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Ocean circulation during the last deglaciation can help to improve the understanding of the mechanisms underlying the ocean circulation. However, previous model-data comparisons suffer from indirect comparison because both reconstruction and climate model have uncertainties. To meet this challenge, my PhD work contributes to the isotope enabled Community Earth System Model (iCESM) project by developing a Neodymium (Nd) model and a Protactinium (231Pa) and Thorium (230Th) in the ocean model of CESM. With the isotope enabled ocean model (iPOP2), I investigated two scientific questions: (1) Deglacial AAIW in the Atlantic. AAIW plays important roles in the global climate system and the global ocean nutrient and carbon cycles. However, neodymium isotopic composition ([epsilon]Nd) reconstructions from different locations from the tropical Atlantic, have led to a debate on the relationship between northward penetration of AAIW into the tropical Atlantic and AMOC variability during the last deglaciation. Our results suggest a coherent response of AAIW and AMOC: when AMOC weakens, the northward penetration and transport of AAIW decreases while its depth and thickness increase. Moreover, the inconsistency among different tropical Atlantic [epsilon]Nd reconstructions is reconciled by considering their corresponding core locations and depths, which were influenced by different water masses in the past. (2) Using 18Oc to reconstruct AMOC. 18Oc gradient can be used to reconstruct density gradient, therefore the AMOC strength. 18Oc from the Florida Straits has been used to reconstruct AMOC evolution during the last deglaciation but the strength of Florida current can also be influenced by surface wind stress. Our model simulation suggests that in the western boundary, the Florida current strength is dominated by AMOC through the last deglaciation, instead of surface wind. However, in the South Atlantic, the basin-wide 18Oc contrast is decoupled from density contrast through the deglaciation in the upper ocean because of the deglacial density contrast change is dominated by salinity, which is caused by the deglacial change of AAIW. Our model suggests that 18Oc contrast across the western boundary is a good indicator for AMOC strength and 18Oc contrast across the whole basin only works for the North Atlantic.
Since the Last Glacial Maximum (LGM, ~ 20,000 years ago) air temperatures warmed, sea level rose roughly 130 meters, and atmospheric concentrations of carbon dioxide increased. This thesis combines global models and paleoceanographic observations to constrain the ocean’s role in storing and transporting heat, salt, and other tracers during this time, with implications for understanding how the modern ocean works and how it might change in the future. • By combining a kinematic ocean model with “upstream” and “downstream” deglacial oxygen isotope time series from benthic and planktonic foraminifera, I show that the data are in agreement with the modern circulation, quantify their power to infer circulation changes, and propose new data locations. • An ocean general circulation model (the MITgcm) constrained to fit LGM sea surface temperature proxy observations reveals colder ocean temperatures, greater sea ice extent, and changes in ocean mixed layer depth, and suggests that some features in the data are not robust. • A sensitivity analysis in the MITgcm demonstrates that changes in winds or in ocean turbulent transport can explain the hypothesis that the boundary between deep Atlantic waters originating from Northern and Southern Hemispheres was shallower at the LGM than it is today.
The oceans play a crucial role in the Earth’s climate system due largely to their ability to store and transport heat. The instrumental record, spanning an order of magnitude of 100 years, is short compared with some of the important timescales of climate variability. To understand the oceans’ role in these long-term changes, proxy data from sediments, ice cores, and corals must be used. Using these proxy data, we examine the evidence for past ocean circulation and sea-level changes before instrumental oceanographic measurements began. We discuss what paleoclimatic data can tell us about past ocean states and what can be learned from ocean and climate models. Particular foci of the chapter are the ocean circulation and sea-level changes during the Quaternary and the Cretaceous, two particularly interesting periods in Earth’s history. The Quaternary covers the past 2.5 million years and is characterized by periodic glaciations, while the Cretaceous, reaching back around 100 million years, had a warm greenhouse climate with a weak temperature gradient between the tropics and the poles.
