponding groundwater to the east forming a shallow lake and creating palustrine conditions to the west our analyses indicate that lake waters were less enriched in 13c or 18o than palustrine waters even though petrologic studies show that the lacustrine carbonates have a complex diagenetic history derived from groundwater rich in dissolved minerals this seemingly contrary observation can be reconciled if an important source of water for the western carbonates was sourced from the pacific ocean and was less evolved whereas eastern limestones precipitated from water which had become depleted in 18o during transport from the east today moisture from the pacific very rarely if ever reaches inland as far as calama indicating that during the late miocene mid pliocene breakdown in the temperature inversion due to weakened or absent upwelling in the cold coastal pacific current allowed moisture from the pacific to reach inland this changing atmospheric circulation pattern may have also affected upper level airflow that today blocks eastern sourced moisture from crossing the andes allowing it to reach farther westward into the atacama region during the late miocene pliocene together these moisture sources were sufficient to produce carbonate rich palustrine and lacustrine environments in the calama basin

application of strontium isotopes to understanding the hydrology and paleohydrology of the altiplano bolivia peru abstract strontium concentrations and strontium isotopic ratios were measured in natural waters and carbonate sediments from throughout the bolivian and peruvian altiplano in order to improve hydrologic and paleohydrologic mass balances with the ultimate goal of better understanding the paleoclimatic history of the central andes rivers flowing into lake titicaca have a wide range of strontium isotopic ratios that exhibit spatial patterns consistent with the lithologies of the different drainage basins because of the limited exchange of water between the two main sub basins of lake titicaca lago grande and lago huinaimarca and between the sub basins of lago huinaimarca there are significant differences in strontium isotopic ratios between the sub basins calculated elemental balances of strontium in lake titicaca are in reasonable agreement with previously published budgets of water and major elements however the strontium isotopic budget indicates that the lake is not in isotopic steady state this also implies that the major element budgets are unlikely to be in steady state lake titicaca had a higher than modern strontium isotopic ratio during the early and middle holocene elevated values persisted in lago grande until at least 2000 cal yr bp consistent with other evidence that modern hydrologic conditions namely overflow were not established until that time an isotopic budget calculated for late pleistocene paleolake tauca in the central altiplano suggests that between 70 and 83 of its riverine inputs were derived from lake titicaca overflow this calculated flow represents about a 30 fold increase over the average discharge of the modern rio desaguadero at desaguadero or a seven fold increase over its average discharge into lago poopo the strontium isotopic budget if complete precludes the possibility that decreased evaporation alone could have accounted for the existence of paleolake tauca

the effects of orography on midlatitude northern hemisphere dry climates abstract the role of mountains in maintaining extensive midlatitude arid regions in the northern hemisphere was investigated using simulations from the gfdl global climate model with and without orography in the integration with mountains dry climates were simulated over central asia and the interior of north america in good agreement with the observed climate in contrast moist climates were simulated in the same regions in the integration without mountains during all season but summer large amplitude stationary waves occur in response to the tibetan plateau and rocky mountains the midlatitude dry regions are located upstream of the troughs of these waves where general subsidence and relatively infrequent storm development occur and precipitation is thus inhibited in summer this mechanism contributes to the dryness of interior north america as a stationary wave trough remains east of the rockies but is not effective in eurasia due to seasonal changes in the atmospheric circulation the dryness

tectonics and climate of the southern central andes the history of the southern central andes including the world s second largest plateau and adjacent intermontane basins and ranges of the eastern cordillera and the northern sierras pampeanas of argentina and bolivia impressively documents the effects of tectonics and topography on atmospheric circulation patterns the development of orographic barriers and their influence on erosion and landscape evolution at various timescales protracted aridity in the orogen interior has facilitated the creation and maintenance of the puna altiplano plateau contraction and range uplift filling of basins and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior which caused the eastward migration of deformation into the foreland and successive aridification the uplift of the andean orogen has also had a far reaching influence on atmospheric and moisture transport patterns in south america this is documented by the onset of humid climate conditions on the eastern side of the andes in late miocene time which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen and changes in tectonic processes in the andean orogenic wedge

el nino southern oscillation signal in the world s highest elevation tree ring chronologies from the altiplano central andes article i nfo el nino southern oscillation enso is the largest source of inter annual variability operating in the earth s climate system with enormous ecological social and economic impacts as the instrumental record of enso variability is limited to the past 150 yr the search for proxies sensitive to enso has become a priority task in recent years it is vital to expand the sparse network of annually resolved and exactly dated enso sensitive proxies to entirely capture the spatial variability of the enso coupled ocean atmospheric phenomenon several tree ring records have been used to reconstruct past enso variability however none of them are from tropical south america the polylepis tarapacana woodlands form the world s highest elevation treeline trees grow on the altiplano plateau between 4000 5200 m elevation 16 23 s the climate variability in this climatically transitional tropical subtropical region is strongly modulated by enso two chronologies of p tarapacana along the western cordillera in the tropical central andes were analyzed to determine the strength of the enso signal present in these tree ring records the growth of p tarapacana has a strong common signal amongst trees in a single site and between sites across the altiplano ring formation is induced by climatic conditions during the previous and current growing season december february at the time of the strongest enso influences on the peruvian coast tree growth variations show opposite relationships with climate between consecutive growing seasons during the current growing season ring width variations show positive and negative relationships with temperature and precipitation respectively both tree ring chronologies are significantly correlated with austral spring summer august february sst in the nino3 4 region and show oscillatory modes within the classical enso bandwidth our results support the idea that p tarapacana chronologies from the south central tropical andes provide high resolution records extremely sensitive to enso in the tropical pacific and represent an important component to be considered in future multiproxy enso reconstructions

