ALBERT

Description: Herein, we publish the simulated global annual mean temperature (THO), salinity (SAO), ice compactness (SICOMO), Atlantic Meridional Overturning Circulation (AMOC), Global Meridional Overturning Circulation (GMOC), zonal velocity (UKO), meridional velocity (VKE), 10m u-velocity (u10), 10m v-velocity (v10), mixed layer depth (zmld), horizontal barotropic streamfunction (PSIUWE) and sealevel (ZO) over a time period of 100 years retrieved from equilibrium climate simulations for the Miocene (~23-15 Ma) and use different Greenland-Scotland Ridge (GSR) and Fram Strait (FS) sill depths as a representative for different tectonic settings that occur during the subsidence interval and utilized in the publication by Hossain et al. (2020). The climate data has been produced with COSMOS (ECHAM5/JSBACH/MPIOM/OASIS3), utilized at a resolution of T31 in the atmosphere (19 hybrid sigma-pressure levels) and a resolution of GR30 (bipolar orthogonal curvilinear grid, formal resolution of ~3.0°x1.8°) in the ocean (40 z-coordinate levels). The model setup refers to boundary conditions (incl. changes in orography, bathymetry, physical land surface characteristics, ice sheets, atmospheric CO2) representative for the Miocene. Details on setup and identifiers of Miocene model simulations can be found in Table 1 and Supplementary Table 1 of Hossain et al., 2020.

Description: During the Middle Miocene climate transition about 14 million years ago, the Antarctic ice sheet expanded to near-modern volume. Surprisingly, this ice sheet growth was accompanied by a warming in the surface waters of the Southern Ocean, whereas a slight deep-water temperature increase was delayed by more than 200 thousand years. Here we use a coupled atmosphere-ocean model to assess the relative effects of changes in atmospheric CO2 concentration and ice sheet growth on regional and global temperatures. In the simulations, changes in the wind field associated with the growth of the ice sheet induce changes in ocean circulation, deep-water formation and sea-ice cover that result in sea surface warming and deep-water cooling in large swaths of the Atlantic and Indian ocean sectors of the Southern Ocean. We interpret these changes as the dominant ocean surface response to a 100-thousand-year phase of massive ice growth in Antarctica. A rise in global annual mean temperatures is also seen in response to increased Antarctic ice surface elevation. In contrast, the longer-term surface and deep-water temperature trends are dominated by changes in atmospheric CO2 concentration. We therefore conclude that the climatic and oceanographic impacts of the Miocene expansion of the Antarctic ice sheet are governed by a complex interplay between wind field, ocean circulation and the sea-ice system.

Description: Herein, we publish the simulated global annual mean surface air temperatures (tsurf), zonal (UKO) and meridional (VKE) velocities, temperature (THO), salinity (SAO) and horizontal barotropic streamfunction (PSIUWE) over a time period of 100 years retrieved from equilibrium climate simulations for testing the sensitivity of the Greenland-Scotland Ridge and utilized in the publication by Stärz et al. (2017). The climate data has been produced with COSMOS (ECHAM5/JSBACH/MPIOM/OASIS3), utilized at a resolution of T31 in the atmosphere (19 hybrid sigma-pressure levels) and a resolution of GR30 (bipolar orthogonal curvilinear grid, formal resolution of ~3.0°x1.8°) in the ocean (40 z-coordinate levels). The model setup refers to boundary conditions (incl. changes in orography, bathymetry, physical land surface characteristics, ice sheets, atmospheric CO2) representative for the Miocene. Further information on the model setup and the model scenarios, including identifiers, is given in the Supplementary Table 1 of Stärz et al. (2017).

Description: Reduced surface-deep ocean exchange and enhanced nutrient consumption by phytoplankton in the Southern Ocean have been linked to lower glacial atmospheric CO2. However, identification of the biological and physical conditions involved and the related processes remains incomplete. Here we specify Southern Ocean surface-subsurface contrasts using a new tool, the combined oxygen and silicon isotope measurement of diatom and radiolarian opal, in combination with numerical simulations. Our data do not indicate a permanent glacial halocline related to melt water from icebergs. Corroborated by numerical simulations, we find that glacial surface stratification was variable and linked to seasonal sea-ice changes. During glacial spring-summer, the mixed layer was relatively shallow, while deeper mixing occurred during fall-winter, allowing for surface-ocean refueling with nutrients from the deep reservoir, which was potentially richer in nutrients than today. This generated specific carbon and opal export regimes turning the glacial seasonal sea-ice zone into a carbon sink.

