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Synoptic maps at 12Z from 26 to January 31, 2021. Each map includes 500 hPa geopotential height (contours, every 50 mgp), precipitable water (color shades, in mm) and 700 hPa wind speed (values exceeding 15 m/s are shaded in grey). The small, dashed box indicates central Chile. Data source: ERA5. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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A major storm impacted the subtropical Andes during 28–31 January 2021 producing 4-days accumulated precipitation up to 100 mm over central-south Chile. These are high accumulations even for winter events but the storm occurred in the middle of the summer when precipitation in virtually absent, conferring it an extraordinary character. Similar stor...
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... synoptic evolution during 26-31 January 2021 is illustrated in Fig. 6 by maps every 24 h of the 500 hPa geopotential height (contours), precipitable water (colors) and 700 hPa wind speed (grey shading outlines wind maxima). At 12 UTC 26 January, the aforementioned deep ridge at midlatitudes and closed low farther north were well defined over the central south Pacific (Fig. 6a). By this time a tongue of ...
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... 26-31 January 2021 is illustrated in Fig. 6 by maps every 24 h of the 500 hPa geopotential height (contours), precipitable water (colors) and 700 hPa wind speed (grey shading outlines wind maxima). At 12 UTC 26 January, the aforementioned deep ridge at midlatitudes and closed low farther north were well defined over the central south Pacific (Fig. 6a). By this time a tongue of moist air (PW > 40 mm) had made its way from lower latitudes into the central subtropical Pacific. Wind convergence downstream of the high/low couplet acted to enhance the meridional temperature gradient in the free troposphere at midlatitudes, most marked around 90 • W (Sup. Fig. 4). As a result, a westerly ...
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... 90 • W (Sup. Fig. 4). As a result, a westerly wind jet developed in the next 24 h extending from the eastern Pacific toward the Chilean coast centered at 35-40 • S. The core of the jet was at around 300 hPa but strong zonal flow also prevailed in the middle and lower troposphere, promoting the rapid eastward extension of a moist air filament (Fig. 6b). This Zonal Atmospheric River (ZAR) made landfall along the coast of Chile at about 39 • S by the end of 28 January ( Fig. 6c). At this time the ZAR was collocated with the jet axis and to the north of a weak baroclinic zone in the lower and middle troposphere at about 38 • S (not shown). Weakly stable conditions during the ZAR ...
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... the Chilean coast centered at 35-40 • S. The core of the jet was at around 300 hPa but strong zonal flow also prevailed in the middle and lower troposphere, promoting the rapid eastward extension of a moist air filament (Fig. 6b). This Zonal Atmospheric River (ZAR) made landfall along the coast of Chile at about 39 • S by the end of 28 January ( Fig. 6c). At this time the ZAR was collocated with the jet axis and to the north of a weak baroclinic zone in the lower and middle troposphere at about 38 • S (not shown). Weakly stable conditions during the ZAR passage fostered the strong ascent in the vicinity of the Andean foothills (Sup. Fig. 5) increasing the orographic precipitation ...
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... midday of 30 January, the ZAR reached its northernmost latitude (32 • S) and begun to fade rapidly as per the decrease in IVT (Fig. 4c). Nevertheless, moisture remained high and only reached pre-storm (Fig. 4b). At the same time, a trough began to develop along the coast of southern-central Chile (Fig. 6d). The development of cyclonic relative vorticity (ξ < 0) at mid-levels occurred rapidly during the last two days of the storm (Sup. Fig. 6) and was more evident between 700 and 500 hPa where a closed low centered 36 • S, 75 • W (just off the coast) formed during the morning of January 31. The depression at mid-levels was located well to ...
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... rapidly as per the decrease in IVT (Fig. 4c). Nevertheless, moisture remained high and only reached pre-storm (Fig. 4b). At the same time, a trough began to develop along the coast of southern-central Chile (Fig. 6d). The development of cyclonic relative vorticity (ξ < 0) at mid-levels occurred rapidly during the last two days of the storm (Sup. Fig. 6) and was more evident between 700 and 500 hPa where a closed low centered 36 • S, 75 • W (just off the coast) formed during the morning of January 31. The depression at mid-levels was located well to the north of the polar jet so it can be described as a cut-off low (COL), although of short duration (<24 h) and with a limited ...
