TY - JOUR A1 - Castino, Fabiana A1 - Bookhagen, Bodo A1 - Strecker, Manfred T1 - River-discharge dynamics in the Southern Central Andes and the 1976-77 global climate shift JF - Geophysical research letters N2 - Recent studies have shown that the 1976-77 global climate shift strongly affected the South American climate. In our study, we observed a link between this climate shift and river-discharge variability in the subtropical Southern Central Andes. We analyzed the daily river-discharge time series between 1940 and 1999 from small to medium mountain drainage basins (10(2)-10(4) km(2) ) across a steep climatic and topographic gradient. We document that the discharge frequency distribution changed significantly, with higher percentiles exhibiting more pronounced trends. A change point between 1971 and 1977 marked an intensification of the hydrological cycle, which resulted in increased river discharge. In the upper Rio Bermejo basin of the northernmost Argentine Andes, the mean annual discharge increased by 40% over 7 years. Our findings are important for flood risk management in areas impacted by the 1976-77 climate shift; discharge frequency distribution analysis provides important insights into the variability of the hydrological cycle in the Andean realm. Y1 - 2016 U6 - https://doi.org/10.1002/2016GL070868 SN - 0094-8276 SN - 1944-8007 VL - 43 SP - 11679 EP - 11687 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Castino, Fabiana A1 - Bookhagen, Bodo A1 - Strecker, Manfred T1 - Rainfall variability and trends of the past six decades (1950-2014) in the subtropical NW Argentine Andes JF - Climate dynamics : observational, theoretical and computational research on the climate system N2 - The eastern flanks of the Central Andes are characterized by deep convection, exposing them to hydrometeorological extreme events, often resulting in floods and a variety of mass movements. We assessed the spatiotemporal pattern of rainfall trends and the changes in the magnitude and frequency of extreme events (ae95th percentile) along an E-W traverse across the southern Central Andes using rain-gauge and high-resolution gridded datasets (CPC-uni and TRMM 3B42 V7). We generated different climate indices and made three key observations: (1) an increase of the annual rainfall has occurred at the transition between low (< 0.5 km) and intermediate (0.5-3 km) elevations between 1950 and 2014. Also, rainfall increases during the wet season and, to a lesser degree, decreases during the dry season. Increasing trends in annual total amounts characterize the period 1979-2014 in the arid, high-elevation southern Andean Plateau, whereas trend reversals with decreasing annual total amounts were found at low elevations. (2) For all analyzed periods, we observed small or no changes in the median values of the rainfall-frequency distribution, but significant trends with intensification or attenuation in the 95th percentile. (3) In the southern Andean Plateau, extreme rainfall events exhibit trends towards increasing magnitude and, to a lesser degree, frequency during the wet season, at least since 1979. Our analysis revealed that low (< 0.5 km), intermediate (0.5-3 km), and high-elevation (> 3 km) areas respond differently to changing climate conditions, and the transition zone between low and intermediate elevations is characterized by the most significant changes. KW - Extreme rainfall KW - South American Monsoon System KW - Central Andes KW - Quantile regression KW - Rain gauges KW - CPC-uni KW - TRMM KW - Orographic barrier Y1 - 2017 U6 - https://doi.org/10.1007/s00382-016-3127-2 SN - 0930-7575 SN - 1432-0894 VL - 48 SP - 1049 EP - 1067 PB - Springer CY - New York ER - TY - JOUR A1 - Castino, Fabiana A1 - Bookhagen, Bodo A1 - Strecker, Manfred T1 - Oscillations and trends of river discharge in the southern Central Andes and linkages with climate variability JF - Journal of hydrology N2 - This study analyzes the discharge variability of small to medium drainage basins (10(2)-10(4) km(2)) in the southern Central Andes of NW Argentina. The Hilbert-Huang Transform (HHT) was applied to evaluate non-stationary oscillatory modes of variability and trends, based on four time series of monthly normalized discharge anomaly between 1940 and 2015. Statistically significant trends reveal increasing discharge during the past decades and document an intensification of the hydrological cycle during this period. An