precipitation changes over the eastern bolivian andes inferred from speleothem δ18o records for the last 1400 years abstract here we present high resolution 18 o records obtained from speleothems collected in the eastern bolivian andes the stable isotope records are related to the regional to large scale atmospheric circulation over south america and allow interpreting changes in 18 o during the last 1400 yr as a function of changes in precipitation regimes over the southern tropical andes two distinct phases with more negative 18 o values interpreted as periods of increased convective activity over the eastern andean cordillera in bolivia are observed concomitantly with periods of global climate anomalies during the last millennium such as the medieval climate anomaly mca and the little ice age lia respectively changes in the bolivian 18 o record during the lia are apparently related to a southward displacement of the intertropical convergence zone itcz which acts as a main moisture driver to intensify convection over the tropical continent during the mca however the increased convective activity observed in the bolivian record is likely the result of a different mechanism which implies moisture sourced mainly from the southern tropical atlantic this interpretation is consistent with paleoclimate records further to the north in the tropical andes that show progressively drier conditions during this time period indicating a more northerly position of the itcz the transition period between the mca and the lia shows a slight tendency toward increased 18 o values indicating weakened convective activity our results also reveal a non stationary anti phased behavior between the 18 o reconstructions from bolivia and northeastern brazil that confirms a continental scale east west teleconnection across south america during the lia

climate in the western cordillera of the central andes over the last 4300 years abstract the distichia peat core obtained in the carhuasanta valley near nevado mismi cordillera chila provides information on climatic and environmental conditions over the last 4300 years the relative changes in the stable carbon isotope composition of plant remains preserved in the core reflect major temperature fluctuations in the western cordillera of the southern peruvian andes these temperature variations can be additionally linked with the changes in precipitation patterns by analysing c and c n ratio in the core relatively warm and moist conditions prevailed from 4280 to 3040 cal yrs bp bc 2330 1090 with a short colder dry episode around 3850 cal yrs bp bc 1900 the most prominent climate changes recorded in the peat occurred between 3040 and 2750 cal yrs bp bc 1090 800 when the initial warming turned to a rapid cooling to temperatures at least 2 c lower than the mean for the late holocene initially drier conditions within this event turned to a short wet phase after 2780 cal yrs bp bc 830 when the temperature increased again this event coincides with significant changes in peat and ice core records in the central andes matching the timing of the global climate event around 2 8 cal ka bp climatic conditions in the study area became relatively dry and stable after the event for about 800 years highly variable temperatures and humidity prevailed during the last 2000 years when an extended warm and relatively humid period occurred between 640 and 155 cal yrs bp ad 1310 1795 followed by predominantly colder and drier conditions the established 13 c peat record represents the first continuous proxy for the temperature in the southern peruvian andes dated by the ams 14 c distichia peat is wide spread in the andes and the proposed approach can be applied elsewhere in high altitudes where no other traditional climate proxies are available

surface expression of eastern mediterranean slab dynamics neogene topographic and structural evolution of the southwest margin of the central anatolian plateau turkey 1 the southwest margin of the central anatolian plateau has experienced multiple phases of topographic growth including the formation of localized highs prior to the late miocene that were later affected by wholesale uplift of the plateau margin our new biostratigraphic data limit the age of uplifted marine sediments at the southwest plateau margin at 1 5 km elevation to 7 17 ma and regional lithostratigraphic correlations imply that the age is 6 7 ma single grain ca tims u pb zircon analyses from a reworked ash within the marine sediments yield dates as young as 10 6 ma indicating a maximum age that is consistent with the biostratigraphy our structural measurements within the uplifted region and fault inversion modeling agree with previous findings in surrounding regions with early contraction followed by strike slip and extensional deformation during uplift focal mechanisms from shallow earthquakes show that the extensional phase has continued to the present broad similarities in the change in the tectonic stress regime after 8 ma and the onset of surface uplift after 7 ma imply that deep seated process es caused post 7 ma uplift the geometry of lithospheric slabs beneath the plateau margin pliocene to recent alkaline volcanism and the uplift pattern with accompanying normal faulting point toward slab tearing and localized heating at the base of the lithosphere as a probable mechanism for post 7 ma uplift of the southwest margin considering previous work in the region there appears to be an important link between slab dynamics and surface uplift throughout the anatolian plateau s southern margin