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... January 31. The depression at mid-levels was located well to the north of the polar jet so it can be described as a cut-off low (COL), although of short duration (<24 h) and with a limited expression in the upper troposphere. Such development was promoted by southerly winds downstream of the ridge over the south Pacific near the Chilean coast ( Fig. 6e and f). Specifically, we verified that meridional advection of planetary vorticity was the leading element causing cyclonic development off central Chile in this part of the storm. Just to the north of the mid-level COL, strong NNW flow advected cyclonic vorticity toward the continent causing broader ascent over central Chile, somewhat ...
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... distinct precipitation features are more akin to winter events (Falvey and Garreaud, 2007;Viale and Nuñez, 2011) and reflect the leading driver of the event: a zonal atmospheric river (ZAR) just to the north of a weak baroclinic zone over the southeast Pacific (Fig. 6). The axis of the ZAR landfalled at about 39 • S on 28 January and moved northward to reach 32 • S two days later (Fig. 3b). Tropospheric deep, strong westerlies forced the ascent of moist laden air parcels over the windward side of the subtropical Andes resulting in widespread precipitation over central Chile, a marked orographic ...
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... reservoir into midlatitudes, and equatorward flow downstream of the blocking high steepened the north-south temperature contrast over the eastern Pacific. These two ingredients, acting for more than a week prior to the central Chile storm, provided the conditions for the formation of the ZAR that rapidly moved eastward to reach South America (Fig. ...
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... (Figs. 3a and 4a). Downpours tended to occur simultaneous along the Andes foothills and higher terrain during evening and early night hours, accompanied by hail and lightning, indicative of a convective nature. At the same time, our synoptic analyses revealed the formation of cut-off low (COL) off the coast of central Chile in the wake of the ZAR (Fig. 6). The COL development was linked to negative planetary vorticity advection by wind blowing from south to north along the Chilean coast that also advected cold air at mid-levels. As seen in previous events ( Garreaud and Fuenzalida, 2007) the COL fostered broad scale ascent over central Chile and unstable thermodynamic conditions (e.g., ...
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Citations
... This event became remarkable when it flooded Central Chile during the dry season (Valenzuela et al., 2022). As it comes onshore in Chile, some moisture continues to be sourced (Figure 3b) as the same regions as during ARgenesis (Figure 3a), with a long tail for moisture sources trailing along the AR's path, 1,000's km away. ...
Atmospheric Rivers (ARs) are synoptic‐scale conduits for poleward transport of heat and water, often associated with extreme rainfall. Using NASA surface heat flux observations and climate model simulations, we assess whether ARs are “rivers”, transporting heat and moisture over longer distances, or mostly local convergence. The observations indicate that ARs reduce extratropical surface energy fluxes, even during early development. This damping of surface fluxes during ARs is also simulated in the Goddard Institute for Space Studies ModelE, 2.1 (GISS‐E2.1) nudged to (MERRA2) reanalysis winds. Furthermore, water provenance tracers in GISS‐E2.1 identify the moisture source for ∼7,500 ARs globally during 2018–2022 as farther upstream and equatorward compared to climatology. These results quantitatively show that ARs source relatively less moisture from the surface beneath them and more from a greater distance than during non‐AR times.
... The angle of the along-AR main axis relative to the Andes varies widely. ARs are generally oriented in the NW-SE direction forming an acute angle with the Andes (tilted ARs; [8]), but in some cases, the moist stream impinges nearly perpendicular to the mountains, referred to as zonal atmospheric rivers (ZARs; [19]). Winter storms in central Chile with unusually high air temperatures often feature a ZAR [15], and some high-impact events in the recent past have been caused by this type of AR (e.g., [20]). ...
... The angle of the along-AR main axis relative to the Andes varies widely. ARs are generally oriented in the NW-SE direction, forming an acute angle with the Andes (tilted ARs; [8]), but in some cases, the moist stream impinges nearly perpendicular to the mountains, referred to as zonal atmospheric rivers (ZARs; [19]). Winter storms in central Chile with unusually high air temperatures often feature a ZAR [15], and some high-impact events in the recent past have been caused by this type of AR (e.g., [20]). ...
