1. Control geológico y climático del sistema Andino-Amazónico y de su biodiversidad

Los excepcionales recursos naturales de la Amazonia son el fruto de una larga historia geológica y climática en la que los Andes juegan un papel determinante. Desde su creación hasta la actualidad, el Impacto de esta cadena montañosa sobre el ambiente de la reglón ha dado forma a las faunas y floras sucesivas y sus distribuciones. Regulador de las precipitaciones y de la fantástica biodiversidad animal y vegetal de la región, el sistema geoclimático andino-amazónico debe ser considerado como un elemento fundamental a la hora de estudiar el impacto del cambio climático en la región.Les ressources naturelles exceptionnelles de l’Amazonie sont le fruit d’une longue histoire géologique et climatique où les Andes jouent un rôle déterminant. Depuis son apparition jusqu’á nos jours, l’impact de cette chaîne de montagne sur l’environnement de la région a donné forme aux faunes et flores successives et leurs distributions. Régulateur des précipitations et de la fantastique biodiversité de la région, le système géo-climatique andino-amazonien doit être considéré comme un élément fondamental si l’on souhaite étudier l’impact du changement climatique dans la région.The extraordinary natural resources of the Amazon region are the result of a long geological and climatic history, where the Andes play a decisive role. This mountain range has impacted on the environment of the region, arraying Flora and Fauna, and its successive distribution since its origin to the present. As a regulator of rainfalls and of the fantastic animal and vegetal biodiversity of the region, the Andean-Amazon geo-climate system must be considered as an essential element to study the Climate Change impact in the region

Impacts of topography and land use changes on the air surface temperature and precipitation over the central Peruvian Andes

This paper focuses on the representation of the air surface temperature and precipitation using high spatiotemporal simulations (3 km-1 h) of the WRF3.7.1 model in the central Peruvian area. It covers, from east to west, the coastal zone, the western slope of the Andes, the Andean Mantaro basin (500-5000 masl), and the Andes-Amazon transition region in the eastern Andes. The study covers the January months from 2004 to 2008. Three experiments were conducted using different topography and land use data sources: (1) a control simulation using the default WRF topography and land use datasets from the United States Geological Survey (USGS); (2) a simulation changing only the topography by using the SRTM topography dataset; and (3) a simulation changing the land use data of (2) by a new dataset adapted from Eva et al. (2004). SRTM topography performed better than the control simulation for representing the actual altitudes of 57 meteorological stations that were used for precipitation and surface air temperature data. As a result, the simulations of experiments (2) and (3) produced lower bias values than that of (1). Topography change (experiment (2)) showed improvements in temperature bias that were directly associated with linear modifications of -5.6 and -6.7 degrees C.km(-1) in minimum and maximum temperature, respectively. Increasing (decreasing) precipitation with topography or land use change was clearly controlled by changes in the moisture flux patterns and its convergence (divergence) in the Andes-Amazon transition. On the western slope, precipitation increase could be associated with the increase in easterly flow by the smaller altitudes of the Andes mountains in SRTM topography and by increasing evaporation with new land use. Inside the Mantaro Basin, low level moisture flux seems to control the rainfall changes. Overall, relative changes (positive or negative) in precipitation due to topography or land use change could reach values above 25%

Evidencing decadal and interdecadal hydroclimatic variability over the Central Andes

In this study we identified a significant low frequency variability (8 to 20 years) that characterizes the hydroclimatology over the Central Andes. Decadal-interdecadal variability is related to the central-western Pacific Ocean (R-2 = 0.50) and the zonal wind at 200 hPa above the Central Andes (R-2 = 0.66). These two oceanic-atmospheric variables have a dominant decadal-interdecadal variability, and there is a strong relationship between them at a low frequency time scale (R-2 = 0.66). During warming decades in the central-western Pacific Ocean, westerlies are intensified at 200 hPa above the Central Andes, which produce decadal periods of hydrological deficit over this region. In contrast, when the central- western Pacific Ocean is cooler than usual, easterly anomalies prevail over the Central Andes, which are associated with decades of positive hydrological anomalies over this region. Our results indicate that impacts of El Nino on hydrology over the Central Andes could be influenced by the low frequency variability documented in this study

Summer precipitation variability over Southeastern South America in a global warming scenario