misr based passive optical bathymetry from orbit with few cm level of accuracy on the salar de uyuni bolivia abstract we demonstrate that under ideal circumstances passive optical measurements can yield surface water depth estimates with an accuracy of a few centimeters our target area is the salar de uyuni in bolivia it is a large active salt flat or playa which is maintained as an almost perfectly level and highly reflective surface by annual flooding to a mean depth of 30 50 cm we use misr data to estimate spatial and temporal variations in water depth during the waning portion of the 2001 flooding cycle we use a single icesat laser altimetry profile to calibrate our water depth model though the salt surface is probably the smoothest surface of its size on earth with less that 30 cm rms height variations over an area of nearly 104 km2 it is not completely featureless topography there includes a peripheral depression or moat around the edge of the salt and several sets of prominent parallel ridges with 5 km wavelength and 30 cm amplitude the process by which these features form is still not well characterized

a preliminary study of summertime tropospheric circulation patterns over south america estimated from cloud winds abstract cloud winds obtained from sms i geostationary satellite images for the southern hemisphere summer time are used to describe the tropospheric circulation over tropical and subtropical south america these data provide good spatial and temporal coverage over south america for at least two levels in the troposphere the summertime circulation derived from cloud winds is similar in pattern to that obtained from sparse conventional winds but is more intense the major features of the time averaged circulation at the upper level 300 100 mb are an intense quasi stationary anticyclone trough system extending over most of south america north of 35 s a band of easterly flow near 10 s and strong cross equatorial northward flow between 60 and 80 w longitudes at the lower level 900 700 mb the major features are the northerly and northwesterly jet type flow along the eastern slopes of the central andes and northeasterly trades over much of the remainder of the continental region to the north and cast

a late miocene subtropical dry flora from the northern altiplano bolivia abstract a variety of evidence suggests that the altiplano of the central andes the second highest and largest plateau on earth underwent significant uplift in the late miocene pliocene the most important datum supporting recent uplift is a collection of the 10 66 0 06 ma jakokkota flora from west central bolivia which implies a paleoelevation no more than 1600 1200 m today the site has an elevation of almost 4000 m in order to test the reliability of this estimate the present study analyzes a new collection of the jakokkota flora from a lacustrine unit that is 0 2 0 5 myr younger than the previously analyzed collection from a fluvial unit climate estimates based on leaf morphology for the two collections are statistically indistinguishable the combined flora has a mean annual temperature of 21 5 2 0 c and a mean annual precipitation of 550 180 mm the similarity of the climate estimates for the two floras suggests that there was not a significant climate change between them nor a significant bias in the leaf morphology due to differing taphonomic processes the climate estimate for the combined flora thus presents a representative picture of the late miocene climate of the northern altiplano if one assumes that the climate of the tropics has not changed significantly since the late miocene as is suggested by marine isotopic data then the paleoclimate of the jakokkota flora implies a paleoelevation of 1160 600 m thus the jakokkota flora supports the hypothesis of a young age for the altiplano

in stream wetland deposits megadroughts and cultural change in the northern atacama desert chile a key concern regarding current and future climate change is the possibility of sustained droughts that can have profound impacts on societies as such multiple paleoclimatic proxies are needed to identify megadroughts the synoptic climatology responsible for these droughts and their impacts on past and future societies in the hyperarid atacama desert of northern chile many streams are characterized by perennial flow and support dense in stream wetlands these streams possess sequences of wetland deposits as fluvial terraces that record past changes in the water table we mapped and radiocarbon dated a well preserved sequence of in stream wetland deposits along a 4 3 km reach of the rio san salvador in the calama basin to determine the relationship between regional climate change and the incision of in stream wetlands the rio san salvador supported dense wetlands from 11 1 to 9 8 6 4 to 3 5 2 8 to 1 3 and 1 0 to 0 5 ka and incised at the end of each of these intervals comparison with other in stream wetland sequences in the atacama desert and with regional paleoclimatic archives indicates that in stream wetlands responded similarly to climatic changes by incising during periods of extended drought at 9 8 3 5 1 3 and 0 5 ka

monsoons itczs and the concept of the global monsoon earth s tropical and subtropical rainbands such as intertropical convergence zones itczs and monsoons are complex systems governed by both large scale constraints on the atmospheric general circulation and regional interactions with continents and orography and coupled to the ocean monsoons have historically been considered as regional large scale sea breeze circulations driven by land sea contrast more recently a perspective has emerged of a global monsoon a global scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation this results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone regional subsystems are embedded in this global monsoon localized by surface boundary conditions parallel with this much theoretical progress has been made on the fundamental dynamics of the seasonal hadley cells and convergence zones via the use of hierarchical modeling approaches including aquaplanets here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings after summarizing the dynamical and energetic balances that distinguish an itcz from a monsoon we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets limitations of current theories are discussed including the need for a better understanding of the influence of zonal asymmetries and transients on the large scale tropical circulation