Ensemble Empirical Mode Decomposition (EEMD) analysis revealed that discharge variability in this region can be best described by five quasi-periodic statistically significant oscillatory modes, with mean periods varying from 1 to 20 y. Moreover, we show that discharge variability is most likely linked to the phases of the Pacific Decadal Oscillation (PDO) at multi-decadal timescales (similar to 20 y) and, to a lesser degree, to the Tropical South Atlantic SST anomaly (TSA) variability at shorter timescales (similar to 2-5 y). Previous studies highlighted a rapid increase in discharge in the southern Central Andes during the 1970s, inferred to have been associated with the global 1976-77 climate shift. Our results suggest that the rapid discharge increase in the NW Argentine Andes coincides with the periodic enhancement of discharge, which is mainly linked to a negative to positive transition of the PDO phase and TSA variability associated with a long-term increasing trend. We therefore suggest that variations in discharge in this region are largely driven by both natural variability and the effects of global climate change. We furthermore posit that the links between atmospheric and hydrologic processes result from a combination of forcings that operate on different spatiotemporal scales. (C) 2017 Elsevier B.V. All rights reserved. KW - River discharge KW - Central Andes KW - Empirical Mode Decomposition KW - PDO KW - Climate variability KW - Global climate change Y1 - 2017 U6 - https://doi.org/10.1016/j.jhydrol.2017.10.001 SN - 0022-1694 SN - 1879-2707 VL - 555 SP - 108 EP - 124 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Castino, Fabiana A1 - Bookhagen, Bodo A1 - De la Torre, Alejandro T1 - Atmospheric dynamics of extreme discharge events from 1979 to 2016 in the southern Central Andes JF - Climate dynamics : observational, theoretical and computational research on the climate system N2 - During the South-American Monsoon season, deep convective systems occur at the eastern flank of the Central Andes leading to heavy rainfall and flooding. We investigate the large- and meso-scale atmospheric dynamics associated with extreme discharge events (> 99.9th percentile) observed in two major river catchments meridionally stretching from humid to semi-arid conditions in the southern Central Andes. Based on daily gauge time series and ERA-Interim reanalysis, we made the following three key observations: (1) for the period 1940-2016 daily discharge exhibits more pronounced variability in the southern, semi-arid than in the northern, humid catchments. This is due to a smaller ratio of discharge magnitudes between intermediate (0.2 year return period) and rare events (20 year return period) in the semi-arid compared to the humid areas; (2) The climatological composites of the 40 largest discharge events showed characteristic atmospheric features of cold surges based on 5-day time-lagged sequences of geopotential height at different levels in the troposphere; (3) A subjective classification revealed that 80% of the 40 largest discharge events are mainly associated with the north-northeastward migration of frontal systems and 2/3 of these are cold fronts, i.e. cold surges. This work highlights the importance of cold surges and their related atmospheric processes for the generation of heavy rainfall events and floods in the southern Central Andes. KW - South American monsoon system KW - cold surges KW - orographic barrier KW - mesoscale convective systems KW - extreme discharge KW - daily-discharge time KW - series ERA-interim Y1 - 2019 U6 - https://doi.org/10.1007/s00382-020-05458-1 SN - 0930-7575 SN - 1432-0894 VL - 55 IS - 11-12 SP - 3485 EP - 3505 PB - Springer CY - Berlin ; Heidelberg [u.a.] ER - TY - THES A1 - Castino, Fabiana T1 - Climate variability and extreme hydro-meteorological events in the Southern Central Andes, NW Argentina T1 - Klimavariabilität und extreme hydro-meteorologische Ereignisse in den südlichen Zentralanden, NW Argentinien N2 - Extreme hydro-meteorological events, such as severe droughts or heavy rainstorms, constitute primary manifestations of climate variability and exert a critical impact on the natural environment and human society. This is particularly true for high-mountain areas, such as the eastern flank of the southern Central Andes of NW Argentina, a region impacted by deep convection processes that form the basis of extreme events, often resulting