South American Low-level Jet Research Articles

Influence of Eastern, Central and Mix El Niño on the variability of rainfall in southeastern South America

El Niño/Southern Oscillation (ENSO) plays a very important role in modulating rainfall variability in South America. Although the effects of conventional ENSO are well known, there are significant variations in rainfall patterns when sea surface temperature anomalies manifest over the different regions along the equatorial Pacific, namely, Niño 3 (East), Niño 3 and Niño 4 (Mix), and Niño 4 (Central) ENSO. This work aims to analyze the variability of precipitation in southeastern South America associated with different types of El Niño (EN). For this, composite analyzes of precipitation, winds at low and high levels, vertical velocity and geopotential height were carried out. The results indicate that the rainfall pattern associated with Eastern Pacific (EP) El Niño shows the classic features expected from EN described in previous literature, i.e., a stronger and well-defined positive precipitation anomaly over southeast South America, while Central Pacific (CP) El Niño shows weaker and even an opposite sign over the region. Mix (MX) El Niño events present a spatial pattern of precipitation anomalies similar to EP El Niño events, but with lower intensity in the study area. The heterogeneity of EN-related precipitation effects results from a change in large-scale atmospheric circulation patterns: the Walker cell in the tropics, the Rossby waves in the extratropics, and the circulation of low and high-level jet streams. EP El Niño events feature a configuration similar to the South American Low-Level Jet, the strengthening of the subtropical jet, better structured Rossby waves and the Pacific-South America pattern. These characteristics are observed in MX El Niños, but in a weaker way compared to EP El Niños. CP El Niño events, on the other hand, present a weakening and spatial displacement of the aforementioned conditions, which is reflected in negative precipitation anomalies in most seasons. Thus, more significant rainfall variations are noted between the EP El Niños and CP El Niños, with a predominance of positive and negative rainfall anomalies in the study area, respectively.

Influences of the South American Low-Level Jet on the Convective Environment in Central Argentina Using a Convection-Permitting Simulation

Abstract This study documents the spatial and temporal distribution of the South American low-level jet (SALLJ) and quantifies its impact on the convective environment using a 6.5-month convection-permitting simulation during the Remote Sensing of Electrification, Lightning, And Mesoscale/Microscale Processes with Adaptive Ground Observations and Clouds, Aerosols, and Complex Terrain Interactions (RELAMPAGO-CACTI) campaigns. Overall, the simulation reproduces the observed SALLJ characteristics in central Argentina near the Sierras de Córdoba (SDC), a focal point for terrain-focused upscale growth. SALLJs most frequently occur in the summer with maxima to the northwest and east of the SDC and minima over the higher terrain. The shallower SALLJs (<1750 m) have a strong overnight skew, while the elevated jets are more equally spread throughout the day. SALLJ periods often have higher amounts of low-level moisture and instability compared to non-SALLJ periods, with these impacts increasing over time when the SALLJ is present and decreasing afterward. The SALLJ may enhance low-level wind shear magnitudes (particularly when accounting for the jet height); however, enhancement is somewhat limited due to the presence of speed shear in most situations. SALLJ periods are associated with low-level directional shear favorable for organized convection and an orientation of cloud-layer wind shear parallel to the terrain, which could favor upscale growth. A case study is shown in which the SALLJ influenced both the magnitude and direction of wind shear concurrent with convective upscale growth near the SDC. This study highlights the complex relationship between the SALLJ and its impacts during periods of widespread convection. Significance Statement Areas of enhanced low-level winds, or low-level jets, likely promote favorable conditions for upscale growth, the processes by which storms grow larger. Central Argentina is an ideal place to study the influence of low-level jets on upscale growth as storms often stay connected to the Sierras de Córdoba Mountain range, growing over a relatively small area. This study uses model data to describe the distribution and impact of the South American low-level jet on the storm environment. The South American low-level jet is frequently found near the Sierras de Córdoba, and moisture and convective instability increase when it is present. However, the jet’s impact on other conditions important for upscale growth, such as vertical wind shear, is not as straightforward.

