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  • 1
    Online-Ressource
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    South American Summer Monsoon variability over the last millennium in paleoclimate records and isotope-enabled climate models
    Copernicus GmbH ; 2022
    In:  Climate of the Past Vol. 18, No. 9 ( 2022-09-02), p. 2045-2062
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 18, No. 9 ( 2022-09-02), p. 2045-2062
    Kurzfassung: Abstract. The South American Summer Monsoon (SASM) is the main driver of regional hydroclimate variability across tropical and subtropical South America. It is best recorded on paleoclimatic timescales by stable oxygen isotope proxies, which are more spatially representative of regional hydroclimate than proxies for local precipitation alone. Network studies of proxies that can isolate regional influences lend particular insight into various environmental characteristics that modulate hydroclimate, such as atmospheric circulation variability and changes in the regional energy budget as well as understanding the climate system sensitivity to external forcings. We extract the coherent modes of variability of the SASM over the last millennium (LM) using a Monte Carlo empirical orthogonal function (MCEOF) decomposition of 14 δ18O proxy records and compare them with modes decomposed from isotope-enabled climate model data. The two leading modes reflect the isotopic variability associated with (1) thermodynamic changes driving the upper-tropospheric monsoon circulation (Bolivian High–Nordeste Low waveguide) and (2) the latitudinal displacement of the South Atlantic Convergence Zone (SACZ). The spatial characteristics of these modes appear to be robust features of the LM hydroclimate over South America and are reproduced both in the proxy data and in isotope-enabled climate models, regardless of the nature of the model-imposed external forcing. The proxy data document that the SASM was characterized by considerable temporal variability throughout the LM, with significant departures from the mean state during both the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). Model analyses during these periods suggest that the local isotopic composition of precipitation is primarily a reflection of upstream rainout processes associated with monsoon convection. Model and proxy data both point to an intensification of the monsoon during the LIA over the central and western parts of tropical South America and indicate a displacement of the South Atlantic Convergence Zone (SACZ) to the southwest. These centennial-scale changes in monsoon intensity over the LM are underestimated in climate models, complicating the attribution of changes on these timescales to specific forcings and pointing toward areas of important model development.
    Materialart: Online-Ressource
    ISSN: 1814-9332
    URL: Article
    DOI: 10.5194/cp-18-2045-2022
    Sprache: Englisch
    Verlag: Copernicus GmbH
    Publikationsdatum: 2022
    ZDB Id: 2217985-9
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  • 2
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    The South American monsoon variability over the last millennium in climate models
    Copernicus GmbH ; 2016
    In:  Climate of the Past Vol. 12, No. 8 ( 2016-08-19), p. 1681-1691
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 12, No. 8 ( 2016-08-19), p. 1681-1691
    Kurzfassung: Abstract. In this paper we assess South American monsoon system (SAMS) variability in the last millennium as depicted by global coupled climate model simulations. High-resolution proxy records for the South American monsoon over this period show a coherent regional picture of a weak monsoon during the Medieval Climate Anomaly and a stronger monsoon during the Little Ice Age (LIA). Due to the small external forcing during the past 1000 years, model simulations do not show very strong temperature anomalies over these two specific periods, which in turn do not translate into clear precipitation anomalies, in contrast with the rainfall reconstructions in South America. Therefore, we used an ad hoc definition of these two periods for each model simulation in order to account for model-specific signals. Thereby, several coherent large-scale atmospheric circulation anomalies are identified. The models feature a stronger monsoon during the LIA associated with (i) an enhancement of the rising motion in the SAMS domain in austral summer; (ii) a stronger monsoon-related upper-tropospheric anticyclone; (iii) activation of the South American dipole, which results in a poleward shift of the South Atlantic Convergence Zone; and (iv) a weaker upper-level subtropical jet over South America. The diagnosed changes provide important insights into the mechanisms of these climate anomalies over South America during the past millennium.
    Materialart: Online-Ressource
    ISSN: 1814-9332
    URL: Article
    DOI: 10.5194/cp-12-1681-2016
    Sprache: Englisch
    Verlag: Copernicus GmbH
    Publikationsdatum: 2016
    ZDB Id: 2217985-9
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  • 3
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    Hemispherically asymmetric volcanic forcing of tropical hydroclimate during the last millennium
    Copernicus GmbH ; 2016
    In:  Earth System Dynamics Vol. 7, No. 3 ( 2016-08-23), p. 681-696
    Details
    In: Earth System Dynamics, Copernicus GmbH, Vol. 7, No. 3 ( 2016-08-23), p. 681-696
    Kurzfassung: Abstract. Volcanic aerosols exert the most important natural radiative forcing of the last millennium. State-of-the-art paleoclimate simulations of this interval are typically forced with diverse spatial patterns of volcanic forcing, leading to different responses in tropical hydroclimate. Recently, theoretical considerations relating the intertropical convergence zone (ITCZ) position to the demands of global energy balance have emerged in the literature, allowing for a connection to be made between the paleoclimate simulations and recent developments in the understanding of ITCZ dynamics. These energetic considerations aid in explaining the well-known historical, paleoclimatic, and modeling evidence that the ITCZ migrates away from the hemisphere that is energetically deficient in response to asymmetric forcing.Here we use two separate general circulation model (GCM) suites of experiments for the last millennium to relate the ITCZ position to asymmetries in prescribed volcanic sulfate aerosols in the stratosphere and related asymmetric radiative forcing. We discuss the ITCZ shift in the context of atmospheric energetics and discuss the ramifications of transient ITCZ migrations for other sensitive indicators of changes in the tropical hydrologic cycle, including global streamflow. For the first time, we also offer insight into the large-scale fingerprint of water isotopologues in precipitation (δ18Op) in response to asymmetries in radiative forcing. The ITCZ shifts away from the hemisphere with greater volcanic forcing. Since the isotopic composition of precipitation in the ITCZ is relatively depleted compared to areas outside this zone, this meridional precipitation migration results in a large-scale enrichment (depletion) in the isotopic composition of tropical precipitation in regions the ITCZ moves away from (toward). Our results highlight the need for careful consideration of the spatial structure of volcanic forcing for interpreting volcanic signals in proxy records and therefore in evaluating the skill of Common Era climate model output.