in floods, a variety of mass movements, and hillslope processes. This region is characterized by pronounced E-W gradients in topography, precipitation, and vegetation cover, spanning low to medium-elevation, humid and densely vegetated areas to high-elevation, arid and sparsely vegetated environments. This strong E-W gradient is mirrored by differences in the efficiency of surface processes, which mobilize and transport large amounts of sediment through the fluvial system, from the steep hillslopes to the intermontane basins and further to the foreland. In a highly sensitive high-mountain environment like this, even small changes in the spatiotemporal distribution, magnitude and rates of extreme events may strongly impact environmental conditions, anthropogenic activity, and the well-being of mountain communities and beyond. However, although the NW Argentine Andes comprise the catchments for the La Plata river that traverses one of the most populated and economically relevant areas of South America, there are only few detailed investigations of climate variability and extreme hydro-meteorological events. In this thesis, I focus on deciphering the spatiotemporal variability of rainfall and river discharge, with particular emphasis on extreme hydro-meteorological events in the subtropical southern Central Andes of NW Argentina during the past seven decades. I employ various methods to assess and quantify statistically significant trend patterns of rainfall and river discharge, integrating high-quality daily time series from gauging stations (40 rainfall and 8 river discharge stations) with gridded datasets (CPC-uni and TRMM 3B42 V7), for the period between 1940 and 2015. Evidence for a general intensification of the hydrological cycle at intermediate elevations (~ 0.5 – 3 km asl) at the eastern flank of the southern Central Andes is found both from rainfall and river-discharge time-series analysis during the period from 1940 to 2015. This intensification is associated with the increase of the annual total amount of rainfall and the mean annual discharge. However, most pronounced trends are found at high percentiles, i.e. extreme hydro-meteorological events, particularly during the wet season from December to February.An important outcome of my studies is the recognition of a rapid increase in the amount of river discharge during the period between 1971 and 1977, most likely linked to the 1976-77 global climate shift, which is associated with the North Pacific Ocean sea surface temperature variability. Interestingly, after this rapid increase, both rainfall and river discharge decreased at low and intermediate elevations along the eastern flank of the Andes. In contrast, during the same time interval, at high elevations, extensive areas on the arid Puna de Atacama plateau have recorded increasing annual rainfall totals. This has been associated with more intense extreme hydro-meteorological events from 1979 to 2014. This part of the study reveals that low-, intermediate, and high-elevation sectors in the Andes of NW Argentina respond differently to changing climate conditions. Possible forcing mechanisms of the pronounced hydro-meteorological variability observed in the study area are also investigated. For the period between 1940 and 2015, I analyzed modes of oscillation of river discharge from small to medium drainage basins (102 to 104 km2), located on the eastern flank of the orogen. First, I decomposed the relevant monthly time series using the Hilbert-Huang Transform, which is particularly appropriate for non-stationary time series that result from non-linear natural processes. I observed that in the study region discharge variability can be described by five quasi-periodic oscillatory modes on timescales varying from 1 to ~20 years. Secondly, I tested the link between river-discharge variations and large-scale climate modes of variability, using different climate indices, such as the BEST ENSO (Bivariate El Niño-Southern Oscillation Time-series) index. This analysis reveals that, although most of the variance on the annual timescale is associated with the South American Monsoon System, a relatively large part of river-discharge variability is linked to Pacific Ocean variability (PDO phases) at multi-decadal timescales (~20 years). To a lesser degree, river discharge variability is also linked to the Tropical South Atlantic (TSA) sea surface temperature anomaly at multi-annual