International audienceDecember-January-February (DJF) rainfall variability in southeastern South America (SESA) is studied in 18 coupled general circulation models from the WCRP/CMIP3 dataset, for present climate and the SRES-A1B climate change scenario. The analysis is made in terms of properties of the first leading pattern of rainfall variability in the region, characterized by a dipole-like structure with centers of action in the SESA and South Atlantic Convergence Zone (SACZ) regions. The study was performed to address two issues: how rainfall variability in SESA would change in a future climate and how much of that change explains the projected increasing trends in the summer mean rainfall in SESA identified in previous works. Positive (negative) dipole events were identified as those DJF seasons with above (below) normal rainfall in SESA and below (above) normal rainfall in the SACZ region. Results obtained from the multi-model ensemble confirm that future rainfall variability in SESA has a strong projection on the changes of seasonal dipole pattern activity, associated with an increase of the frequency of the positive phase. In addition, the frequency increase of positive dipole phase in the twenty first century seems to be associated with an increase of both frequency and intensity of positive SST anomalies in the equatorial Pacific, and with a Rossby wave train-like anomaly pattern linking that ocean basin to South America, which regionally induces favorable conditions for moisture transport convergence and rainfall increase in SESA. © 2011 Springer-Verlag

Impact of projected SST changes on summer rainfall in southeastern South America

International audienceRecent studies have shown that global warming and associated sea-surface temperature (SST) changes may trigger an important rainfall increase in southeastern South America (SESA) during the austral summer (December-January-February, DJF). The goal of this paper is to provide some insight into processes which may link global and SESA changes. For this purpose, a "two-way nesting" system coupling interactively the regional and global versions of the LMDZ4 atmospheric model is used to study the response to prescribed SST changes. The regional model is a variable-grid version of the global model, with a zoom focused over South America. An ensemble of simulations forced by distinct patterns of DJF SST changes has been carried out using a decomposition of full SST changes into their longitudinal and latitudinal components. The full SST changes are based on projections for the end of the twenty-first century from a multi-model ensemble of WCRP/CMIP3. Results confirm the presence of a major rainfall dipole structure, characterized by an increase in SESA and a decrease in the South Atlantic Convergence Zone region. Rainfall changes found in the WCRP/CMIP3 models are largely explained as a response of this dipole structure to the zonally-asymmetric (or longitudinal) component of SST changes. The rainfall response to the zonal-mean (or latitudinal) SST changes (including the global warming signal itself) shows an opposite contribution. The processes explaining the role of zonally-asymmetric SST changes involve remote effects of SST warming over the equatorial Indian and Pacific oceans inducing an atmospheric wave-train extended across the South Pacific into South America. © 2013 Springer-Verlag Berlin Heidelberg

Influence of South America orography on summertime precipitation in Southeastern South America. Climate Dynamics

Impacts of the main South American orographic structures (the Andes, the Brazilian Plateau and the Guiana shield) on the regional climate and associated global teleconnection are investigated through numerical experiments in which some of these features are suppressed. Simulations are performed with a ‘‘two-way nesting’’ system coupling interactively the regional and global versions of the LMDZ4 atmospheric general circulation model. At regional scale, the simulations confirm previous studies, showing that both the Andes and the Brazilian Plateau exert a control on the position and strength of the South Atlantic convergence zone (SACZ), mainly through their impact on the low-level jet and the coastal branch of the subtropical anticyclones. The northern topography of South America appears to be crucial to determine the leading mode of rainfall variability in eastern South America, which manifests itself as a dipole-like pattern between Southeastern South America and the SACZ region. The suppression of South America orography also shows global-scale effects, corresponding to an adjustment of the global circulation system. Changes in atmospheric circulation and precipitation are found in remote areas on the globe, being the consequences of various teleconnection mechanisms. When the Brazilian Plateau and the Andes are suppressed, there is a decrease of precipitation in the SACZ region, associated with a weakening of the large-scale ascendance. Changes are described in terms of anomalies in the Walker circulation, meridional displacements of the mid-latitude jet stream, Southern annular mode anomalies and modifications of Rossby wave train teleconnection processes.Fil: Junquas, C.. Instituto Geofísico del Perú ; Perú. Centre National de la Recherche Scientifique; FranciaFil: Li, L.. Institut Pierre Simon Laplace; Francia. Centre National de la Recherche Scientifique; FranciaFil: Vera, Carolina Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; ArgentinaFil: Le Treut, H.. Institut Pierre Simon Laplace; Francia. Centre National de la Recherche Scientifique; FranciaFil: Takahashi, K.. Instituto Geofísico del Perú ; Per

Recent changes in the precipitation-driving processes over the southern tropical Andes/western Amazon