seismic anisotropy from compositional banding in granulites from the deep magmatic arc of fiordland new zealand abstract we present calculated seismic velocities and anisotropies of mafic granulites and eclogites from the cretaceous deep lower crust 40 65 km of fiordland new zealand both rock types show a distinct foliation defined by cm scale compositional banding seismic properties are estimated using the asymptotic expansion homogenisation finite element aeh fe method that unlike the commonly used voigt reuss hill homogenisation incorporates the phase boundary network into calculations the predicted mean p and s wave velocities are consistent with previously published data for similar lithologies from other locations e g kohistan arc although we find higher than expected anisotropies a v p 5 0 8 0 a v s 3 0 6 5 and substantial s wave splitting along foliation planes in granulites this seismic signature of granulites results from a density and elasticity contrast between cm scale pyroxene garnet stringers and plagioclase matrix rather than from crystallographic orientations alone banded eclogites do not show elevated anisotropies as the contrast in density and elastic constants of garnet and pyroxene is too small the origin of compositional banding in fiordland granulites is primarily magmatic and structures described here are expected to be typical for the base of present day magmatic arcs hence we identify a new potential source of anisotropy within this geotectonic setting

geochronology and stratigraphy of late pleistocene lake cycles on the southern bolivian altiplano implications for causes of tropical climate change large paleolakes 33 000 60 000 km2 that once occupied the high altitude poopo coipasa and uyuni basins in southern bolivia 18 22 s provide evidence of major changes in low latitude moisture in these now dry or oligosaline basins extensive natural exposure reveals evidence for two deep lake and several minor lake cycles over the past 120 k y fifty three new u th and 87 new 14c dates provide a chronologic framework for changes in lake level deposits from the ouki deep lake cycle are extensively exposed in the poopo basin but no deep lakes are apparent in the record between 98 and 18 1 ka the ouki lake cycle was 80 m deep and nineteen u th dates place this deep lake cycle between 120 and 98 ka shallow lakes were present in the terminal uyuni basin between 95 and 80 ka salinas lake cycle at ca 46 inca huasi lake cycle and between 24 and 20 5 ka sajsi lake cycle the tauca deep lake cycle occurred between 18 1 and 14 1 ka resulting in the deepest 140 m and largest lake in the basin over the past 120 ka multiple 14c and u th dates constrain the highest stand of lake tauca along a topographically conspicuous shoreline between 16 4 and 14 1 ka a probable post tauca lake cycle the coipasa produced a 55 m deep lake that is tentatively dated between 13 and 11 ka we suggest that paleolakes on the bolivian altiplano expanded in response to increased moisture in the amazon and enhanced transport of that moisture onto the altiplano by strengthened trade winds or southward displacement of the intertropical convergence zone itcz pole to equator sea surface temperature sst and atmospheric gradients may have influenced the position of the itcz affecting moisture balance over the altiplano and at other locations in the amazon basin links between the position of the itcz and the ca 23 ka precessional solar cycle have been postulated march insolation over the altiplano is a relatively good fit to our lake record but no single season or latitude of solar cycling has yet to emerge as the primary driver of climate over the entire amazon basin temperature may influence altiplano lake levels indirectly as potentially dry glacial periods in the amazon basin are linked to dry conditions on the altiplano intensification of the trade winds associated with la nina like conditions currently brings increased precipitation on the altiplano and deep lake development during the tauca lake cycle coincided with apparently intense and persistent la nina like conditions in the central pacific this suggests that sst gradients in the pacific are also a major influence on deep lake development on the altiplano

late quaternary vegetation and climate history of a perennial river canyon in the rio salado basin 22 s of northern chile abstract plant macrofossils from 33 rodent middens sampled at three sites between 2910 and 3150 m elevation in the main canyon of the rio salado northern chile yield a unique record of vegetation and climate over the past 22 000 cal yr bp presence of low elevation prepuna taxa throughout the record suggests that mean annual temperature never cooled by more than 5 c and may have been near modern at 16 270 cal yr bp displacements in the lower limits of andean steppe and puna taxa indicate that mean annual rainfall was twice modern at 17 520 16 270 cal yr bp this pluvial event coincides with infilling of paleolake tauca on the bolivian altiplano increased enso activity inferred from a marine core near lima abrupt deglaciation in southern chile and heinrich event 1 moderate to large increases in precipitation also occurred at 11 770 9550 central atacama pluvial event 7330 6720 3490 2320 and at 800 cal yr bp desiccation occurred at 14 180 8910 8640 and 4865 cal yr bp compared to other midden sites in the region early holocene desiccation seems to have happened progressively earlier farther south emerging trends from the cumulative midden record in the central atacama agree at millennial timescales with improved paleolake chronologies for the bolivian altiplano implying common forcing through changes in equatorial pacific sea surface temperature gradients