Tornadoes in Southeast South America: Mesoscale to Planetary-Scale Environments

Abstract A multiscale analysis of the environment supporting tornadoes in southeast South America (SESA) was conducted based on a self-constructed database of 74 reports. Composites of environmental and convective parameters from ERA5 were generated relative to tornado events. The distribution of the reported tornadoes maximizes over the Argentine plains, while events are rare close to the Andes and south of Sierras de Córdoba. Events are relatively common in all seasons except in winter. Proximity environment evolution shows enhanced instability, deep-layer vertical wind shear, storm-relative helicity, reduced convective inhibition, and a lowered lifting condensation level before or during the development of tornadic storms in SESA. No consistent signal in low-level wind shear is seen during tornado occurrence. However, a curved hodograph with counterclockwise rotation is present. The Significant Tornado Parameter (STP) is also maximized prior to tornadogenesis, most strongly associated with enhanced CAPE. Differences in the convective environment between tornadoes in SESA and the U.S. Great Plains are discussed. On the synoptic scale, tornado events are associated with a strong anomalous trough crossing the southern Andes that triggers lee cyclogenesis, subsequently enhancing the South American low-level jet (SALLJ) that increases moisture advection to support deep convection. This synoptic trough also enhances vertical shear that, along with enhanced instability, sustains organized convection capable of producing tornadic storms. At planetary scales, the tornadic environment is modulated by Rossby wave trains that appear to be forced by convection near northern Australia. Madden–Julian oscillation phase 3 preferentially occurs 1–2 weeks ahead of tornado occurrence. Significance Statement The main goal of this study is to describe what atmospheric conditions (from local to global scales) are present prior to and during tornadic storms impacting southeast South America (SESA). Increasing potential for deep convection, wind shear, and potential for rotating updrafts, as well as reducing convective inhibition and cloud-base height, are predominant a few hours before and during the events in connection to low-level northerly winds enhancing moisture transport to the region. Remote convective activity near northern Australia appears to influence large-scale atmospheric circulation that subsequently triggers convective storms supporting tornadogenesis 1–2 weeks later in SESA. Our findings highlight the importance of accounting for atmospheric processes occurring at different scales to understand and predict tornado occurrences.

Multiyear versus single‐year El Niño events: Contrasting their impacts on South American seasonal precipitation

AbstractDifferences in the seasonal distribution of precipitation over South America (SA) associated with single‐year (SY) and multiyear (MY) El Niño (EN) events were analysed based on reanalysis data for the 1901–2012 period. The results suggest that Pacific Sea Surface Temperature (SST) anomalies associated with MY EN events interact with the tropical Atlantic and Indian Oceans SST from austral fall to spring, after the event's first year, affecting SA precipitation distribution in subsequent seasons. Compared to SY EN events, which reproduce well the north–south dipolar precipitation anomaly pattern over SA, the MY EN events show differences in the intensity and positioning of precipitation anomalies. Precipitation decreases over northern SA during all seasons are observed for SY and MY EN events except for differences in magnitudes. Variations in the position and longitudinal extension of the downward motions of Walker circulation in response to these events explain these differences. Over southern and southeastern SA, differences in anomaly positioning are more evident. The positive precipitation anomalies over these regions in the austral spring and summer are weakened and southward shifted during the MY EN in relation to those during SY EN events. These variations are associated with the Rossby‐wave train pattern path that depends on the EN and season. Consequently, the associated local atmospheric circulation patterns also depend on the season. The intense (weak) South American low‐level jet (SALLJ) for the first year of MY EN contrasts with the weak or inexistence (intense) SALLJ for the SY EN during summer (springer). Complementary to previous studies, the results indicate that the differences in the intensity and duration of the SY and MY EN events contribute to changes in the EN‐related atmospheric teleconnection pattern that impacts SA precipitation. The results can be useful for climate prediction and monitoring purposes.

