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• 1. [Article] Glacial greenhouse-gas fluctuations controlled by ocean circulation changes

  Earth's climate and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) varied strongly on millennial timescales during past glacial periods. Large and rapid ...

  Citation × Citation Citation Abstract Copy Title: Glacial greenhouse-gas fluctuations controlled by ocean circulation changes Author: Schmittner, Andreas, Galbraith, Eric D. Earth's climate and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) varied strongly on millennial timescales during past glacial periods. Large and rapid warming events in Greenland and the North Atlantic were followed by more gradual cooling, and are highly correlated with fluctuations of N2O as recorded in ice cores. Antarctic temperature variations, on the other hand, were smaller and more gradual, showed warming during the Greenland cold phase and cooling while the North Atlantic was warm, and were highly correlated with fluctuations in CO2. Abrupt changes in the Atlantic meridional overturning circulation (AMOC) have often been invoked to explain the physical characteristics of these Dansgaard–Oeschger climate oscillations, but the mechanisms for the greenhouse-gas variations and their linkage to the AMOC have remained unclear. Here we present simulations with a coupled model of glacial climate and biogeochemical cycles, forced only with changes in the AMOC. The model simultaneously reproduces characteristic features of the Dansgaard–Oeschger temperature, as well as CO2 and N2O fluctuations. Despite significant changes in the land carbon inventory, CO2 variations on millennial timescales are dominated by slow changes in the deep ocean inventory of biologically sequestered carbon and are correlated with Antarctic temperature and Southern Ocean stratification. In contrast, N2O co-varies more rapidly with Greenland temperatures owing to fast adjustments of the thermocline oxygen budget. These results suggest that ocean circulation changes were the primary mechanism that drove glacial CO2 and N2O fluctuations on millennial timescales. Title: Glacial greenhouse-gas fluctuations controlled by ocean circulation changes Author: Schmittner, Andreas, Galbraith, Eric D. Earth's climate and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) varied strongly on millennial timescales during past glacial periods. Large and rapid warming events in Greenland and the North Atlantic were followed by more gradual cooling, and are highly correlated with fluctuations of N2O as recorded in ice cores. Antarctic temperature variations, on the other hand, were smaller and more gradual, showed warming during the Greenland cold phase and cooling while the North Atlantic was warm, and were highly correlated with fluctuations in CO2. Abrupt changes in the Atlantic meridional overturning circulation (AMOC) have often been invoked to explain the physical characteristics of these Dansgaard–Oeschger climate oscillations, but the mechanisms for the greenhouse-gas variations and their linkage to the AMOC have remained unclear. Here we present simulations with a coupled model of glacial climate and biogeochemical cycles, forced only with changes in the AMOC. The model simultaneously reproduces characteristic features of the Dansgaard–Oeschger temperature, as well as CO2 and N2O fluctuations. Despite significant changes in the land carbon inventory, CO2 variations on millennial timescales are dominated by slow changes in the deep ocean inventory of biologically sequestered carbon and are correlated with Antarctic temperature and Southern Ocean stratification. In contrast, N2O co-varies more rapidly with Greenland temperatures owing to fast adjustments of the thermocline oxygen budget. These results suggest that ocean circulation changes were the primary mechanism that drove glacial CO2 and N2O fluctuations on millennial timescales. Close

• 2. [Article] Simulating past, present, and future changes in ENSO : a model evaluation and data-model comparison

  This thesis presents the results of a formal evaluation of a new AOGCM, GENMOM, demonstrating its ability to simulate present-day climate and ENSO dynamics. The model is applied to simulate climate for ...

