LeGrande Allegra N.

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LeGrande
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Allegra N.
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  • Article
    The influence of Indian Ocean atmospheric circulation on Warm Pool hydroclimate during the Holocene epoch
    (American Geophysical Union, 2012-10-04) Tierney, Jessica E. ; Oppo, Delia W. ; LeGrande, Allegra N. ; Huang, Yongsong ; Rosenthal, Yair ; Linsley, Braddock K.
    Existing paleoclimate data suggest a complex evolution of hydroclimate within the Indo-Pacific Warm Pool (IPWP) during the Holocene epoch. Here we introduce a new leaf wax isotope record from Sulawesi, Indonesia and compare proxy water isotope data with ocean-atmosphere general circulation model (OAGCM) simulations to identify mechanisms influencing Holocene IPWP hydroclimate. Modeling simulations suggest that orbital forcing causes heterogenous changes in precipitation across the IPWP on a seasonal basis that may account for the differences in time-evolution of the proxy data at respective sites. Both the proxies and simulations suggest that precipitation variability during the September–November (SON) season is important for hydroclimate in Borneo. The preëminence of the SON season suggests that a seasonally lagged relationship between the Indian
  • Preprint
    Rapid early Holocene deglaciation of the Laurentide ice sheet
    ( 2008-07-24) Carlson, Anders E. ; LeGrande, Allegra N. ; Oppo, Delia W. ; Came, Rosemarie E. ; Schmidt, Gavin A. ; Anslow, Faron S. ; Licciardi, Joseph M. ; Obbink, Elizabeth A.
    The early Holocene deglaciation of the Laurentide Ice Sheet (LIS) is the most recent and best constrained disappearance of a large Northern Hemisphere ice sheet. Its demise is a natural experiment for assessing rates of ice sheet decay and attendant contributions to sea level rise. Here we demonstrate with terrestrial and marine records that the final LIS demise occurred in two stages of rapid melting from ~9.0- 8.5 and 7.6-6.8 kyr BP with the LIS contributing ~1.3 and 0.7 cm yr-1 to sea level rise, respectively. Simulations using a fully coupled atmosphere-ocean general circulation model suggest that increased ablation from enhanced early Holocene boreal summer insolation may have been the predominant cause of the LIS contributions to sea level rise. Although the boreal summer surface radiative forcing of early Holocene LIS retreat is twice that of projections for 2100 C.E. greenhouse gas radiative forcing, the associated summer surface air temperature increase is the same. The geologic evidence for rapid LIS retreat under a comparable forcing provides a prehistoric precedent for a possible large negative mass balance response of the Greenland Ice Sheet by the end of the coming century.
  • Article
    Seawater isotope constraints on tropical hydrology during the Holocene
    (American Geophysical Union, 2007-07-03) Oppo, Delia W. ; Schmidt, Gavin A. ; LeGrande, Allegra N.
    Paleoceanographic data from the low latitude Pacific Ocean provides evidence of changes in the freshwater budget and redistribution of freshwater within the basin during the Holocene. Reconstructed Holocene seawater δ 18O changes compare favorably to differences predicted between climate simulations for the middle Holocene (MH) and for the pre-Industrial late Holocene (LH). The model simulations demonstrate that changes in the tropical hydrologic cycle affect the relationship between δ 18Osw and surface salinity, and allow, for the first time, quantitative estimates of western Pacific salinity change during the Holocene. The simulations suggest that during the MH, the mean salinity of the Pacific was higher because less water vapor was transported from the Atlantic Ocean and more was transported to the Indian Ocean. The salinity of the western Pacific was enhanced further due both to the greater advection of salt to the region by ocean currents and to an increase in continental precipitation at the expense of maritime precipitation, the latter a consequence of the stronger Asian summer monsoon.