Two Case Studies of the Northwestern Argentinean Low: With and without a Coupled Transient Trough

The Northwestern Argentinean Low (NAL) intensifies the zonal component of the geopotential gradient to its east, intensifying the meridional wind and moisture flow from the tropics to the subtropics contibuting importantly to the South American Low-Level Jet and the Chaco Jet. This study compares two situations where the NAL is present in the continent’s subtropics: one where it is coupled with an extratropical transient trough to its south and another without this coupling. The coupled case, called the front case, is stronger in two ways: it is warmer and shows lower geopotential at the center of the NAL. It also shows much stronger moisture transport. Both cases show a similar trait in that moisture transport and the zonal component of the geopotential gradient east of the NAL peak in the coolest hours. The geopotential at the center of the no-front case remains roughly constant throughout the event. For the front case, it attains its minimum value in the six hours preceding the northeastward advance of the transient trough. These results suggest that there are different mechanisms acting at the center of the NAL and at its eastern branch, as well as in the front and no-front cases.

Regionalisation of heat waves in southern South America

This study describes the climatological characteristics of regional heat waves (HWs) over southern South America (SSA) for the warm seasons (October–March) of 1979–2018 based on daily maximum temperature series from 131 weather stations. Clustering of stations with high co-occurrence of simultaneous HW days is employed to identify regional HW events over five homogeneous regions: northern, central-eastern and southern SSA regions, central Argentina, and central Chile. When all regions are considered, we find a mean frequency of ∼4 HWs per year. Transitional regions (northern SSA, central-eastern SSA and central Argentina) are characterised by longer, albeit less intense, HWs than the southernmost region (southern SSA), whereas central Chile events display the lowest duration, intensity and extension. By aggregating these single HW attributes into a combined severity index, a ranking of historical HWs has been obtained, with the March 1980 event standing as the most severe one of SSA. The assessment of long-term changes reveals significant increases in the frequency of regional HW days over central Argentina and central Chile only. Trends in HW characteristics are also region dependent, and the southernmost region is the only one where HW severity has increased significantly.We report similarities and differences in the synoptic circulation patterns associated with regional HW events. Southern SSA HWs have the most distinctive signatures, related to extratropical high-pressure systems blocking the westerly flow. In the remaining regions, HWs are associated with anomalies in the South Atlantic (northern SSA, central-eastern SSA and central Argentina) or South Pacific (central Chile) High, and the intensification of the northerly low-level flow by regional thermal lows and South American Low Level Jet events. Regional HWs often migrate from northern to central-eastern SSA and central Argentina, following the displacement/intensification of the South Atlantic High, which partially explains the similarity of their associated patterns.

Untangling the importance of dynamic and thermodynamic drivers for wet and dry spells across the Tropical Andes

Andean vegetation and agriculture depend on the patterns of rainfall during the South American monsoon. However, our understanding on the importance of dynamic (upper-level wind circulation) as compared to thermodynamic (Amazon basin moisture) drivers for Andes rainfall remains limited. This study examines the effect of these drivers on 3–7 day wet and dry spells across the Tropical Andes and assesses resulting impacts on vegetation. Using reanalysis and remote sensing data from 1985–2018, we find that both dynamic and thermodynamic drivers play a role in determining the rainfall patterns. Notably, we show that the upper-level wind is an important driver of rainfall across the entire Tropical Andes mountain range, but not in the Amazon lowlands, suggesting a crucial role of topography in this relationship. From thermodynamic perspective, we find wet spell conditions to be associated with increased moisture along the Andes’ eastern foothills accompanied by a strengthened South American low-level jet, with moisture lifted into the Andes via topography and convection for all considered regions. Our results suggest that while changes in Amazon basin moisture dominate rainfall changes on daily time scales associated with three day spells, upper-level dynamics play a more important role on the synoptic time scale of 5–7 day spells. Considering impacts on the ground, we find that only 5–7 day spells in the semi-arid Andes have a prolonged effect on vegetation. Our study emphasizes the need to consider both dynamic and thermodynamic drivers when estimating rainfall changes in the Tropical Andes, including in the context of future climate projections.

