Helle Gerhard

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Helle
First Name
Gerhard
ORCID
0000-0001-9988-6638

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  • Preprint
    400 years of summer hydroclimate from stable isotopes in Iberian trees
    ( 2016-11) Andreu-Hayles, Laia ; Ummenhofer, Caroline C. ; Barriendos, Mariano ; Schleser, Gerhard H. ; Helle, Gerhard ; Leuenberger, Markus ; Gutierrez, Emilia ; Cook, Edward R.
    Tree rings are natural archives that annually record distinct types of past climate variability depending on the parameters measured. Here, we use ring-width and stable isotopes in cellulose of trees from the northwestern Iberian Peninsula (IP) to understand regional summer hydroclimate over the last 400 years and the associated atmospheric patterns. Spatial correlations between tree rings and gridded climate products demonstrate that isotope signatures in the targeted Iberian pine forests are very sensitive to water availability during the summer period, and are mainly controlled by stomatal conductance. Non-linear methods based on extreme events analysis allow for capturing distinct seasonal climatic variability recorded by tree-ring parameters and asymmetric signals of the associated atmospheric features. Moreover, years with extreme high (low) values in the tree-ring records were characterised by coherent large-scale atmospheric circulation patterns with reduced (enhanced) moisture transport onto the northwestern IP. These analyses of extremes revealed that high/low proxy values do not necessarily correspond to mirror images in the atmospheric anomaly patterns, suggesting different drivers of these patterns and the corresponding signature recorded in the proxies. Regional hydroclimate features across the broader IP and western Europe during extreme wet/dry summers detected by the northwestern IP trees compare favourably to an independent multicentury sea level pressure and drought reconstruction for Europe. These independent sources of past climate validate our findings that attribute non-linear moisture signals recorded by extreme tree-ring values to distinct large-scale atmospheric patterns and allow for 400-yr reconstructions of the frequency of occurrence of extreme conditions in summer hydroclimate.
  • Article
    Punctuated shutdown of Atlantic Meridional Overturning Circulation during Greenland Stadial 1
    (Nature Publishing Group, 2016-05-19) Hogg, Alan G. ; Southon, John R. ; Turney, Christian S. M. ; Palmer, Jonathan G. ; Bronk Ramsey, Christopher ; Fenwick, Pavla ; Boswijk, Gretel ; Friedrich, Michael ; Helle, Gerhard ; Hughen, Konrad A. ; Jones, Richard ; Kromer, Bernd ; Noronha, Alexandra ; Reynard, Linda ; Staff, Richard ; Wacker, Lukas
    The Greenland Stadial 1 (GS-1; ~12.9 to 11.65 kyr cal BP) was a period of North Atlantic cooling, thought to have been initiated by North America fresh water runoff that caused a sustained reduction of North Atlantic Meridional Overturning Circulation (AMOC), resulting in an antiphase temperature response between the hemispheres (the ‘bipolar seesaw’). Here we exploit sub-fossil New Zealand kauri trees to report the first securely dated, decadally-resolved atmospheric radiocarbon (14C) record spanning GS-1. By precisely aligning Southern and Northern Hemisphere tree-ring 14C records with marine 14C sequences we document two relatively short periods of AMOC collapse during the stadial, at ~12,920-12,640 cal BP and 12,050-11,900 cal BP. In addition, our data show that the interhemispheric atmospheric 14C offset was close to zero prior to GS-1, before reaching ‘near-modern’ values at ~12,660 cal BP, consistent with synchronous recovery of overturning in both hemispheres and increased Southern Ocean ventilation. Hence, sustained North Atlantic cooling across GS-1 was not driven by a prolonged AMOC reduction but probably due to an equatorward migration of the Polar Front, reducing the advection of southwesterly air masses to high latitudes. Our findings suggest opposing hemispheric temperature trends were driven by atmospheric teleconnections, rather than AMOC changes.
  • Preprint
    Decadally resolved lateglacial radiocarbon evidence from New Zealand kauri
    ( 2016-10) Hogg, Alan G. ; Southon, John R. ; Turney, Christian S. M. ; Palmer, Jonathan G. ; Bronk Ramsey, Christopher ; Fenwick, Pavla ; Boswijk, Gretel ; Büntgen, Ulf ; Friedrich, Michael ; Helle, Gerhard ; Hughen, Konrad A. ; Jones, Richard ; Kromer, Bernd ; Noronha, Alexandra ; Reinig, Frederick ; Reynard, Linda ; Staff, Richard ; Wacker, Lukas
    The Last Glacial-Interglacial Transition (LGIT; 15,000-11,000 cal BP) was characterized by complex spatiotemporal patterns of climate change, with numerous studies requiring accurate chronological control to decipher leads from lags in global paleoclimatic, -environmental and archaeological records. However, close scrutiny of the few available tree-ring chronologies and 14C-dated sequences composing the IntCal13 radiocarbon calibration curve, indicates significant weakness in 14C calibration across key periods of the LGIT. Here, we present a decadally-resolved atmospheric 14C record derived from New Zealand kauri spanning the Lateglacial from ~13,100 - 11,365 cal BP. Two floating kauri 14C time series, curve-matched to IntCal13, serve as a radiocarbon backbone through the Younger Dryas. The floating Northern Hemisphere (NH) 14C datasets derived from the YD-B and Central European Lateglacial Master tree-ring series are matched against the new kauri data, forming a robust NH 14C time series to ~14,200 cal BP. Our results show that IntCal13 is questionable from ~12,200 - 11,900 cal BP and the ~10,400 BP 14C plateau is approximately five decades too short. The new kauri record and re-positioned NH pine 14C series offer a refinement of the international 14C calibration curves IntCal13 and SHCal13, providing increased confidence in the correlation of global paleorecords.