albedo changes milankovitch forcing and late quaternary climate changes in the central andes late quaternary humidity changes resulted in substantial modifications of the land surface characteristics in the altiplano of the atacama desert central andes reconstructions of surface albedo top of atmosphere toa albedo and short wave net radiation in the andes of northern chile for 20 14 10 7 and 0 ka suggest that surface and toa albedo increased substantially during periods of relatively humid environmental conditions i e with large palaeolakes glaciers and dense vegetation the decrease of summer shortwave net radiation and seasonality during the late glacial early holocene humid phase 14 to 10 ka due to earth s surface and atmospheric characteristics added to the effect of orbitally driven negative deviations of southern hemisphere austral summer insolation and minimum seasonality at 20 s therefore in situ radiative forcing is in contrast to the northern hemisphere tropics not a suitable explanation for enhanced convective precipitation and ultimately humid climatic conditions our results suggest that late quaternary humidity changes on the altiplano reflect a collective response to 1 environmental changes in the source area of the moisture e g re expansion of the rain forest and increased release of latent heat over amazonia and the chaco warm sea surface temperatures in the e pacific and 2 large scale circulation patterns and wave structures in the upper troposphere strength and position of the bolivian high divergent flow stimulating convection over the altiplano or that they even reflect a response to 3 interhemispherical teleconnections

linear response of a stratified tropical atmosphere to convective forcing abstract the three dimensional response of the tropical atmosphere to an isolated heat source is investigated using a primitive equation model linearized about a resting basic state on an equatorial plane the model equations are solved by applying vertical and horizontal normal mode transforms the thermal forcing was chosen to simulate the convection which occurs over tropical south america during the southern hemisphere summer the vertical dependence of the forcing is such that the heating is zero at the top and bottom boundaries with a variable level of maximum beating the model results with steady forcing are compared with the average circulation over tropical south america for a 19 day period in the southern hemisphere summer of 1979 and it is shown that the model reproduces many aspects of the observed circulation the partition of the energy of the steady state response between the vertical modes is calculated and it is shown that the internal mode with an equivalent depth of 208 m is domina

the el nino impact on the summer monsoon in brazil regional processes versus remote influences the el nino impact on brazil s summer monsoon has not been adequately assessed through seasonal analysis because it shows significant subseasonal variations in this study the el nino influence on the summer monsoon circulation rainfall and temperature is analyzed with monthly resolution using data from a dense network of stations the expected precipitation percentiles during the monsoon season of el nino en events are calculated as well as anomalies of surface temperature and thermodynamic parameters this information is analyzed jointly with anomaly composites of several circulation parameters the analysis shows that some precipitation and circulation anomalies which are consistent and important during part of the season are smoothed out in a seasonal analysis there are abrupt changes of anomalies within the summer monsoon season suggesting the prevalence of regional processes over remote influences during part of the season the probable role of remote influences and regional processes is assessed the anomalous heat sources associated with el nino perturb the walker and hadley circulations over south america and generate rossby wave trains that produce important effects in the subtropics and extratropics in the early summer monsoon season remotely produced atmospheric perturbations prevail over brazil anticyclonic low level anomalies predominate over central east brazil in the tropics and subtropics due to the subsidence over the amazon and to rossby waves in the subtropics easterly moisture inflow from the atlantic is favored but diverted toward northern south america sa and south brazil there are negative precipitation anomalies in north and central east brazil and positive ones in south brazil these precipitation anomalies are favored by the perturbation in the walker and hadley circulation over the east pacific and south america and by a rossby wave train over southern sa that originates in the eastern pacific in january with the enhancement of the continental subtropical heat low by anomalous surface heating during the spring there is anomalous low level convergence and cyclonic circulation over southeast brazil while at the upper levels anomalies of divergence and anticyclonic circulation prevail this anomalous circulation directs moisture flux toward central east brazil causing moisture convergence in this region a favorable thermodynamic structure enhances precipitation over central east brazil the dry anomalies in north brazil are displaced northward and the anomalies in south brazil almost disappear in february after the above normal precipitation of january the surface temperature anomalies turn negative and the precipitation diminishes in central east brazil there are negative rainfall anomalies in north brazil and in the south atlantic convergence zone sacz and positive ones in south brazil influence function analysis shows that while the anomalies of circulation over southeast brazil in the spring of el nino years are mostly due to remote influences from the tropical east pacific those in january are probably due to local influence during this month the monsoonlike circulation is enhanced simultaneous and lagged correlation analysis of sst and rainfall in central east brazil shows that sst anomalies in the atlantic ocean off the southeastern coast of brazil fluctuate on the same timescale as the circulation and precipitation anomalies