  Citation × Citation Citation Abstract Copy Title: Simulating past, present, and future changes in ENSO : a model evaluation and data-model comparison Author: Alder, Jay R. This thesis presents the results of a formal evaluation of a new AOGCM, GENMOM, demonstrating its ability to simulate present-day climate and ENSO dynamics. The model is applied to simulate climate for the Last Glacial Maximum, deglacial, and Holocene time periods. The model output is evaluated against the best available proxy reconstructions in a detailed data-model comparison. ENSO strength is analyzed in seven paleo simulations and compared to coral and laminated lake sediment proxy records to provide an understanding of how ENSO related mechanisms varied in the past and how they vary under increased atmospheric CO₂ forced global warming. The GENMOM simulated present-day is found to be on par with three models used in the IPCC AR4 assessment and is comparable with reanalysis products (e.g, NCEP2). Atmospheric features such as the jet stream structure and major semipermanent sea level pressure centers are well simulated as is the mean planetary-scale wind structure that is needed to produce the correct position of stormtracks. Most ocean surface currents are reproduced except where they are not resolvable at T31 resolution. Overall, GENMOM captures the observed gradients and spatial distributions of annual surface temperature and precipitation and the simulations are on par with other AOGCMs. Deficiencies in the GENMOM present-day simulation include a warm bias in the surface temperature over the southern oceans, a split in the ITCZ and weaker-thanobserved overturning circulation. GENMOM produces a global temperature bias of 0.6 °C. GENMOM is demonstrated to capture ENSO dynamics similar to eight AOGCMs that were evaluated in the IPCC AR4. The Niño 3 - 4 indices have a standard deviation within 0.3 °C of the observations, indicating GENMOM is producing variability in the tropical Pacifc that is comparable to observations. GENMOM produces present-day ENSO events with an average period of 5.6 years, which is within the 2 – 7 range exhibited in the observed historical record. The mid-Holocene (6ka) and Last Glacial Maximum (LGM, 21ka) simulations are compared to the best available proxy reconstructions for sea surface temperature, precipitation and net moisture to ensure the simulations are plausible. This thesis finds that the model is in good agreement over broad spatial scales, with regional discrepancies between the model and proxy data. Coral and laminated lake sediment proxy records indicate mid-Holocene ENSO strength was reduced by 15 - 60%, offering a scenario in which ENSO-related components can be tested in climates different than present-day, thereby providing context for future changes in ENSO. The mid-Holocene simulations exhibit a 20% reduction of ENSO strength, caused by a precession forced enhancement of the Indian summer monsoon, which strengthened ENSO-related Bjerknes feedbacks. ENSO strength in the LGM is weakened by ~25%, which is not found to be caused by changes in equatorial Pacific dynamics but rather mean state cooling that weakens the tropical thermocline. The 2x and 4x simulations have strongly enhanced and more frequent ENSO events caused by disproportionate warming of the eastern Pacific relative to the western Pacific, which weakens the east-west Pacific surface temperature gradient, allowing larger anomalies, and hence ENSO events, to develop. Title: Simulating past, present, and future changes in ENSO : a model evaluation and data-model comparison Author: Alder, Jay R. This thesis presents the results of a formal evaluation of a new AOGCM, GENMOM, demonstrating its ability to simulate present-day climate and ENSO dynamics. The model is applied to simulate climate for the Last Glacial Maximum, deglacial, and Holocene time periods. The model output is evaluated against the best available proxy reconstructions in a detailed data-model comparison. ENSO strength is analyzed in seven paleo simulations and compared to coral and laminated lake sediment proxy records to provide an understanding of how ENSO related mechanisms varied in the past and how they vary under increased atmospheric CO₂ forced global warming. The GENMOM simulated present-day is found to be on par with three models used in the IPCC AR4 assessment and is comparable with reanalysis products (e.g, NCEP2). Atmospheric features such as the jet stream structure and major semipermanent sea level pressure centers are well simulated as is the mean planetary-scale wind structure that is needed to produce the correct position of stormtracks. Most ocean surface currents are reproduced except where they are not resolvable at T31 resolution. Overall, GENMOM captures the observed gradients and spatial distributions of annual surface temperature and precipitation and the simulations are on par with other AOGCMs. Deficiencies in the GENMOM present-day simulation include a warm bias in the surface temperature over the southern oceans, a split in the ITCZ and weaker-thanobserved overturning circulation. GENMOM produces a global temperature bias of 0.6 °C. GENMOM is demonstrated to capture ENSO dynamics similar to eight AOGCMs that were evaluated in the IPCC AR4. The Niño 3 - 4 indices have a standard deviation within 0.3 °C of the observations, indicating GENMOM is producing variability in the tropical Pacifc that is comparable to observations. GENMOM produces present-day ENSO events with an average period of 5.6 years, which is within the 2 – 7 range exhibited in the observed historical record. The mid-Holocene (6ka) and Last Glacial Maximum (LGM, 21ka) simulations are compared to the best available proxy reconstructions for sea surface temperature, precipitation and net moisture to ensure the simulations are plausible. This thesis finds that the model is in good agreement over broad spatial scales, with regional discrepancies between the model and proxy data. Coral and laminated lake sediment proxy records indicate mid-Holocene ENSO strength was reduced by 15 - 60%, offering a scenario in which ENSO-related components can be tested in climates different than present-day, thereby providing context for future changes in ENSO. The mid-Holocene simulations exhibit a 20% reduction of ENSO strength, caused by a precession forced enhancement of the Indian summer monsoon, which strengthened ENSO-related Bjerknes feedbacks. ENSO strength in the LGM is weakened by ~25%, which is not found to be caused by changes in equatorial Pacific dynamics but rather mean state cooling that weakens the tropical thermocline. The 2x and 4x simulations have strongly enhanced and more frequent ENSO events caused by disproportionate warming of the eastern Pacific relative to the western Pacific, which weakens the east-west Pacific surface temperature gradient, allowing larger anomalies, and hence ENSO events, to develop. Close