Detecting local and regional air pollution from biomass burning at a suburban site

Identifying pollution plumes is challenging in cities where in situ air pollution data are scarce, monitoring stations are placed close to roadways, and the smoke from biomass burning might be of local and regional origin. This work characterizes local and regional smoke events at a suburban site in Londrina, southern Brazil, during the dry season. We measured black carbon at 370 and 800 nm (BC370 and BC880), fine particulate matter (PM2.5), particle number (PN), nitrogen oxides (NOx) and ozone (O3) concentrations with 1–2 min resolutions. Ancillary data, such as meteorological variables, aerosol optical depth (AOD) and air mass backward trajectories, aided identifying the sources. The pollution datasets were analyzed by using descriptive statistics, diurnal cycles, temporal correlations, and conditional probability functions coupled with cluster analysis of backward trajectories. Hourly mean (± standard deviation) concentrations were 1.33 ± 1.28 μg m-3 (BC370), 0.91 ± 0.84 μg m-3 (BC880), 15.90 ± 12.95 μg m-3 (PM2.5), 5691 ± 4872 cm-3 (PN), 7.23 ± 7.53 μg m-3 (NOx) and 49.77 ± 17.66 μg m-3 (O3). The prevalence of aerosols from biomass burning was indicated by Ångström exponent (Å370/880) values predominantly larger than 1 (1.51 ± 0.20). Pollutant concentrations peaked on weekend evenings and, occasionally, scattered along the day due to the short-lived open burning of residential solid waste. These events led to fast changing concentrations, increasing from 282 to 929% between consecutive 2-min measurements, reaching 69.29 μg m-3 (BC370), 456.77 μg m-3 (PM2.5) and 232,500 cm-3 (PN). Emissions from biomass-fueled boilers also contributed to deteriorate the local air quality, under favorable local winds. Three regional events advected pollution from Cerrado (tropical savanna) wildfires lasting two-three days each, and were associated with the South American Low Level Jet. During these event, the AOD reached up to 0.37 and mean PM2.5, BC370, and O3 concentrations were (2.5–3.3), (1.9–2.9) and (1.3–1.5) times higher compared to a clean period. Lowering the population exposure to biomass burning requires immediate action to control local sources, and international cooperation is needed to combat regional wildfires in a warming climate.

Nonlocal Impacts of Soil Moisture Variability in South America: Linking Two Land–Atmosphere Coupling Hot Spots

Abstract The land–atmosphere interactions play an important role in modulating climate variability at different spatial and temporal scales. In South America, two recognized hot spots of soil moisture–atmosphere coupling are located in southeastern South America (SESA) and eastern Brazil. Soil moisture variability may not only alter the climate locally but may also have nonlocal impacts through changes in the regional circulation. Here we explore how these two local coupling hot spots interact with each other, how soil moisture variability modulates the regional circulation, and what is the consequent nonlocal impact on precipitation. To this end, we analyze numerical experiments, performed with a regional climate model for the period October–March of 1983–2012, that allow us to isolate the influence of the soil moisture interannual variability on the regional climate. When the soil moisture–atmosphere interaction is enabled, we find a nonlocal coupling mechanism that links both hot spots at different temporal scales, favoring precipitation in eastern Brazil to the detriment of the precipitation in SESA through shifts in the regional circulation, when compared with a simulation with constrained soil moisture–atmosphere interaction. In northeastern Argentina, a subregion of SESA located at the exit of the South American low-level jet, it was found that the amount of nighttime precipitation is modulated by the proposed nonlocal coupling mechanism. A better understanding of the variability of precipitation due to the influence of land–atmosphere interaction processes may contribute to improving the predictability of precipitation and the interpretation of climate projections.

