Jiang Zhu
Published: 2017
Total Pages: 124
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Paleoclimate reconstructions can help us learn the evolution of climate mean state and variability in the past, understand mechanisms for climate change, and test the climate models that are extensively used for future climate projections. However, reconstructions have one major disadvantage that usually they are measurements of proxy variables (e.g., oxygen isotopes in calcium shells of foraminifera) instead of climate state variables (e.g., ocean temperature), which are directly simulated in traditional coupled climate models. This require that the model-data comparisons are indirect in paleoclimate studies and making it extremely difficult to address model-data discrepancies, especially when both models and reconstructions are subject to substantial biases and uncertainties. To overcome this obstacle, my PhD work involves developing a fully coupled water isotope-enabled Community Earth System Model (iCESM) in collaboration with other scientists. The physical climate of the iCESM is one of the best state-of-the-art fully coupled earth system models. In addition to the regular hydrologic cycle, iCESM can explicitly simulate the transport and transformation of water isotopes (e.g., H218O) in its components--the atmosphere, land, ocean, sea ice and river runoff. The iCESM can well-reproduce the major features of water isotopes in observations, including the tropical amount effect and high latitude temperature effect, as well as the continental and altitude effects in precipitation-d18Op in present day observations. The simulated d18O in seawater (d18Ow) also closely resemble the pattern in observations. Moreover, a simulation of the LGM (21,000 ka before present) shows that the model is able to simulate the glacial-interglacial changes of d18Op in ice cores, d18Ow in porefluid reconstructions, and d18Oc in ocean sediments, suggesting the model is suited for paleoclimate studies. With the water-isotope capability of the iCESM, I have investigated the following scientific questions: (1) How and why the water isotope-temperature relationship in Greenland ice cores varies during abrupt climate changes; (2) Whether the El Niño-Southern Oscillation (ENSO) was stronger or weaker at the LGM than the present day. (1) Isotope-temperature relationship. For more than 50 years, water isotope values (e.g., d18O) in ice cores have provided a tremendous amount of information about the Earth climate history during the late Quaternary. Initially based on a "modern analogue method", d18O changes in ice cores were directly interpreted as variations in local temperature, which was challenged later by independent reconstructions. Although it is now becoming clear that the temporal d18O-temperature relationship could vary both spatially and temporally in ice-core records, how the temporal slope could vary during abrupt climate changes and what is causing these changes still remain unclear. In my PhD study, I have quantitatively studied the changes in d18O-temperature relationship over Greenland in response to varied climatic forcings using the iCESM. I found that the temporal slope in Greenland increases significantly with the amount of meltwater discharged into the northern North Atlantic Ocean, due to the reduced moisture from the nearby oceans and the tracer effect from depleted meltwater (e.g., about -30 ‰). Otherwise, the d18O-temperature relationships (spatial and temporal) are relatively stable in response to greenhouse gas (GHG), ice sheets and mid-Holocene orbital forcing. It is also found that part of the d18O signal in ice cores during meltwater events can be simply attributed to the tracer effect--the propagation of depleted meltwater in the hydrological cycle--instead of any changes in the climate state. These important findings imply that abrupt temperature changes during meltwater events previously inferred from ice cores could have been significantly overestimated. (2) ENSO variability at the LGM. Despite its paramount importance in climate system, the response of ENSO to anthropogenic global warming is still inconclusive in recent climate models. Studying the ENSO strength in the past can serve as a testbed for these climate models used for future projections and provide us the opportunity to investigate possible relationships between ENSO variability and the mean climate states. But, the ENSO strength at the LGM is inconclusive both in current climate models and paleoclimate reconstructions, including those records using the individual foraminifera analysis (IFA) in the eastern equatorial Pacific (EEP). Here, for the first time, I have directly compared modeled water isotopes in the iCESM with the IFA records. Synthesizing evidence from both models and reconstructions, it is found that ENSO at the LGM is most likely weaker than that of the preindustrial, because of the weakened Bjerknes feedbacks. The iCESM suggests that total variance of the IFA records may only reflect changes in the annual cycle instead of ENSO variability as previously assumed. Furthermore, the interpretation of subsurface IFA records can be substantially complicated by the habitat depth of thermocline-dwelling foraminifera and their vertical migration with a temporally varying thermocline. The model suggests an inverse relationship between ENSO variability and zonal SST gradient, thermocline depth and surface winds in equatorial Pacific, consistent with previous theoretical or observation based studies. Results indicate that ENSO variability could be stronger in response to the future anthropogenic global warming.