timescales (~2-5 years). Taken together, these findings exemplify the high degree of sensitivity of high-mountain environments with respect to climatic variability and change. This is particularly true for the topographic transitions between the humid, low-moderate elevations and the semi-arid to arid highlands of the southern Central Andes. Even subtle changes in the hydro-meteorological regime of these areas of the mountain belt react with major impacts on erosional hillslope processes and generate mass movements that fundamentally impact the transport capacity of mountain streams. Despite more severe storms in these areas, the fluvial system is characterized by pronounced variability of the stream power on different timescales, leading to cycles of sediment aggradation, the loss of agriculturally used land and severe impacts on infrastructure. N2 - Extreme hydro-meteorologische Ereignisse, wie langanhaltende Dürren oder Starkregen, gelten als Haupterscheinungsform der Klimavariabilität und haben einen entscheidenden Einfluss auf Umwelt und Gesellschaft. Dies gilt im Besonderen für die großen Gebirgsregionen dieser Erde. In einer extrem sensiblen Hochgebirgsregion wie den NW argentinischen Anden haben selbst geringe Veränderungen in der Intensität solcher Extremereignisse, sowie deren Häufigkeit und räumliche Verteilung, nicht nur einen großen Einfluss auf die Landschaftsentwicklung; flussabwärtsliegende Gemeinden sind zudem wirtschaftlich als auch humanitär einem hohen Risiko ausgesetzt. Die vorliegende Arbeit befasst sich im Wesentlichen mit der räumlich-zeitlichen Verteilung von Niederschlags- und Abflussmengen über den Zeitraum der letzten sieben Jahrzente, mit besonderem Fokus auf extreme hydro-meteorologische Ereignisse der subtropischen Zentralanden NW-Argentiniens. Um räumliche und zeitliche statistisch signifikante Trends der Niederschlags- und Abflussmengen bestimmen und quantifizieren zu können, finden in dieser Arbeit verschiedene Methoden Anwendung, in denen hochaufgelöste Zeitreihen von Niederschlags- und Abflussmengenstationen mit diversen Rasterdatensätzen von 1940 bis 2015 kombiniert werden. Über den betrachteten Zeitraum hinweg lässt sich eine allgemeine Intensivierung des hydrologischen Kreislaufes auf mittleren Höhen (500 - 3.000 m ü. NN) belegen. Diese Intensivierung steht einerseits im Zusammenhang mit dem Ansteigen von Extremwetterereignissen, besonders während der Regenzeit von Dezember bis Februar. Der beobachtete Anstieg in der Intensität dieser Wetterlagen deutet auf einen Zusammenhang der schweren Regenstürme und der Verlandung der Abflussbecken in der untersuchten, intermontanen Region hin. Ein rapider Anstieg in der Abflussmenge in Flüssen um bis zu 40% ist für den Zeitraum zwischen 1971 und 1977 dokumentiert. Dieser steht höchstwahrscheinlich mit der globalen Klimaverschiebung von 1976-77 in Zusammenhang, welche wiederum durch die Variabilität der Oberflächentemperatur des Pazifischen Ozeans beeinflusst wird. Nach diesem starken Anstieg können jedoch abnehmende Trends in Niederschlags- wie auch Abflussmengen auf niedrigen und mittleren Höhen der Ostflanken der Anden beobachtet werden. Im Gegensatz dazu belegen ein Anstieg der jährlichen Gesamtniederschlagsmenge sowie der Magnitude von extremen hydro-meterologischen Ereignissen in hohen Höhenlagen der trockenen Puna de Atacama Plateaus, dass niedrige, mittlere und hohe Sektoren der NW argentinischen Anden unterschiedlich auf Änderungen des Klimas reagieren. Schlussendlich kann die in der Region beobachtete, stark ausgeprägte Variabilität in der Hydro-meteorologie über jährliche Zeiträume zum größten Teil mit dem Südamerikanischen Monsunsystem erklärt werden. Jedoch sind große Anteile in der Variabilität der Abflussmenge auch stark an die Pazifische Dekaden-Oszillation (PDO) in Zeiträumen über mehrereJahrzehnte (~20 Jahre) gekoppelt und zu einem geringeren Anteil auch an die Meeresoberflächentemperatur-Anomalie des tropischen Südatlantiks (TSA) über mehrjährige Zeiträume hinweg (~2-5 Jahre). Interessanterweise wurden nur weniger stark ausgeprägte Zusammenhänge zwischen der Abflussvariabilität und El Niño-Southern Oscillation in unserem Untersuchungsgebiet gefunden. KW - climate variability KW - extreme hydrometeorological events KW - Central Andes KW - Klimavariabilität KW - extreme hydro-meteorologische Ereignisse KW - Zentralanden Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-396815 ER -