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Jiang Zhu, PhD Defense

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

Investigate Paleoclimate Questions Using iCESM — the ENSO Response to Global Cooling/warming and the Ice-core Thermometer

Room 811 AOSS, May 1, 2017, 3:30 PM

Abstract

In my PhD study, I have investigated two questions using the water isotope-enabled Community Earth System Model (iCESM): (1) How does the ENSO respond to global cooling/warming? (2) How good is the ice-core δ18O thermometer? (1) ENSO response to global cooling/warming. Despite its paramount importance in climate system, the response of ENSO to anthropogenic global warming is still inconclusive in climate models, so is the ENSO response to the glacial global cooling at the last glacial maximum (LGM) in reconstructions and models. Here, for the first time, model simulated oxygen isotopes are directly compared with the foraminifera δ18O records for the LGM ENSO. I find that the LGM ENSO is most likely weaker than the preindustrial, due to the weaker atmosphere-ocean coupling in a colder climate with a deeper thermocline. The iCESM further suggests that δ18O records using the individual foraminifera analysis could be complicated by possible changes in seasonality and habitat depth of foraminifera. This study indicates that we could have an intensified ENSO for the future warming. (2) The ice-core δ18O thermometer. For more than 50 years, ice-core δ18O has provided a tremendous amount of information about the Earth climate history during the late Quaternary. However, questions still remain about a quantitative interpretation of ice-core δ18O. Here, δ18O–temperature relationship over Greenland in response to varied climatic forcings, including greenhouse gas, ice sheet, orbital parameter and meltwater, is quantitatively studied. I find that temporal slope responds most significantly to the meltwater forcing. Meltwater discharge in the northern North Atlantic can increase the temporal slope considerably, because of the reduced moisture from nearby oceans. It is also found that part of the ice-core δ18O changes during meltwater events can be simply attributed to the tracer effect—the propagation of depleted meltwater in the hydrological cycle—without involving any changes in climate states. These findings imply that abrupt temperature changes during meltwater events previously inferred from ice cores could have been significantly overestimated.



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