spatio temporal variations in polylepis tarapacana radial growth across the bolivian altiplano during the 20th century abstract we document the dendroclimatological potential of polylepis tarapacana to estimate past temporal and spatial variations in precipitation across the bolivian altiplano 17 23 s the p tarapacana chronologies which presently range between 110 and 705 years in length represent the highest altitude tree ring records worldwide interannual variations in fourteen tree ring chronologies located between 3900 and 4850 m elevation were compared using correlation and principal component analyses there is a strong common signal among the chronologies for the common interval ad 1890 1999 three dominant patterns of tree growth across the bolivian altiplano were identified the first pattern which explains 52 of the total variance in tree growth is associated with years of above or below average tree growth across the whole region the second and third patterns which are related to significantly lower common variance 13 and 8 respectively are associated with contrasting positive and negative anomalies of growth between the northern and southern sectors and between the central western sector and the rest of the region respectively in order to determine the climatic significance of the spatial variations of p tarapacana growth the interannual patterns in tree growth were compared with spatial anomalies in precipitation across the region uniform patterns of above below average tree growth are associated with positive negative precipitation anomalies in the preceding year that are uniformly distributed across the whole altiplano in the westernmost areas of the altiplano summer rainfall accounts for more than 80 of total annual precipitation therefore relationships between tree growth and summer or annual precipitation are in most cases similar contrasting patterns of tree growth are related to opposite anomalies in precipitation during the preceding year in the different sub regions in the altiplano north versus south central versus north and south climatic preconditioning of tree growth was also identified for individual years for instance extremely reduced growth in the year 1967 reflects dry conditions in the two preceding years 1965 1966 long term intervals 3 or more years with reduced tree growth resulted from several consecutive years with low precipitation our results indicate that the network of tree ring chronologies of p tarapacana in the bolivian altiplano has the potential to provide annually resolved precipitation reconstructions in this region for the past 5 7 centuries this is a valuable source to document decadal to century scale precipitation variability in the altiplano and its relation to large scale ocean atmosphere circulation features

recharge to groundwater in the turi basin northern chile an evaluation based on tritium and chloride mass balance techniques summary the turi basin contains an unconfined phreatic aquifer within the toconce formation volcano sedimentary sequence that passes eastwards under the quaternary volcanic chain and outcrops in the altiplano the phreatic aquifer is floored by impermeable welded miocene ignimbrites precipitation captured by the volcanic edifices infiltrates through high permeability soils to recharge the phreatic aquifer the hydrochemistry of the groundwater in the phreatic aquifer shows little evidence of mixing with geothermal water and therefore approximates to a closed system tritium values demonstrate that recharge is occurring under current climatic conditions and takes around 40 years to pass 10 km from recharge areas on the slopes of the volcanoes and the altiplano to the center of the basin the chloride mass balance technique has been used to quantify precipitation recharge at 15 500 m3 d 1 this is significantly less than the flow through the phreatic aquifer 58 000 86 000 m3 d 1 calculated from its physical properties and the difference is interpreted to be due to subsurface inflow through the toconce formation from the altiplano the results of this study have wider application throughout the andean hydrological environment of northern chile

relationship between amazon and high andes rainfall 1 a regional climate model is used to study the connections between amazon and high andes rainfall the modeling effort focuses on understanding the rainfall response during the last glacial maximum lgm 21 000 years ago as paleoproxy evidence generally suggests that the high andes were wet during this period while conditions over the amazon were drier although there is more uncertainty about the latter analysis of the yearlong present day and full lgm integrations reveals that the delayed onset of amazon convection in the lgm during austral spring is associated with an enhancement of rainfall over the high andes in the absence of convection over the amazon in the austral spring zonal geopotential height gradients over the amazon strengthen resulting in anomalous easterly flow between the amazon and the high andes instead of the low level flow paralleling the andes e g northwesterly the flow impinges more directly onto the eastern slopes of the andes during the lgm where it rises and transports moisture from the amazon up into the mountains enhancing convection over the andes this circulation mechanism explains how the moisture and rainfall over the high andes of peru and bolivia can increase such as the case for the lgm despite large scale drying over south america present day observations are then explored for 1984 2002 and 2003 to identify whether mechanisms identified in the paleoclimate modeling are observable in the present day climate comparison of these particular springs reveals a response analogous to the lgm model response suggesting that this mechanism that connects tropical atlantic sea surface temperatures amazon convection and high andes rainfall is operating in the present climate

canyon incision and knickpoint propagation recorded by apatite 4he 3he thermochronometry abstract in many regions of the world deeply incised canyons demonstrate the net effects of physical processes active at earth s surface in response to surface uplift low temperature thermochronometry can constrain the timing and rates of bedrock incision which is necessary for relating canyon incision to surface uplift over geological timescales we analyzed four samples from the cotahuasi ocona canyon system in southwest peru using 4 he 3 he thermochronometry and we present a new inversion model to identify continuous low temperature cooling histories that are consistent with the observed data derived cooling histories limit the onset of fluvial incision to 13 to 8 ma this is in agreement with previously reported interpretations based on a three dimensional thermal model interpolation to a much larger set of conventional apatite u th he ages however because 4 he 3 he thermochronometry constrains an independent cooling history for each sample the results also permit testing of landscape evolution models with greater spatial variability in exhumation compared to those models that can be tested by a geographically scattered set of conventional u th he ages the different cooling histories of the four samples require asynchronous incision along at least part of the canyon system that is best explained by headward propagation of fluvial incision by knickpoint migration