The extratropical west coast of South America has one of the largest frequencies of landfalling atmospheric rivers (ARs), with dozens of events per season that account for ~50% of the annual precipitation and can produce extreme rainfall events in south-central Chile. Most ARs form an acute angle with the Andes, but, in some cases, the moist stream impinges nearly perpendicular to the mountains, referred to as zonal atmospheric rivers (ZARs). Enhanced surface-based and upper-air measurements in Concepcion (36.8° S), as well as numerical simulations, were used to characterize a ZAR and a meridionally oriented AR in July 2022. They represent extremes of the broad distribution of winter storms in this region and exhibit key features that were found in a composite analysis based on larger samples of ZARs and tilted ARs. The latter is associated with an upper-level trough, broad-scale ascent, extratropical cyclone, and cold front reaching southern Chile. Instead, ZARs are associated with tropospheric-deep, strong zonal flow and a stationary front across the South Pacific, with ascent restricted upstream of the Andes. Consequently, ZARs have minimum precipitation offshore but a marked orographic precipitation enhancement and exhibit relatively warm temperatures, thus resulting in an augmented risk of hydrometeorological extreme events.
... Atmospheric rivers (AR) are transient, narrow, and elongated channels in the atmosphere that are known to transport water vapor and heat poleward from the tropics (Zhu & Newell, 1998) . In Central Chile, ARs contribute about 50% of the annual rainfall (Viale et al., 2018) and they are also responsible for extreme rainfall events when water vapor is intercepted by the steep Andes orography, creating hazards such as floods and landslides (Rutllant et al., 2023;Valenzuela et al., 2022;Valenzuela & Garreaud, 2019) . Considering that these ARs have to cross through a large gradient of sea surface temperature from the Central Pacific towards cold coastal waters (e.g. ...
Atmospheric rivers (ARs) are known to produce both beneficial and extreme rainfall, leading to natural hazards in Chile. Motivated to understand moisture transport during AR events, this study performs a moisture budget analysis along 50 zonally elongated ARs reaching the western coast of South America. We identify the convergence of moist air masses of tropical/subtropical origin along the AR as the primary source of vertically integrated water vapor (IWV). Over the open ocean, moisture convergence is nearly balanced by precipitation. The advection of moisture along the AR, although smaller compared to mass convergence, significantly increases toward the landfalling region. The near conservation of IWV over the open ocean, observed by tracking a Lagrangian atmospheric column along the ARs, is the explanation behind the seemingly tropical origin of ARs in time-lapse visualizations of IWV
... In central Chile, most precipitation events are related to mid-latitude baroclinic perturbations and are usually accompanied by atmospheric rivers (ARs), which contribute up to 40% of extreme precipitation amounts (Aceituno et al. 2021;Valenzuela and Garreaud 2019;Viale et al., 2018). Although most EPEs occur during the austral winter, some events are observed during the warm season (Viale and Garreaud 2014;Valenzuela et al. 2022), mainly associated with COLs. On the other hand, precipitation events are evenly distributed during the year in southern and Austral Chile (Aceituno et al. 2021). ...
We characterize trends in maximum seasonal daily precipitation (seasonal Rx1day), minimum (Tn), and maximum (Tx) daily temperatures during days with precipitation over continental Chile for the period 1979–2017, using surface stations and the AgERA5 gridded product derived from the ERA5 reanalysis dataset. We also examine seasonal trends of Sea Surface Temperature (SST), Precipitable Water (PW), Convective Available Potential Energy (CAPE), Eddy Kinetic Energy (EKE), Atmospheric Rivers (ARs) frequency, and upper air observations to seek possible mechanisms that explain precipitation trends. Our results show an increase in seasonal Rx1day during fall in the south part of Northern Chile (15–30°S) and during fall and winter in Austral Chile (45–57°S), and mostly negative trends in Central Chile (30–36°S), where a few locations with positive trends along the coast during summer. Temperature trends presented cooling patterns north of 33°S in almost all the seasons (< -2 °C/dec), while warming trends prevail south of 38°S (> 1 °C/dec). The highest values in Tn trends are obtained on the western slopes of the Andes around 30°S. We also explore temperature scaling in surface stations, finding strong positive super Clausius Clapeyron with Tn, especially between fall and spring in the 33–40°S region. Sounding observations in five stations across Chile suggest warming trends at 23.5°, 33°S, and 53°S, with a stabilization effect by enhanced warming in the upper troposphere, while presenting cooling trends in Puerto Montt (41.5°S). Seasonal trends in PW reveal moistening along southern Peru and northern Chile during spring and summer. Positive trends in CAPE are observed over 35–40°S (austral summer and fall) and the north Altiplano (autumn). SST analyses reveal strong cooling around 30°S in winter, which may explain the negative trends in seasonal Rx1day in central Chile. A warming spot on the northern Peruvian coast during fall may be responsible for humidification in front of Northern Chile, particularly during summer and fall. Positive EKE trends are detected south of 40°S, being stronger and reaching almost all of the coast during spring. ARs frequency unveils negative trends up to -5 days/dec during summer and positive trends of 1 day/dec in 40°- 50°S coastal regions during spring. More generally, the results presented here shed light on the main large-scale processes driving recent trends in precipitation extremes across continental Chile.