Climate Patterns and Their Influence in the Cordillera Blanca, Peru, Deduced from Spectral Analysis Techniques

Climate patterns are natural processes that drive climate variability in the short, medium, and long term. Characterizing the patterns behind climate variability is essential to understand the functioning of the regional atmospheric system. Since investigations typically reveal only the link and extent of the influence of climate patterns in specific regions, the magnitude of that influence in meteorological records usually remains unclear. The central Peruvian Andes are affected by most of the common climate patterns of tropical areas, such as Intertropical Convergence Zone (ITCZ), Sea Surface Temperature (SST), solar irradiance, Madden Julian Oscillation (MJO), Pacific Decadal Oscillation (PDO), and El Niño Southern Oscillation (ENSO). They are also affected by regional processes that are exclusive from South America, such as the South American Low-Level Jet (SALLJ), South American Monsoon System (SAMS), Bolivian High (BH), and Humboldt Current. The aim of this research is to study the climate variability of precipitation, maximum and minimum temperature records over Cordillera Blanca (Peru), and its relationship with the intensity and periodicity of the common climate patterns that affect this region. To achieve this aim, a spectral analysis based on Lomb’s Periodogram was performed over meteorological records (1986–2019) and over different climate pattern indexes. Results show a coincidence in periodicity between MJO and SALLJ, with monthly cycles for precipitation and temperature (27-day, 56-day, and 90-day cycles). Moreover, the most intense periodicities, such as annual (365 days) and biannual (182 and 122 days) cycles in meteorological variables, possibly would be led by ITCZ and ENSO together, as well as a combination of the Humboldt Current and SALLJ. Additionally, interannual periodicities (3-year, 4.5-year, 5.6–7-year and 11-year cycles) would have coincidence with the ENSO–solar combination, while the longest cycles (16 years) could match PDO variability.

Observational study of the South American low‐level jet during the SALLJEX

AbstractThe South American low‐level jet (SALLJ) is a narrow northerly wind speed maximum present just above the boundary layer. It is an important component of the tropical–extratropical heat and moisture exchange in South America and can favour deep moist convection in southeastern South America. The main objective of this study is to analyse the SALLJ characteristics at 21 upper‐air stations deployed between the Tropics and the subtropics from just east of the Andes to the eastern plains during the SALLJ Experiment (SALLJEX). The greatest wind speed occurs between 300 and 2,000 m AGL between 0300 and 1200 UTC, mainly in connection with greater northerly winds during an anticlockwise rotation of the wind from sunset to sunrise, thus suggesting the important role of the inertial oscillation in the wind's diurnal cycle. The spatial variation of the LLJ throughout the SALLJEX network shows a weakening of the maximum wind speed from stations near the Andes towards the Plains suggesting the presence of the LLJ core just east of the Andes around Santa Cruz de la Sierra's latitude (17°48′S). Weak, moderate, and strong SALLJ categories defined from a local maximum northerly wind speed threshold at each station are defined to analyse the relationship between the SALLJ intensity and the thermodynamic properties of the lower layers of the atmosphere. Strong SALLJs are frequently observed at night‐time, while weak SALLJs are likely to occur at any time of the day. Strong cases have deeper and less stable nocturnal boundary layers, which could be due to the SALLJ warm advection near the time of wind speed maximum (0600 UTC). Deeper convective boundary layers and higher low‐level temperatures observed at 1800 UTC prior to strong nocturnal SALLJs can potentially lead to larger amplitudes of inertial oscillation and contribute to generating stronger SALLJs.

Dynamics of Precipitation Anomalies in Tropical South America

In this study, precipitation in Tropical South America in the 1931–2016 period is investigated by means of Principal Component Analysis and composite analysis of circulation fields. The associated dynamics are analyzed using the 20th century ERA-20C reanalysis. It is found that the main climatic processes related to precipitation anomalies in Tropical South America are: (1) the intensity and position of the South Atlantic Convergence Zone (SACZ); (2) El Niño Southern Oscillation (ENSO); (3) the meridional position of the Intertropical Convergence Zone (ITCZ), which is found to be related to Atlantic Sea Surface Temperature (SST) anomalies; and (4) anomalies in the strength of the South American Monsoon System, especially the South American Low-Level Jet (SALLJ). Interestingly, all of the analyzed anomalies are related to processes that operate from the Atlantic Ocean, except for ENSO. Results from the present study are in agreement with the state of the art literature about precipitation anomalies in the region. However, the added strength of the longer dataset and the larger study area improves the knowledge and gives new insights into how climate variability and the resulting dynamics are related to precipitation in Tropical South America.