Analyzing December-February (DJF) precipitation in the southern tropical Andes-STA (12 circle S}; > 3000 m.a.s.l) allows revisiting regional atmospheric circulation features accounting for its interannual variability over the past 35 years (1982-2018). In a region where in-situ rainfall stations are sparse, the CHIRPS precipitation product is used to identify the first mode of interannual DJF precipitation variability (PC1-Andes). A network of 98 rain-gauge stations further allows verifying that PC1-Andes properly represents the spatio-temporal rainfall distribution over the region; in particular a significant increase in DJF precipitation over the period of study is evident in both in-situ data and PC1-Andes. Using the ERA-Interim data set, we found that aside from the well-known relationship between precipitation and upper-level easterlies over the STA, PC1-Andes is also associated with upward motion over the western Amazon (WA), a link that has not been reported before. The ascent over the WA is a component of the meridional circulation between the tropical North Atlantic and western tropical South America-WTSA (80 circle W). Indeed, the precipitation increase over the last 2 decades is concomitant with the strengthening of this meridional circulation. An intensified upward motion over the WA has moistened the mid-troposphere over WTSA, and as a consequence, a decreased atmospheric stability between the mid- and the upper troposphere is observed over this region, including the STA. We further show that, over the last 15 years or so, the year-to-year variability of STA precipitation (periodicity < 8 years) has been significantly associated with upward motion over the WA, while upper-level easterlies are no longer significantly correlated with precipitation. These observations suggests that the STA have experienced a transition from a dry to a wet state in association with a change in the dominant mode of atmospheric circulation. In the former dominant state, zonal advection of momentum and moisture from the central Amazon, associated with upper-level easterlies, is necessary to develop convection over the STA. Since the beginning of the 21st century, DJF precipitation over the STA seems to respond directly and primarily to upward motion over the WA. Beyond improving our understanding of the factors influencing STA precipitation nowadays, these results point to the need of exploring their possible implications for the long-term evolution of precipitation in a context of global warming

Understanding the influence of orography on the precipitation diurnal cycle and the associated atmospheric processes in the central Andes

In the tropical Andes, the identification of the present synoptic mechanisms associated with the diurnal cycle of precipitation and its interaction with orography is a key step to understand how the atmospheric circulation influences the patterns of precipitation variability on longer time-scales. In particular we aim to better understand the combination of the local and regional mechanisms controlling the diurnal cycle of summertime (DJF) precipitation in the Northern Central Andes (NCA) region of Southern Peru. A climatology of the diurnal cycle is obtained from 15 wet seasons (2000-2014) of 3-hourly TRMM-3B42 data (0.25A degrees x 0.25A degrees) and swath data from the TRMM-2A25 precipitation radar product (5 km x 5 km). The main findings are: (1) in the NCA region, the diurnal cycle shows a maximum precipitation occurring during the day (night) in the western (eastern) side of the Andes highlands, (2) in the valleys of the Cuzco region and in the Amazon slope of the Andes the maximum (minimum) precipitation occurs during the night (day). The WRF (Weather Research and Forecasting) regional atmospheric model is used to simulate the mean diurnal cycle in the NCA region for the same period at 27 km and 9 km horizontal grid spacing and 3-hourly output, and at 3 km only for the month of January 2010 in the Cuzco valleys. Sensitivity experiments were also performed to investigate the effect of the topography on the observed rainfall patterns. The model reproduces the main diurnal precipitation features. The main atmospheric processes identified are: (1) the presence of a regional-scale cyclonic circulation strengthening during the afternoon, (2) diurnal thermally driven circulations at local scale, including upslope (downslope) wind and moisture transport during the day (night), (3) channelization of the upslope moisture transport from the Amazon along the Apurimac valleys toward the western part of the cordillera

Circulation Patterns and Associated Rainfall Over South Tropical South America: GCMs Evaluation During the Dry‐To‐Wet Transition Season

International audienceSouth Tropical South America (STSA), extended approximately between 10°N-30°S and 90°W-30°W, is a wide region where diverse interactions among biomass, land surface processes and atmospheric convection take place. These interactions modulate the local and regional climate and directly impact on the socio-environmental activities (Fu et al., 2013; Reis et al., 2018; Zhang et al., 2015). STSA hosts the Amazonia-the world's largest rainforest and one of the major sources of evapotranspiration-playing a critical role in the global balances of energy, water, moisture and carbon (Gatti et al., 2021; Llopart et al., 2020). The region presents unique biodiversity and geographical patterns, mainly due to the interaction of the Amazonia and the Andes mountain range, which have deep implications in the atmospheric dynamics, moisture transport and river discharge not only throughout STSA but also in remote regions of the continent (Arias, Garreaud, et al., 2021; Espinoza et al., 2020; Sierra et al., 2021). Precipitation (PP) over STSA presents a marked spatio-temporal variability, strongly controlled by the South American Monsoon System (SAMS), with rainfall maxima during its active phase during the austral summer (Marengo et al., 2012; Vera et al., 2006). The monsoonal circulation-which develops in response to seasonal changes in thermal land-sea contrasts-is connected to different documented atmospheric features, including a NW-SE band of convergence and convective activity over the southeast of South America and the adjacent South Atlantic ocean known as the South Atlantic Convergence Zone (SACZ), an anticyclonic center locate