... For the statistical modelling, a common period with the largest number of stations with at least 75 % of the measurements was sought. Using a common period reduces the bias of earthquakes, volcanic eruptions, storms (Valenzuela et al., 2022) and climate variability (Masiokas et al., 2010) that could have affected nearby basins in a period in which at least one has no data. In addition to these processes, glaciers also can have significant variability and trends in sediment export (Fernandez et al., 2011;Vergara et al., 2022a). ...
Understanding how the environmental factors that determine erosion operate is essential to know the past and future evolution of the Earth and adequately manage natural resources. In this work, the main controlling factors of the current sediment export in the western side of the extratropical Andes (20°-55°S) are investigated using multidecadal series of specific suspended sediment yields from 42 rivers, as well as land cover, climatic, cryospheric, topographic, seismological and geological data. Through an automatic selection of Generalized Additive Models based on predictability and complexity, it was found that the combined effects of extreme runoff, glacier cover and channel steepness explain ~90% of the spatial variability of sediment export. The runoff effect varies from positive for hyper-arid and semi-arid settings to slightly negative for moderate and wet settings, probably because of an associated increase in vegetation coupled with a recurrence and magnitude of floods that becomes greater than the time it takes for the basins to generate sediment. Cryosphere and channel steepness influences were also observed on the average suspended sediment concentration, which is less associated with water availability and more associated with erodibility and sediment availability.
... The cold air behind the surface front is a region of extremely dry air, also explaining the decrease of PW near the continent in the climatological sense. Some individual storms can even produce precipitation without the need for the atmospheric river to land over the continent, for instance, in the recent case of 2021, where most of the precipitation occurred after the IVT maximum near the coast, where water vapor previously transported by an atmospheric river was organized by convective instability due to a cut-off low (Valenzuela et al., 2022). Regarding the source of water vapor, Langhamer et al. (2018) used a Lagrangian approach and found that the main source of moisture for precipitation in the Patagonia icefields is located along a diagonal parallel to the SPCZ. ...
Plain Language Summary
Changes in the amount of rain Central Chile gets from year to year is often related to how the atmosphere behaves over the Southern Pacific Ocean. Specifically, it is connected to the interaction between storm tracks and certain wind patterns that respond to changes in the amount of rainfall in tropical areas. Our research shows that during the warm part of a natural climate cycle known as ENSO, there is more moisture transported from subtropical areas. This leads to more occurrences of what are known as atmospheric rivers, which are essentially large streams of water vapor in the sky. It also leads to a greater amount of water vapor in the air and more of this vapor being moved across the Pacific. These changes mean that there is more rain and atmospheric rivers, which carry a lot of moisture and stick around for a long time, affecting Central Chile. So our work is helping to reveal how changes in the rainfall in tropical areas can influence the amount of rain that falls in distant places like Central Chile.
... These storms have caused flooding and loss of human lives over mountainous coastal regions in winter (Curry et al., 2019;R. Garreaud & Rutllant, 1996;Lavers et al., 2011) and even in summer months (Valenzuela et al., 2022). Since these storms are still less studied than typical cold fronts and have the potential of causing important hydrological disasters, the second goal of this study is to characterize the cloud properties of high-FL cold fronts affecting central Chile and quantify their differences compared to low-FL cold fronts. ...
