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Jiaxu Zhang

Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison

Understanding the deglacial evolution of deep Atlantic water masses in an isotope-enabled ocean model

Room 811 AOSS, May 6, 2016, 2:45 PM


The deep Atlantic water masses evolution during the early period of the last deglaciation (Heinrich Stadial 1; 17,500 years ago) is a key to unraveling the ocean's role in the global carbon cycle and climate. Yet the deep water source in response to a weakening of the Atlantic Meridional Overturning Circulation remains a subject of debate. Although many recent studies used benthic foraminiferal δ18O to provide new insights into early deglacial deep circulation, these efforts lacked a dynamic framework and concluded on speculative notes. My research develops the simulation capability of several geotracers, notably δ18O, in a state-of-the-art ocean general circulation model (POP2), and uses this isotope-enabled model to conduct a transient simulation of the last 22,000 years under realistic forcing. I find quantitatively similar evolution of δ18O and other geotracers simulated in the model and in paleoceanographic reconstructions. Analysis of model diagnostics suggests a new scenario of water mass deglacial evolution. In response to North Atlantic freshwater forcing during HS1, input into the ocean interior from northern and southern sources both decreased dramatically, with the former reducing more. The deep North Atlantic was filled with previously formed, isolated water, which was renewed slowly from the south. Meanwhile, the deep North Atlantic warmed significantly due to enhanced downward diapycnal mixing. This dynamic warming, rather than changes of the water δ18O, is the main cause of the observed lead of the deglacial North Atlantic δ18O decrease over that of the South Atlantic. This evolution scenario may help to explain the early rise of the deglacial atmospheric CO2, as well as the following abrupt onset of Northern Hemisphere warming.

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