Processus physiques associés à l'augmentation des précipitations d'été dans le Sud-Est de l'Amérique du Sud dans un scénario de réchauffement climatique

Southeastern South America (SESA) is one of the few subtropical regions where WCRP/CMIP3 coupled general circulation models project significant summer rainfall increases by the end of the twenty first century, in a global warming scenario. This signal is revealed to be associated with a frequency increase of positive phase of the leading pattern of austral summer rainfall variability in the region. The positive phase manifests as austral summer rainfall above (below) normal in the SESA (South Atlantic convergence zone) region. Moreover, the rainfall pattern change is shown to be associated with positive anomalies of the sea surface temperature (SST) in the equatorial Pacific. This result is confirmed by numerical sensitivity experiments performed with the LMDZ4 " two-way nesting " interactive climate models system, which also showed that the projected rainfall increase in SESA is mainly associated to the zonally asymmetric pattern of the tropical SST warming.Le Sud-Est de l'Amérique du Sud (SESA) est l'une des rares régions subtropicales où les modèles climatiques du WCRP/CMIP3 projettent une augmentation significative des précipitations en été austral pour la fin du XXIème siècle, dans un scénario de réchauffement climatique. Ce signal est associé à une augmentation de la fréquence des étés identifiés comme des phases positives du mode dominant de la variabilité des précipitations dans la région, ce mode étant défini par des précipitations au-dessus (en-dessous) de la normale dans le SESA (zone de convergence d'Atlantique Sud). Cette tendance est associée à une augmentation de la température de surface de la mer (SST) dans le Pacifique équatorial. Cela est confirmé par des expériences de simulation numérique effectuées avec le système interactif two-way nesting du LMDZ4, qui montrent aussi que l'augmentation projetée des précipitations dans le SESA est associée au signal zonalement asymétrique du réchauffement des SST

Extreme austral winter precipitation events over the South-American Altiplano: regional atmospheric features

International audienceThe South American Altiplano has a marked dry season during the austral winter (June to August, JJA). However, during this season synoptic meteorological conditions triggering heavy precipitation can damage socioeconomic activities, often causing the loss of human lives. Using daily in-situ precipitation data from 39 rain-gauge stations over the northern Altiplano (18∘S -15∘S ; >3000 m.a.s.l.) for the JJA season, we computed the historical percentile 90 (p90) and we identified extreme rainy days with precipitation higher than p90 in the 1980-2010 period. We identified 100 winter extreme precipitation events (WEPEs) over this region that can last between one to 16 days. The K-means analysis was applied to anomalies of geopotential height at 500 hPa from ERA-Interim data during the initial day or Day(0) of WEPEs lasting 1 day (42 cases), 2 days (19) and more than 2 days (39). We found 59 WEPEs characterized by an upper-level trough over the Peruvian-Chilean coast. At 850 hPa, these 59 WEPEs are also associated with cold surges along the eastern Central Andes, indicating an association between the upper-level trough and the cold surge in developing deep convection over the northern Altiplano. A lead-lag composite analysis further showed a significant lower- and mid-tropospheric moistening over the western Amazon 2 days before the onset of these 59 WEPEs, due to low-level northerly wind anomalies originating over equatorial South America. The other 41 WEPEs are associated with a low-level southerly wind regime crossing the equator and a mid-and upper-level low-pressure system over the Peruvian-Chilean coast. While the low-level southerly regime enhances mid-tropospheric moisture transport from the equator towards the Altiplano due to the developed shallow meridional circulation when propagating equatorward, a low-pressure system promotes intensification of upward motion, boosting the upslope moisture transport from the lowlands to the east of the Central Andes towards the Altiplano

Valley–Mountain Circulation Associated with the Diurnal Cycle of Precipitation in the Tropical Andes (Santa River Basin, Peru)