New Insights into the South American Low-Level Jet from RELAMPAGO Observations

AbstractThe Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations (RELAMPAGO) campaign produced unparalleled observations of the South American low-level jet (SALLJ) in central Argentina with high temporal observations located in the path of the jet and upstream of rapidly growing convection. The vertical and temporal structure of the jet is characterized using 3-hourly soundings launched at two fixed sites near the Sierras de Córdoba (SDC), along with high-resolution reanalysis data. Objective SALLJ identification criteria are applied to each sounding to determine the presence, timing, and vertical characteristics of the jet. The observations largely confirm prior results showing that SALLJs most frequently come from the north, occur overnight, and peak in the low levels, though SALLJs notably peaked higher near the end of longer-duration events during RELAMPAGO. This study categorizes SALLJs into shorter-duration events with jet cores peaking overnight in the low levels and longer 5–6-day events with elevated jets near the end of the period that lack a clear diurnal cycle. Evidence of both boundary layer processes and large-scale forcing were observed during shorter-duration events, whereas synoptic forcing dominated the longer 5–6-day events. The highest amounts of moisture and larger convective coverage east of the SDC occurred near the end of the 5–6-day SALLJ events.Significance StatementThe South American low-level jet (SALLJ) is an area of enhanced northerly winds that likely contributes to long-lived, widespread thunderstorms in Southeastern South America (SESA). This study uses observations from a recent SESA field project to improve understanding of the variability of the SALLJ and the underlying processes. We related jet occurrence to upper-level environmental patterns and differences in the progression speed of those patterns to varying durations of the jet. Longer-duration jets were more elevated, transported moisture southward from the Amazon, and coincided with the most widespread storms. These findings enable future research to study the role of the SALLJ in the life cycle of storms in detail, leading to improved storm prediction in SESA.

81Kr reveals one-million-year-old groundwater at the Atlantic coast of Argentina as a record of Mid-Pleistocene climate

Sedimentary deposits in both Colorado Basin and Salado Basin, two geological basins originated after the Atlantic rifting in the coastal area of Argentina, contain deep confined aquifers bearing thermal waters. Previous studies on the paleogroundwater of this region have been limited by the radiocarbon dating range. In this study 81Kr dating is applied to reveal groundwater ages up to 1.1 Ma, so far the oldest found in South America. Groundwater in the Colorado Basin, previously assigned to the Last Glacial Maximum (LGM), is reassigned to a period from Marine Isotope Stage 1 to 5 (MIS 1 to MIS 5). The age information combined with water stable isotopes and noble gases suggests the possible existence of the South-American-Low-Level-Jet (SALLJ) in this period, that had carried the recycled water vapor from the tropical Amazonia to locations further south compared to the present time. Also, the inferred recharge conditions in both basins during Mid-Pleistocene agree with the very dry climate in this period suggested by previous studies.

Amazonian Moisture Recycling Revisited Using WRF With Water Vapor Tracers

AbstractPrevious studies have estimated that 25%–35% of Amazonian precipitation comes from evapotranspiration (ET) within the basin. However, due to simplifying assumptions of traditional models, these studies primarily focus on large spatial and temporal scales. This study is the first to analyze the moisture of Amazonian origin at the annual to daily timescale in four different subregions of the Amazon. We analyze the sources, sinks and stores of moisture that originates as Amazonian ET. To do this, we use the Weather Research and Forecast (WRF) regional meteorological model with the added capability of water vapor tracers to track this moisture. Moisture of Amazonian origin shows strong annual and semi‐annual signals, with contrasting behavior between the northern and southern parts of the basin. The tracers reveal a strong diurnal cycle of Amazonian water vapor which had not been previously reported. This signal is related to the diurnal cycle of ET, convective precipitation and advected moisture. ET's contribution to atmospheric moisture increases from early morning into the afternoon. Some of this moisture is rained out through convective storms in the early evening. Later in the night and following morning, strong winds associated with the South American Low Level Jet advect moisture downwind. The beating pattern becomes apparent when visualizing the Amazonian water vapor as an animation.