Description: An ensemble of new, high-resolution records of surface ocean hydrography from the Indian-Atlantic oceanic gateway, south of Africa, demonstrates recurrent and high-amplitude salinity oscillations in the Agulhas Leakage area during the penultimate glacial-interglacial cycle. A series of millennial-scale salinification events, indicating strengthened salt leakage into the South Atlantic, appear to correlate with abrupt changes in the North Atlantic climate and Atlantic Meridional Overturning Circulation (AMOC). This interhemispheric coupling, which plausibly involved changes in the Hadley Cell and midlatitude westerlies that impacted the interocean transport at the tip of Africa, suggests that the Agulhas Leakage acted as a source of negative buoyancy for the perturbed AMOC, possibly aiding its return to full strength. Our finding points to the Indian-to-Atlantic salt transport as a potentially important modulator of the AMOC during the abrupt climate changes of the Late Pleistocene.

Description: One of the most abrupt and yet unexplained past rises in atmospheric CO2 (10 p.p.m.v. in two centuries) occurred in quasi-synchrony with abrupt northern hemispheric warming into the Bølling/Allerød, 14,600 years ago. Here we use a U/Th-dated record of atmospheric D14C from Tahiti corals to provide an independent and precise age control for this CO2 rise. We also use model simulations to show that the release of old (nearly 14C-free) carbon can explain these changes in CO2 and D14C. The D14C record provides an independent constraint on the amount of carbon released (125 Pg C). We suggest, in line with observations of atmospheric CH4 and terrigenous biomarkers, that thawing permafrost in high northern latitudes could have been the source of carbon, possibly with contribution from flooding of the Siberian continental shelf during meltwater pulse 1A. Our findings highlight the potential of the permafrost carbon reservoir to modulate abrupt climate changes via greenhouse-gas feedbacks.

Description: Here we publish simulated Late Cretaceous (~70 Ma) annual mean surface temperatures (tsurf) with different (280-1680 ppm) CO2 levels, annual mean surface temperatures with different gateway configurations between the Arctic Ocean and North proto-Atlantic basin, as well as summer (JJA) and winter (DJF) mean surface temperatures with 840 ppm CO2 level. All data were averaged over the period of 100 years. The climate data has been produced with COSMOS (ECHAM5/MPIOM/OASIS3). The atmosphere component ECHAM5 was run in the resolution of T31/L19, while the ocean model MPIOM in the resolution of GR30 (bipolar orthogonal curvilinear grid with a formal resolution of ~3.0°x1.8°). OASIS3 is responsible for coupling between the atmosphere and ocean components. The Late Cretaceous setup is based on the Markwick and Valdes (2004, doi:10.1016/j.palaeo.2004.06.015) paleogeography. More details about a model setup can be found in Niezgodzki et al. (2017, doi:10.1002/2016PA003055). Additionally, we have prepared the .xlsx file where we provide the details of the locations of data points as well as reconstructed temperatures as used in a model-data comparison. These data are the modified data from Upchurch et al. (2015, doi:10.1130/G36802.1). For more details about the procedure used to modify the original data set, see Niezgodzki et al. (2017, doi:10.1002/2016PA003055).

Description: We present ice thickness and bedrock height from simulations of the Miocene Antarctic ice sheet (AIS), using the 3D thermodynamical Parallel Ice Sheet Model (PISM) version 0.7.3. The applied climate forcing consists of temperature and precipitation anomalies with respect to a preindustrial reference simulation, obtained from simulations using the atmosphere-ocean general circulation model COSMOS. These anomalies are added to an ERA-40/WOD-09 base climate. COSMOS was run using preindustrial settings, and Miocene settings with CO2 levels of 278 ppm (low), 450 ppm (medium), and 600 ppm (high). The steady state simulations using PISM are started with present-day bedrock conditions, with a present-day AIS (Bedmap2), and isostatically rebounded after removal of the ice. Additional simulations are started with the Wilson et al. (2012) late-Eocene bedrock topography reconstruction. The steady state simulations are conducted by applying the same climate forcing over 200 kyr. The transient simulations are performed by using an index method to interpolate between different forcing climate states. In these runs, the forcing climate state is gradually varied over quasi-orbital timescales of 400 kyr, 40 kyr, 1600 kyr, or 200 kyr. Details are given in the accompanying publication. For more information or data, please contact L.B. Stap mailto:lstap@awi.de. Dataset 2019_Stap_steadystate_startfrom_presentdayAIS.nc contains the data to plot Fig. 1. Dataset 2019_Stap_PISM_transient.xlsx contains the data to plot Fig. 2 and 3. Datasets 2019_Stap_steadystate_startfrom_noice.nc and 2019_Stap_steadystate_startfrom_Wilson_noice.nc contain additional data plotted in Fig. 2 and 3.