The Cordillera Blanca (central Andes of Peru) represents the largest concentration of tropical glaciers in the world. The atmospheric processes related to precipitations are still scarcely studied in this region. The main objective of this study is to understand the atmospheric processes of interaction between local and regional scales controlling the diurnal cycle of precipitation over the Santa River basin located between the Cordillera Blanca and the Cordillera Negra. The rainy season (December–March) of 2012–2013 is chosen to perform simulations with the WRF (Weather Research and Forecasting) model, with two domains at 6 km (WRF-6 km) and 2 km (WRF-2 km) horizontal resolutions, forced by ERA5. WRF-2 km precipitation shows a clear improvement over WRF-6 km in terms of the daily mean and diurnal cycle, compared to in situ observations. WRF-2 km shows that the moisture from the Pacific Ocean is a key process modulating the diurnal cycle of precipitation over the Santa River basin in interaction with moisture fluxes from the Amazon basin. In particular, a channeling thermally orographic flow is described as controlling the afternoon precipitation along the Santa valley. In addition, in the highest parts of the Santa River basin (in both cordilleras) and the southern part, maximum precipitation occurs earlier than the lowest parts and the bottom of the valley in the central part of the basin, associated with the intensification of the channeling flow by upslope cross-valley winds during mid-afternoon and its decrease during late afternoon/early night

Valley–Mountain Circulation Associated with the Diurnal Cycle of Precipitation in the Tropical Andes (Santa River Basin, Peru)

The Cordillera Blanca (central Andes of Peru) represents the largest concentration of tropical glaciers in the world. The atmospheric processes related to precipitations are still scarcely studied in this region. The main objective of this study is to understand the atmospheric processes of interaction between local and regional scales controlling the diurnal cycle of precipitation over the Santa River basin located between the Cordillera Blanca and the Cordillera Negra. The rainy season (December–March) of 2012–2013 is chosen to perform simulations with the WRF (Weather Research and Forecasting) model, with two domains at 6 km (WRF-6 km) and 2 km (WRF-2 km) horizontal resolutions, forced by ERA5. WRF-2 km precipitation shows a clear improvement over WRF-6 km in terms of the daily mean and diurnal cycle, compared to in situ observations. WRF-2 km shows that the moisture from the Pacific Ocean is a key process modulating the diurnal cycle of precipitation over the Santa River basin in interaction with moisture fluxes from the Amazon basin. In particular, a channeling thermally orographic flow is described as controlling the afternoon precipitation along the Santa valley. In addition, in the highest parts of the Santa River basin (in both cordilleras) and the southern part, maximum precipitation occurs earlier than the lowest parts and the bottom of the valley in the central part of the basin, associated with the intensification of the channeling flow by upslope cross-valley winds during mid-afternoon and its decrease during late afternoon/early night

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.IDEX grants of University Grenoble Alpes (UGA) French National Research Agency (ANR) ANR-15-IDEX-02 French National Research Agency (ANR) ANR-18-MPGA-0008 French AMANECER-MOPGA project - IRD ANR-18-MPGA-0008 French National Research Agency (ANR) ANR10 LABX5

Future Projections of Low-Level Atmospheric Circulation Patterns Over South Tropical South America: Impacts on Precipitation and Amazon Dry Season Length

International audienceThe last few decades have shown evidence of a lengthening dry season in southern Amazonia, which is associated with a delay in the onset of the South American Monsoon System (SAMS). Using a pattern recognition framework of atmospheric circulation patterns (CPs), previous studies have identified specific atmospheric situations related to the onset of the SAMS. Here, we analyze the future changes in the CPs that largely define the main hydro-climatological states of Tropical South America. We evaluated the CP changes that occurred between two periods: historical (1970-2000) and future (2040-2070), using six General Circulation Models (GCMs) from the Coupled Model Intercomparison Project Phase 6. Future GCM projections show significant spatio-temporal changes in the CPs associated with the dry season in southern Amazonia during the mid-21st century. These changes are related to both a late onset of the SAMS and an early demise of the SAMS. Particularly, the CP methodology allowed for a better understanding of the behavior of the southern Amazon dry season under future conditions, showing an increase in the frequency of the CPs typically observed during the dry season. The occurrence of dry days in the Amazon basin during the austral winter of the mid-21st century increases by 19.4% on average, with respect to the historical period. This methodology also identified a future increase in the frequency of dry CPs, both at the beginning of the dry-to-wet transition period (8%) and at the end of the wet-to-dry transition season (11%)