Determination of climatic conditions related to precipitation anomalies in the Tropical Andes by means of the random forest algorithm and novel climate indices

AbstractUnderstanding precipitation and its relation with atmospheric and oceanic conditions is vital in the face of climate change. This is crucial in the Tropical Andes (TA) because millions of people depend on water originated in the cordillera. Unfortunately, the paucity of meteorological monitoring that exists in mountainous regions is accentuated in the tropics. In this context, climate indices, remotely sensed, and gridded datasets, are useful tools to study climate and precipitation in the TA, and additional climate indices can be calculated from reanalysis datasets. The combination of this information with traditional indices has the potential to improve our understanding of precipitation. Our objective was to use the k‐means algorithm to regionalize precipitation in the TA (different regions have different climate), and then use the random forest algorithm to study the variables related to precipitation in each of these regions in seasonal timescales. Here, we show the suitability of the random forest algorithm to reveal climate processes and the high potential of the novel climate indices to improve the regressions. We found that convective available potential energy was the most important variable for precipitation in the northern TA, except for the Chocó, where v at 850 hPa was the most important one. Meanwhile, vertical integral of divergence of moisture flux was the most important one in the southern TA. Interestingly, in the DJF season when the South American low‐level jet (SALLJ) is more active, u and v at 850 hPa showed their lowest relative importance and the total column of water vapour showed its maximum, this could indicate that precipitation anomalies are controlled by atmospheric moisture availability rather than by the speed of the SALLJ during DJF. These results deepen our understanding of precipitation anomalies in the TA and the related oceanic and atmospheric conditions. The proposed methodology was proven to be suitable and it could be used in the future to test and formulate new hypotheses, and to forecast seasonal precipitation.

Deforestation impacts on Amazon-Andes hydroclimatic connectivity

Amazonian deforestation has accelerated during the last decade, threatening an ecosystem where almost one third of the regional rainfall is transpired by the local rainforest. Due to precipitation recycling, the southwestern Amazon, including the Amazon-Andes transition region, is particularly sensitive to forest loss. This study evaluates the impacts of Amazonian deforestation on the hydro-climatic connectivity between the Amazon and the eastern tropical Andes during the austral summer (December–January–February) in terms of hydrological and energetic balances. Using 10-years high-resolution simulations (2001–2011) with the Weather Research and Forecasting Model, we analyze control and deforestation scenario simulations. Regionally, deforestation leads to a reduction in the surface net radiation, evaporation, moisture convergence and precipitation (~ 20%) over the entire Amazon basin. In addition, during this season, deforestation increases the atmospheric subsidence over the southern Amazon and weakens the regional Hadley cell. Atmospheric stability increases over the western Amazon and the tropical Andes inhibiting convection in these areas. Consequently, major deforestation impacts are observed over the hydro-climate of the Amazon-Andes transition region. At local scale, nighttime precipitation decreases in Bolivian valleys (~ 20–30%) due to a strong reduction in the humidity transport from the Amazon plains towards the Andes linked to the South American low-level jet. Over these valleys, a weakening of the daytime upslope winds is caused by local deforestation, which reduces the turbulent fluxes at lowlands. These alterations in rainfall and atmospheric circulation could impact the rich Andean ecosystems and its tropical glaciers.

Recent changes in the atmospheric circulation patterns during the dry-to-wet transition season in south tropical South America (1979-2020): Impacts on precipitation and fire season