    Materialart: Online-Ressource
    ISSN: 2190-4987
    URL: Article
    URL: Article
    DOI: 10.5194/esd-7-681-2016
    DOI: 10.5194/esd-7-681-2016-supplement
    Sprache: Englisch
    Verlag: Copernicus GmbH
    Publikationsdatum: 2016
    ZDB Id: 2578793-7
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  • 4
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    Influence of North Pacific decadal variability on the western Canadian Arctic over the past 700 years
    Copernicus GmbH ; 2017
    In:  Climate of the Past Vol. 13, No. 4 ( 2017-04-21), p. 411-420
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 13, No. 4 ( 2017-04-21), p. 411-420
    Kurzfassung: Abstract. Understanding how internal climate variability influences arctic regions is required to better forecast future global climate variations. This paper investigates an annually-laminated (varved) record from the western Canadian Arctic and finds that the varves are negatively correlated with both the instrumental Pacific Decadal Oscillation (PDO) during the past century and also with reconstructed PDO over the past 700 years, suggesting drier Arctic conditions during high-PDO phases, and vice versa. These results are in agreement with known regional teleconnections, whereby the PDO is negatively and positively correlated with summer precipitation and mean sea level pressure respectively. This pattern is also evident during the positive phase of the North Pacific Index (NPI) in autumn. Reduced sea-ice cover during summer–autumn is observed in the region during PDO− (NPI+) and is associated with low-level southerly winds that originate from the northernmost Pacific across the Bering Strait and can reach as far as the western Canadian Arctic. These climate anomalies are associated with the PDO− (NPI+) phase and are key factors in enhancing evaporation and subsequent precipitation in this region of the Arctic. Collectively, the sedimentary evidence suggests that North Pacific climate variability has been a persistent regulator of the regional climate in the western Canadian Arctic. Since projected sea-ice loss will contribute to enhanced future warming in the Arctic, future negative phases of the PDO (or NPI+) will likely act to amplify this positive feedback.
    Materialart: Online-Ressource
    ISSN: 1814-9332
    URL: Article
    URL: Article
    DOI: 10.5194/cp-13-411-2017
    DOI: 10.5194/cp-13-411-2017-supplement
    Sprache: Englisch
    Verlag: Copernicus GmbH
    Publikationsdatum: 2017
    ZDB Id: 2217985-9
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  • 5
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    The influence of volcanic eruptions on the climate of tropical South America during the last millennium in an isotope-enabled general circulation model
    Copernicus GmbH ; 2016
    In:  Climate of the Past Vol. 12, No. 4 ( 2016-04-15), p. 961-979
    Details
    In: Climate of the Past, Copernicus GmbH, Vol. 12, No. 4 ( 2016-04-15), p. 961-979
    Kurzfassung: Abstract. Currently, little is known on how volcanic eruptions impact large-scale climate phenomena such as South American paleo-Intertropical Convergence Zone (ITCZ) position and summer monsoon behavior. In this paper, an analysis of observations and model simulations is employed to assess the influence of large volcanic eruptions on the climate of tropical South America. This problem is first considered for historically recent volcanic episodes for which more observations are available but where fewer events exist and the confounding effects of El Niño–Southern Oscillation (ENSO) lead to inconclusive interpretation of the impact of volcanic eruptions at the continental scale. Therefore, we also examine a greater number of reconstructed volcanic events for the period 850 CE to present that are incorporated into the NASA GISS ModelE2-R simulation of the last millennium. An advantage of this model is its ability to explicitly track water isotopologues throughout the hydrologic cycle and simulating the isotopic imprint following a large eruption. This effectively removes a degree of uncertainty associated with error-prone conversion of isotopic signals into climate variables, and allows for a direct comparison between GISS simulations and paleoclimate proxy records. Our analysis reveals that both precipitation and oxygen isotope variability respond with a distinct seasonal and spatial structure across tropical South America following an eruption. During austral winter, the heavy oxygen isotope in precipitation is enriched, likely due to reduced moisture convergence in the ITCZ domain and reduced rainfall over northern South America. During austral summer, however, more negative values of the precipitation isotopic composition are simulated over Amazonia, despite reductions in rainfall, suggesting that the isotopic response is not a simple function of the "amount effect". During the South American monsoon season, the amplitude of the temperature response to volcanic forcing is larger than the rather weak and spatially less coherent precipitation signal, complicating the isotopic response to changes in the hydrologic cycle.
    Materialart: Online-Ressource
    ISSN: 1814-9332
    URL: Article
    URL: Article
    DOI: 10.5194/cp-12-961-2016
    DOI: 10.5194/cp-12-961-2016-supplement
    Sprache: Englisch
    Verlag: Copernicus GmbH
    Publikationsdatum: 2016
    ZDB Id: 2217985-9
    Standort Signatur Einschränkungen Verfügbarkeit
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