new insights into the rainfall variability in the tropical andes on seasonal and interannual time scales in this study we analyze the atmospheric mechanisms associated with the main rainfall patterns in the tropical andes 20 circ hbox s 1 circ hbox n on seasonal and interannual time scales using a homogeneous and high spatial resolution precipitation data set 0 05 circ times 0 05 circ at monthly time step chirps 1981 2016 in situ precipitation from 206 rain gauge stations power spectrum and eof analysis we identify three andean regions characterized by specific seasonal and interannual rainfall modes the equatorial andes ea 5 circ hbox s 1 circ hbox n the transition zone tz 8 circ hbox s 5 circ hbox s and the southern tropical andes sta 20 circ hbox s 8 circ hbox s on seasonal time scales the main mode of precipitation in the ea and sta are characterized by a unimodal regime while the tz is represented by a bimodal regime the ea and the tz share the same wet season in the february april period which is associated with a weakened walker cell the southerly position of the intertropical convergence zone itcz and a strong westward humidity transport from the equatorial amazon this latter mechanism and a reduced elevation of the andes are associated with the october november wet season in the tz the presence of the bolivian high and the northward extension of the low level jet are associated with the precipitation over andean regions between 20 circ s and 8 circ s in the december march period on interannual time scales extreme monthly wet events emwe in the sta tz are related to convection over the western equatorial amazon during the december march february april period showing an atmospheric relationship between the amazon and the andes extreme monthly dry events emde in the tz and in the ea during the february april period are related to a strengthened walker cell especially in the eastern pacific in addition emwe emde in the ea are associated with an anomalous southward northward displaced eastern pacific itcz moreover we find a relationship between precipitation at higher elevations in the andes north of 10 circ hbox s and easterly winds at 200 hpa during february april emwe finally extreme monthly events in the ea sta are related to sea surface temperature anomalies in the western central equatorial pacific

the great atacama flood of 2001 and its implications for andean hydrology in february 2001 widespread flooding occurred throughout the atacama desert of northern chile and southern peru it was particularly severe in the rio loa basin where roads and bridges were disrupted and the town of calama inundated the instantaneous peak flow in the rio salado a tributary of the rio loa reached 310 m3 s 1 an order of magnitude higher than any previously recorded event the flood is estimated to have a return period of 100 200 years and is shown to have been caused by intense long duration rainfall in the western cordillera associated with la nina the surface water response is typical of arid areas and highly dependent on antecedent conditions but is quite different in perennial and ephemeral catchments ephemeral flood flows suffer high transmission losses recharging phreatic aquifers perennial rivers have lower runoff coefficients but baseflow levels remained high after the event for several months due to bank storage rebound and interflow extremely high energies of 3000 w m 2 were generated by the floods in the cordillera becoming less in the precordillera and downstream erosion and sediment transport were consequently highest in the upper and middle reaches of the rivers with mixed erosion deposition in the lowest reach the new insights gained from the interpretation and quantification of this event have important implications for palaeoenvironmental analysis hazard management water resource evaluation and the palaeohydrological evolution of the andes copyright 2005 john wiley sons ltd

a new estimate of the holocene lowstand level of lake titicaca central andes and implications for tropical palaeohydrology new evidence from piston cores and high resolution seismic reflection data shows that water levels in lake titicaca were as much as 100 m below the present level during the early to mid holocene between 6 and 3 8 14c kyr bp climatological and modelling studies indicate that lake titicaca rainfall depends on convective activity in upwind amazonia the lake level data therefore suggest a drier amazon basin during this time this view is bolstered by an excellent match between the titicaca lake level curve and decreased methane concentrations in greenland ice previously ascribed to drying of low latitude wetlands blunier et al 1995 the postglacial history of lake titicaca fits a global pattern of lake level change in the tropics characterized by opposite phasing between the southern and northern hemispheres this pattern is most likely the result of orbital controls over the intensity of summer insolation

hydrologic variation during the last 170 000 years in the southern hemisphere tropics of south america despite the hypothesized importance of the tropics in the global climate system few tropical paleoclimatic records extend to periods earlier than the last glacial maximum lgm about 20 000 years before present we present a well dated 170 000 year time series of hydrologic variation from the southern hemisphere tropics of south america that extends from modern times through most of the penultimate glacial period alternating mud and salt units in a core from salar de uyuni bolivia reflect alternations between wet and dry periods the most striking feature of the sequence is that the duration of paleolakes increased in the late quaternary this change may reflect increased precipitation geomorphic or tectonic processes that affected basin hydrology or some combination of both the dominance of salt between 170 000 and 140 000 yr ago indicates that much of the penultimate glacial period was dry in contrast to wet conditions in the lgm our analyses also suggest that the relative influence of insolation forcing on regional moisture budgets may have been stronger during the past 50 000 years than in earlier times d 2003 university of washington all rights reserved