AbstractWe analyze the characteristics of atmospheric variations over tropical South America using the pattern recognition framework of weather typing or atmospheric circulation patterns (CPs). During 1979-2020, nine CPs are defined in the region, using a k-means algorithm based on daily unfiltered 850 hPa winds over 0035°N-30°S, 90°W-30°W. CPs are primarily interpreted as stages of the annual cycle of the low-level circulation. We identified three “winter” CPs (CP7, CP8 and CP9), three “summer” CPs (CP3, CP4 and CP5) and three “transitional” CPs (CP1, CP2 and CP6). Significant long-term changes are detected during the dry-to-wet transition season (July-October) over south tropical South America (STSA). One of the wintertime patterns (CP9) increases from 20% in the 1980s to 35% in the last decade while the “transitional” CP2 decreases from 13% to 7%. CP9 is characterized by enhancement of the South American Low-Level Jet and increasing atmospheric subsidence over STSA. CP2 is characterized by southerly cold-air incursions and anomalous convective activity over STSA. The years characterized by high (low) frequency of CP9 (CP2) during the dry-to-wet transition season are associated with a delayed South American Monsoon onset and anomalous dry conditions over STSA. Consistently, a higher frequency of CP9 intensifies the fire season over STSA (1999-2020). Over the Brazilian states of Maranhão, Tocantins, Goiás and São Paulo, the seasonal frequency of CP9 explains around 35%-44% of the interannual variations of fire counts.

The competing effects of terrestrial evapotranspiration and raindrop re-evaporation on the deuterium excess of continental precipitation

The deuterium excess (d-excess) of precipitation, which tracks kinetic fractionations during water phase changes, has been used to trace the regions and conditions of oceanic moisture sources, in particular from polar ice-core records. Still, many observations suggest that precipitation d-excess varies significantly across terrestrial environments, both above and below the global average value 10. These variations are often interpreted to reflect either moisture recycling via terrestrial evapotranspiration or sub-cloud raindrop re-evaporation, respectively. Despite being frequently mentioned in literature, however, little work has been carried out to quantify these two competing effects on the widespread variations of d-excess. Here, we use a one-dimensional model of water vapor transport to interrogate the relative controls on d-excess of continental precipitation. We show that when the water vapor gradient is coupled with decreasing temperature, d-excess increases with net rainout and δ18O depletion along the model transect, while the magnitude of increase is controlled by the water balance, evaporation/transpiration ratio, and transport type. Raindrop re-evaporation functions as an additional flux of recycled moisture and further increases the d-excess downwind. Alternatively, when the water vapor gradient is coupled with decreasing relative humidity, d-excess may decrease along the model transect wherein upwind evapotranspiration is overwhelmed by local raindrop re-evaporation effects. This local effect becomes even stronger under a regime of turbulent eddy transport with high transpiration fractions, resulting in a pronounced decrease of d-excess without notable changes in δ18O. Finally, we demonstrate that model processes capture the isotopic variations in precipitation across the altitudinal gradient of the Andes as well as the South American low-level jet zone. Broadly, this study presents a novel framework for understanding the dynamical controls of precipitation d-excess and for linking spatial isotopic variations with ecohydrological fluxes and processes in both modern and paleo-environments.

Variability of the low-level circulation of the South American Monsoon analysed with complex networks

Understanding the variability of low-level atmospheric circulation regimes is key for understanding the dynamics of monsoon systems. The South American Monsoon is characterized by strong year-long trade winds that are channeled southward into the South American Low-Level Jet after crossing the Amazon basin, which in turn is elementary for the moisture transport to Southern South America. In this study, we utilize streamflow wind networks, a type of climate networks that tracks the local flow of the wind field, together with the analysis of composites of wind, precipitation, and geopotential height fields, to investigate the variability of the South American low-level circulation. The streamflow wind networks are used here as they are able to directly track the wind flow and encode its spatiotemporal characteristics in their topology. We focus on intraseasonal variations in terms of active and break monsoon phases on the one hand, and on the interannual variability in terms of the impacts of the El Niño-Southern Oscillation on the other hand. Our findings highlight the importance of the South American Low-Level Jet, its spatial position and variability. Our study reveals the relation of the active and break regimes to anomalous high- and low-pressure systems over the southern Atlantic that are connected to Rossby wave trains from the southern Pacific, as well as the impact of these regimes on the cross-equatorial low-level flow. In addition, the streamflow networks that we use demonstrate significant shifts of the dominant wind flow pattern during El Niño and La Niña episodes.