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• 1. [Article] Impact of the Ocean’s Overturning Circulation on Atmospheric CO2

  A coupled climate-carbon cycle model and ice core CO2 data from the last glacial period are used to explore the impact of changes in ocean circulation on atmospheric CO2 concentrations on millennial time ...

  Citation × Citation Citation Abstract Copy Title: Impact of the Ocean’s Overturning Circulation on Atmospheric CO2 Author: Brook, Edward J., Schmittner, Andreas, Ahn, Jinho A coupled climate-carbon cycle model and ice core CO2 data from the last glacial period are used to explore the impact of changes in ocean circulation on atmospheric CO2 concentrations on millennial time scales. In the model, stronger wind driven circulation increases atmospheric CO2. Changes in the buoyancy driven deep overturning in the Atlantic affect atmospheric CO2 only indirectly through their effect on Southern Ocean stratification. In simulations with an abrupt and complete shutdown of the Atlantic overturning, stratification in the Southern Ocean decreases due to salinification of surface waters and freshening of the deep sea. Deeper mixed layers and steeper isopycnals lead to outgassing of CO2 in the Southern Ocean and hence gradually increasing atmospheric CO2 concentrations on a multi-millennial time scale. The rise in CO2 terminates at the time of rapid resumption of deep water formation and warming in the North Atlantic, and CO2 levels subsequently gradually decrease. These model responses and a strong correlation between simulated atmospheric CO2 and Antarctic surface air temperatures with little or no time lag are consistent with newly synchronized ice core data from the last ice age. Sensitivity experiments reveal that the amplitude of the response of atmospheric CO2 is sensitive to the model background climatic state and decreases in a colder climate owing to smaller changes in the overturning. Title: Impact of the Ocean’s Overturning Circulation on Atmospheric CO2 Author: Brook, Edward J., Schmittner, Andreas, Ahn, Jinho A coupled climate-carbon cycle model and ice core CO2 data from the last glacial period are used to explore the impact of changes in ocean circulation on atmospheric CO2 concentrations on millennial time scales. In the model, stronger wind driven circulation increases atmospheric CO2. Changes in the buoyancy driven deep overturning in the Atlantic affect atmospheric CO2 only indirectly through their effect on Southern Ocean stratification. In simulations with an abrupt and complete shutdown of the Atlantic overturning, stratification in the Southern Ocean decreases due to salinification of surface waters and freshening of the deep sea. Deeper mixed layers and steeper isopycnals lead to outgassing of CO2 in the Southern Ocean and hence gradually increasing atmospheric CO2 concentrations on a multi-millennial time scale. The rise in CO2 terminates at the time of rapid resumption of deep water formation and warming in the North Atlantic, and CO2 levels subsequently gradually decrease. These model responses and a strong correlation between simulated atmospheric CO2 and Antarctic surface air temperatures with little or no time lag are consistent with newly synchronized ice core data from the last ice age. Sensitivity experiments reveal that the amplitude of the response of atmospheric CO2 is sensitive to the model background climatic state and decreases in a colder climate owing to smaller changes in the overturning. Close

• 2. [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

• 3. [Article] Large fluctuations of dissolved oxygen in the Indian and Pacific oceans during Dansgaard-Oeschger oscillations caused by variations of North Atlantic Deep Water Subduction: Paleoceanography

  Paleoclimate records from glacial Indian and Pacific oceans sediments document millennial-scale fluctuations of subsurface dissolved oxygen levels and denitrification coherent with North Atlantic temperature ...

  Citation × Citation Citation Abstract Copy Title: Large fluctuations of dissolved oxygen in the Indian and Pacific oceans during Dansgaard-Oeschger oscillations caused by variations of North Atlantic Deep Water Subduction: Paleoceanography Author: Schmittner, Andreas Paleoclimate records from glacial Indian and Pacific oceans sediments document millennial-scale fluctuations of subsurface dissolved oxygen levels and denitrification coherent with North Atlantic temperature oscillations. Yet the mechanism of this teleconnection between the remote ocean basins remains elusive. Here we present model simulations of the oxygen and nitrogen cycles that explain how changes in deepwater subduction in the North Atlantic can cause large and synchronous variations of oxygen minimum zones throughout the Northern Hemisphere of the Indian and Pacific oceans, consistent with the paleoclimate records. Cold periods in the North Atlantic are associated with reduced nutrient delivery to the upper Indo-Pacific oceans, thereby decreasing productivity. Reduced export production diminishes subsurface respiration of organic matter leading to higher oxygen concentrations and less denitrification. This effect of reduced oxygen consumption dominates at low latitudes. At high latitudes in the Southern Ocean and North Pacific, increased mixed layer depths and steepening of isopycnals improve ocean ventilation and oxygen supply to the subsurface. Atmospheric teleconnections through changes in wind-driven ocean circulation modify this basin-scale pattern regionally. These results suggest that changes in the Atlantic Ocean circulation, similar to those projected by climate models to possibly occur in the centuries to come because of anthropogenic climate warming, can have large effects on marine ecosystems and biogeochemical cycles even in remote areas. Title: Large fluctuations of dissolved oxygen in the Indian and Pacific oceans during Dansgaard-Oeschger oscillations caused by variations of North Atlantic Deep Water Subduction: Paleoceanography Author: Schmittner, Andreas Paleoclimate records from glacial Indian and Pacific oceans sediments document millennial-scale fluctuations of subsurface dissolved oxygen levels and denitrification coherent with North Atlantic temperature oscillations. Yet the mechanism of this teleconnection between the remote ocean basins remains elusive. Here we present model simulations of the oxygen and nitrogen cycles that explain how changes in deepwater subduction in the North Atlantic can cause large and synchronous variations of oxygen minimum zones throughout the Northern Hemisphere of the Indian and Pacific oceans, consistent with the paleoclimate records. Cold periods in the North Atlantic are associated with reduced nutrient delivery to the upper Indo-Pacific oceans, thereby decreasing productivity. Reduced export production diminishes subsurface respiration of organic matter leading to higher oxygen concentrations and less denitrification. This effect of reduced oxygen consumption dominates at low latitudes. At high latitudes in the Southern Ocean and North Pacific, increased mixed layer depths and steepening of isopycnals improve ocean ventilation and oxygen supply to the subsurface. Atmospheric teleconnections through changes in wind-driven ocean circulation modify this basin-scale pattern regionally. These results suggest that changes in the Atlantic Ocean circulation, similar to those projected by climate models to possibly occur in the centuries to come because of anthropogenic climate warming, can have large effects on marine ecosystems and biogeochemical cycles even in remote areas. Close