linear stationary wave simulations of the time mean climatological flow abstract a spectral coordinate model linearized about a zonal mean flow is used to investigate the maintenance of the december february climatological stationary waves the model is generally run with 15 levels in the vertical and a t31 truncation the sum of the response to orography diabatic heating and transient eddy flux convergences specified from data gives a generally good northern hemisphere simulation at 200 mb with some deterioration in the lower troposphere the same is true for the southern hemisphere but only when the forcing terms from recent years are used the response is broken down into that associated with particular orographic features tropical and midlatitude heating and transient vorticity and heat flux convergence all of these are shown to be significant in particular orography accounts for approximately 30 of the total 200 mb planetary wave response in midlatitudes the diabatic heating maxima in the two storm tracks are important this heating is related to latent and se

on the origin of the bolivian high and related circulation features of the south american climate abstract the climatological structure in the upper tropospheric summertime circulation over south america is diagnosed using a gcm with and without south american topography a linear model and observational data emphasis is placed on understanding the origin of observed features such as the bolivian high and the accompanying nordeste low to the east results from the linear model indicate that these two features are generated in response to precipitation over the amazon basin central andes and south atlantic convergence zone with african precipitation also playing a crucial role in the formation of the nordeste low the direct mechanical and sensible heating effects of the andes are minimal acting only to induce a weak lee trough in midlatitudes and a shallow monsoonal circulation over the central andes in the gcm the effects of the andes include a strengthening of the bolivian high and northward shift of the nordeste low primarily through changes in the precipitation field the position of

vegetation precipitation controls on central andean topography climatic controls on fluvial landscapes are commonly characterized in terms of mean annual precipitation however physical erosion processes are driven by extreme events and are therefore more directly related to the intensity duration and frequency of individual rainfall events climate also influences erosional processes indirectly by controlling vegetation in this study we explore how interdependent climate and vegetation properties affect landscape morphology at the scale of the andean orogen the mean intensity duration and frequency of precipitation events are derived from the trmm 3b42v7 product relationships between mean hillslope gradients and precipitation event metrics mean annual precipitation vegetation and bedrock lithology in the central andes are examined by correlation analyses and multiple linear regression our results indicate that mean hillslope gradient correlates most strongly with percent vegetation cover r 0 56 where vegetation cover is less than 95 mean hillslope gradients increase with mean annual precipitation r 0 60 and vegetation cover r 0 69 where vegetation cover is dense 95 mean hillslope gradients increase with increasing elevation r 0 74 decreasing inter storm duration r 0 69 and decreasing precipitation intensity by 0 5 mm d 1 r 0 56 thus we conclude that at the orogen scale climate influences on topography are mediated by vegetation which itself is dependent on mean annual precipitation r 0 77 observations from the central andes are consistent with landscape evolution models in which hillslope gradients are a balance between rock uplift climatic erosional efficiency and erosional resistance of the landscape determined by bedrock lithology and vegetation

on the likelihood of tropical extratropical cloud bands in the south indian convergence zone during enso events abstractthe southern hemisphere subtropical convergence zones are important regions of rainfall in the subtropics the south indian ocean convergence zone sicz has the strongest seasonality and exhibits substantial interannual variability in strength and position during austral summer on synoptic time scales the sicz is a preferred region for the formation of tropical extratropical te cloud bands with local maxima over the southern african mainland and madagascar this study investigates how the seasonality in satellite observed cloud band frequency emerges from the interplay between the asynchronous seasonal cycles in convective instability and upper level flow as represented by reanalysis data these atmospheric mean states are diagnosed with a gross convective instability metric and a method to distinguish between subtropical and eddy driven jet axes month by month analysis of these diagnostics elucidates how mean state perturbations during enso events modify cloud band likelihood typically 1

vegetation invasions into absolute desert a 45 th000 yr rodent midden record from the calama salar de atacama basins northern chile lat 22 24 s plant macrofossils percentage abundance of grass taxa fecal pellet d13 c and plant cuticle contents from 49 fossil rodent middens dated by 14 c record changes in local vegetation and precipitation since 45 ka calibrated or measured to thousands of calendar years before present in the central atacama desert lat 228 248s of northern chile the midden sites are along the hyperarid upper margin 2400 3100 m of the absolute desert in an extreme environment sparsely vegetated by annual herbs and halophytic shrubs conditions between 40 and 22 ka may have been at least intermittently dry and possibly cooler as implied by four middens with low species richness we infer a large increase in summer rainfall between 16 2 and 10 5 ka on the basis of the lowering of steppe grasses by as much as 1000 m prominence of c4 grasses and summer annuals high species richness and displacement of northern species at least 50 km south of their modern ranges the precipitation increase was greatest for a cluster of middens between 11 8 and 10 5 ka abrupt drying evident in a dramatic decrease in grass abundance occurred after 10 5 ka at all four midden localities increased percentages of