New insights into the rainfall variability in the tropical Andes on seasonal and interannual time scales

International audienc

Changes in the surface and atmospheric water budget due to projected Amazon deforestation: Lessons from a fully coupled model simulation

International audienceThe Amazon forest has a complex interaction with climate at different spatial and temporal scales. This means that alterations in land use could modify the regional water cycle, including the surface and atmospheric water budget. However, little is known about how these changes occur seasonally and in a spatially distributed manner in the most vulnerable regions, such as the southern Amazon. In this study, the local to regional effects of future Amazon deforestation on the surface and atmospheric water budget components are investigated by twin numerical experiments using the Regional Earth System Model of the 'Institute Pierre Simone Laplace' (RegIPSL) for 19 yr (2001-2019). The results show that significant changes in precipitation and actual evapotranspiration in the southern Amazon (south of 5°S) are associated with surrounding areas with a deforested ratio higher than 40%. During the onset of the wet season (September-November) the largest changes in convective processes are manifested by opposite atmospheric dynamic in adjacent regions (dipole), associated with. This dynamic is associated with wind orientation and the different sizes of the straight corridors of continuous deforestation (pathways). The dipole manifests itself as a suppression of convection in the upwind sector, while convection increases in the downwind sector of the deforestation pathway. For medium-sized deforestation pathways (∼350 km) convection changes are related to dynamic processes (decrease in surface roughness). In large-sized pathways (∼500 km) the mechanisms causing convective changes are combined, dynamic and thermal (increase in surface temperature). In deforested regions there is an average increase of terrestrial water storage dynamics and runoff ∼10 times higher than in non-deforested regions. Furthermore, the atmosphere becomes ∼8 times drier in deforested regions than in non-deforested regions. Our findings indicate a new perspective regarding a comprehensive modeling approach to understand potential changes in the surface and atmospheric water cycle in different regions of Amazonia and in different seasons due to future deforestation and thus provide new insights into their spatial and temporal variability at sub-regional scales

A regional view of the linkages between hydro‐climatic changes and deforestation in the Southern Amazon

International audienceIn the last four decades, the Southern Amazon (south of 8S) has shown changes in the spatial and temporal patterns of its hydro-climatic components, leading to drier conditions. Due to climate and land-use changes, this region is considered as a zone under biophysical transition processes. Previous studies have documented a complex interaction between climate and deforestation either on a large-scale or based on limited in situ data, typically covering the Brazilian Amazon. In this study, we analyse the relationships between hydro-climate, the surface waterenergy partitioning and an index of regional forest cover change for the period 1981–2018. Additionally, we discretized three regions covering the Bolivian Amazon and the southern portions of the Peruvian and Brazilian Amazon due to their differences in the evolution of land use. In the Bolivian region, a high ratio of forest cover change, exceeding 40–50%, is related to a significant tendency to become water-limited. This change is associated with decreased rainfall, increased potential evapotranspiration and decreased actual evapotranspiration. Regardless of the region analysed, those that are characterized by a high ratio of forest cover change (>40–50%) show growing imbalance between increasing potential and decreasing actual evapotranspiration. However, in the Peruvian and Brazilian regions, hydroclimatic conditions remain energy-limited due to minor rainfall changes. The observed differences in surface water-energy partitioning behaviour evidence a complex dependence of both sub-regional (i.e., land cover changes) and large-scale (i.e., strengthening of the Walker and Hadley circulations) conditions. Ourfindings indicate a clear link between hydro-climatic changes and deforestation, providing a new perspective on their spatial variability on a sub-regional scale