MARGO - Multiproxy Approach for the Reconstruction of the Glacial Ocean surface summarizes the results of the MARGO international working group, with the aim to develop an updated and harmonised reconstruction of sea surface temperatures and sea-ice extent of the Last Glacial Maximum oceans. The MARGO approach differs from previous efforts by developing and consistently applying measures of various aspects of reconstruction reliability, and by combining faunal and geochemical proxies. In 14 papers, the volume provides a comprehensive review of earlier work and a series of new, proxy-specific reconstructions based on census counts of planktonic foraminifera, diatoms, radiolaria and dinoflagellate cysts as well as on Mg/Ca measurements in planktonic foraminifera. The approach of harmonising the calibration and application of different proxies is described in detail, various paleothermometry techniques and their results are compared and the challenge of treating sparsely sampled data as the basis for ocean circulation models is addressed. The use of stable oxygen isotope composition of foraminiferal shells as a proxy for past sea water composition is comprehensively reassessed, and a new approach to the transfer function paleothermometer is presented. This volume represents a landmark contribution to the understanding of ice-age oceanography as well as the proxies used to reconstruct past ocean states. The results will form the basis for forcing and validation of ocean circulation models. New regional reconstructions of Last Glacial Maximum ocean temperatures and sea ice cover Compilation of new calibration and fossil datasets as well as documentation of techniques and approaches to paleoenvironmental reconstructions Comparison of techniques, proxies and modelling approaches
Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 126. Until a few decades ago, scientists generally believed that significant large-scale past global and regional climate changes occurred at a gradual pace within a time scale of many centuries or millennia. A secondary assumption followed: climate change was scarcely perceptible during a human lifetime. Recent paleoclimatic studies, however, have proven otherwise: that global climate can change extremely rapidly. In fact, there is good evidence that in the past at least regional mean annual temperatures changed by several degrees Celsius on a time scale of several centuries to several decades.
Changes in the Antarctic Circumpolar Current (ACC) transport during the last deglaciation cycle are thought to have played an important role on global climate variability. A better understanding of ACC transport at the Last Glacial Maximum (LGM) would allow better assessment of ACC dynamics and past global-scale climatic variations. However, estimates of ACC transport vary widely among some LGM coupled ocean-atmosphere climate model simulations and proxy studies. As the ACC is in the thermal wind balance, oxygen isotopic ratios (d18O) in the foraminifera (d18Oc) are often used as a proxy to reconstruct past density variation and thus ACC transport. Here we test the ACC transport and its dynamics at the LGM (20 ka) in a transient simulation of the last deglaciation with the CESM (C-iTRACE) and examine the ability of the d18Oc gradient to reconstruct the density gradient to estimate LGM ACC transport. The model simulation suggests that the ACC transport was approximately 60% greater at the LGM (227 Sv) compared with that of today (145 Sv), which mainly results from the baroclinic transport. Furthermore, the d18Oc gradient at the LGM over the Southern Ocean tends to underestimate the LGM density gradient because the d18Oc gradient results primarily from temperature changes but the density gradient is highly affected by salinity changes due to sea ice formation. Therefore, although d18Oc gradient is a powerful tool to reconstruct past density, it likely underestimates the Southern Ocean density gradient at the LGM and it hence to underestimate the ACC transport.
The issue of global warming and climate change is of continuous concern. Since the 1970s, it bas been shown that the pack-ice around the Arctic Ocean is thinning, the margin of permafrost is moving north and the vegetation in the high northern parts of the world is changing (the 'greening' of the Arctic). But are these changes the result of human activity or simply regular variations of the Earth's climate system? Over thousands of years, a continuous archive of iceberg and sea ice drift bas formed in the deep-sea sediments, revealing the place of the ice's origin and allowing a reconstruction of the surface currents and the climate of the past. However, the drift of floating ice from one place to another is not just a passive record of past ocean circulation. It actively influences and changes the surface ocean circulation, thus having a profound effect on climate change. Ice Drift, Ocean Circulation and Climate Change is the first book to focus on the interactions between ice, the ocean and the atmosphere and to describe how these three components of the climate system influence each other. It makes clear the positive contribution of paleoclimatology and paleoceanography and should be read by anyone concerned with global warming and climate change.
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