This study analyzes the cloud properties associated with cold fronts that affected central Chile during May–September 1998–2014 using observational and reanalysis data. Warmer, moister, and more unstable conditions in early winter favor the development of deeper mixed‐phased clouds in cold fronts, having larger rain and ice water contents. Late in the season, cooler, drier, and more stable conditions predominate, favoring the development of a larger percentage of shallower warm‐phased clouds. Years when cold fronts show large environmental humidity and high cloud tops were associated with stronger systems that produced the largest precipitation. The linear positive relationship between cloud tops and precipitable water reported in the Central and Eastern Pacific was also found in storms developing in the Eastern South Pacific. The intensity of cold fronts also determines their cloud development and observed rainfall. High freezing level (FL) cold fronts are more frequent in May and June, generally occur in very high humidity environments, are stronger, include more rain and ice water contents, and allow the development of the highest clouds compared to low FL storms, which are more frequent in August. The strength of low FL storms, instead of their available humidity seems to be the key factor controlling the rain they produce. On the other hand, the intensity of high FL storms seems not to be a decisive factor in controlling its rainfall. Finally, the presence of an atmospheric river seems not to guarantee the rainiest events among high and low FL cold fronts.
... However, they showed a moderate correlation performance in Iran [21]. Other studies evaluated the different IMERG products in Iran [22], Finland [10], Greece [23], South America [24], Brazil [25][26][27], Saudi Arabia [19,28,29], Indonesia [30], Ecuador [31], Pakistan [32] and Chile [15,20,33]. ...
Accurate rainfall measurement is a challenge, especially in regions with diverse climates and complex topography. Thus, knowledge of precipitation patterns requires observational networks with a very high spatial and temporal resolution, which is very difficult to construct in remote areas with complex geological features such as desert areas and mountains, particularly in countries with high topographical variability such as Chile. This study evaluated the performance of the near-real-time Integrated Multi-satellite Retrievals for GPM (IMERG) Early product throughout Chile, a country located in South America between 16° S-66° S latitude. The accuracy of the IMERG Early was assessed at different special and temporal scales from 2015 to 2020. Relative Bias (PBIAS), Mean Absolute Error (MAE), and Root-Mean-Squared Error (RMSE) were used to quantify the errors in the satellite estimates, while the Probability of Detection (POD), False Alarm Ratio (FAR), and Critical Success Index (CSI) were used to evaluate product detection accuracy. In addition, the consistency between the satellite estimates and the ground observations was assessed using the Correlation Coefficient (CC). The spatial results show that the IMERG Early had the best performance over the central zone, while the best temporal performance was detected for the yearly precipitation dataset. In addition, as latitude increases, so do errors. Also, the satellite product tends to slightly overestimate the precipitation throughout the country. The results of this study could contribute towards the improvement of the IMERG algorithms and open research opportunities in areas with high latitudes, such as Chile.
Atmospheric rivers (ARs) are narrow, elongated, synoptic jets of water vapor that play important roles in the global water cycle. The continually developing Tracking Atmospheric Rivers Globally as Elongated Targets (tARget) algorithm identifies AR objects at individual time steps based on thresholding integrated water vapor transport (IVT) and other requirements, and tracks each AR object in time and space. Building on previous versions of tARget, this paper discusses further refinements to the algorithm to better handle ARs in tropical and polar areas, as well as “zonal” ARs which the previous versions of the algorithm were not designed to capture. This further regionally refined algorithm is applied to the ERA5 reanalysis over 1940–2023 at 6 h intervals and a 0.25° × 0.25° horizontal resolution. The AR detection results are evaluated in terms of key AR characteristics. We anticipate this regionally refined global AR database will aid further understanding of ARs such as AR process studies, evaluation of AR simulations and predictions, and assessment of climate change impacts on ARs.
During 29-31 January 2021 (austral summer), an extreme storm event triggered catastrophic debris flows in central Chile (33-36°S). At small and precarious rural settlements in the commune of Malloa in central Chile, debris flows where triggered by a hailstorm. Hail-debris flows, with hail volume concentration near 10%-20%, caused 200 injured individuals and 73 damaged houses. In this study, hail-debris flows where modelled using the FLO2D. software, calibrated against flow velocities and flooded areas obtained from audio-visual records taken by local inhabitants (using cell phones) and a high-accuracy post-event topography obtained with a drone. Results suggest that hail content significantly reduces flow resistance compared to typical debris flows, thus increasing flow velocity, and run out. On the other hand, damage to infrastructure was more related to the materiality of the houses (precarious settlements) than to debris flow severity.
