Late Cretaceous climate simulations with different CO2 levels and subarctic gateway configurations: A model-data comparison

We investigate the impact of different CO2 levels and different subarctic gateway configurations on the surface temperatures during the latest Cretaceous using the Earth System Model COSMOS. The simulated temperatures are compared with the surface temperature reconstructions based on a recent compilation of the latest Cretaceous proxies. In our numerical experiments, the CO2 level ranges from 1 to 6 times the preindustrial (PI) CO2 level of 280 ppm. On a global scale, the most reasonable match between modeling and proxy data is obtained for the experiments with 3 to 5 × PI CO2 concentrations. However, the simulated low- (high-) latitude temperatures are too high (low) as compared to the proxy data. The moderate CO2 levels scenarios might be more realistic, if we take into account proxy data and the dead zone effect criterion. Furthermore, we test if the model-data discrepancies can be caused by too simplistic proxy-data interpretations. This is distinctly seen at high latitudes, where most proxies are biased toward summer temperatures. Additional sensitivity experiments with different ocean gateway configurations and constant CO2 level indicate only minor surface temperatures changes (<~1°C) on a global scale, with higher values (up to ~8°C) on a regional scale. These findings imply that modeled and reconstructed temperature gradients are to a large degree only qualitatively comparable, providing challenges for the interpretation of proxy data and/or model sensitivity. With respect to the latter, our results suggest that an assessment of greenhouse worlds is best constrained by temperatures in the midlatitudes

Astronomically paced changes in overturning circulation in the Western North Atlantic during the middle Eocene

North Atlantic Deep Water (NADW) currently redistributes heat and salt between Earth’s ocean basins, and plays a vital role in the ocean-atmosphere CO2 exchange. Despite its crucial role in today’s climate system, vigorous debate remains as to when deep-water formation in the North Atlantic started. Here, we present datasets from carbonate-rich middle Eocene sediments from the Newfoundland Ridge, revealing a unique archive of paleoceanographic change from the progressively cooling climate of the middle Eocene. Well-defined lithologic alternations between calcareous ooze and clay-rich intervals occur at the ∼41-kyr beat of axial obliquity. Hence, we identify obliquity as the driver of middle Eocene (43.5–46 Ma) Northern Component Water (NCW, the predecessor of modern NADW) variability. High-resolution benthic foraminiferal δ18O and δ13C suggest that obliquity minima correspond to cold, nutrient-depleted, western North Atlantic deep waters. We thus link stronger NCW formation with obliquity minima. In contrast, during obliquity maxima, Deep Western Boundary Currents were weaker and warmer, while abyssal nutrients were more abundant. These aspects reflect a more sluggish NCW formation. This obliquity-paced paleoceanographic regime is in excellent agreement with results from an Earth system model, in which obliquity minima configurations enhance NCW formation

Global and Zonal-Mean Hydrological Response to Early Eocene Warmth

Earth's hydrological cycle is expected to intensify in response to global warming, with a “wet-gets-wetter, dry-gets-drier” response anticipated over the ocean. Subtropical regions (∼15°–30°N/S) are predicted to become drier, yet proxy evidence from past warm climates suggests these regions may be characterized by wetter conditions. Here we use an integrated data-modeling approach to reconstruct global and zonal-mean rainfall patterns during the early Eocene (∼56–48 million years ago). The Deep-Time Model Intercomparison Project (DeepMIP) model ensemble indicates that the mid- (30°–60°N/S) and high-latitudes (>60°N/S) are characterized by a thermodynamically dominated hydrological response to warming and overall wetter conditions. The tropical band (0°–15°N/S) is also characterized by wetter conditions, with several DeepMIP models simulating narrowing of the Inter-Tropical Convergence Zone. However, the latter is not evident from the proxy data. The subtropics are characterized by negative precipitation-evaporation anomalies (i.e., drier conditions) in the DeepMIP models, but there is surprisingly large inter-model variability in mean annual precipitation (MAP). Intriguingly, we find that models with weaker meridional temperature gradients (e.g., CESM, GFDL) are characterized by a reduction in subtropical moisture divergence, leading to an increase in MAP. These model simulations agree more closely with our new proxy-derived precipitation reconstructions and other key climate metrics and imply that the early Eocene was characterized by reduced subtropical moisture divergence. If the meridional temperature gradient was even weaker than suggested by those DeepMIP models, circulation-induced changes may have outcompeted thermodynamic changes, leading to wetter subtropics. This highlights the importance of accurately reconstructing zonal temperature gradients when reconstructing past rainfall patterns

Late Cretaceous climate simulations produced with COSMOS in a coupled atmosphere-ocean configuration in NetCDF format

We provide the results of 11 Late Cretaceous climate simulations (Tab. 1 in Niezgodzki et al. [2019, doi:10.1016/j.gloplacha.2019.03.011]) produced with COSMOS in a coupled atmosphere-ocean configuration. Five of these experiments use a 3 x pre-industrial (PI) CO2 level (840 ppm) while 6 of them were run with 4xPI CO2 (1120 ppm). The experiments with the same CO2 levels differ by gateway configurations between the Arctic Ocean and North proto-Atlantic basin. In spin-up experiments we employ Maastrichtian (~70 Ma) paleogeography of Markwick and Valdes [2004, doi:10.1016/j.palaeo.2004.06.015]. More information about model scenarios and model set-up can be found in Niezgodzki et al. [2019, doi:10.1016/j.gloplacha.2019.03.011]. Here we publish simulated winter (DJF) surface temperatures (tsurf) and salinity (SAO), averaged March-April (Ma-Ap) sea surface temperature (THO) and SAO and monthly sea-ice compactness (SICOMO) of each experiment. Additionally, for two gateway configurations we show DJF and summer (JJA) 10m meridional (v10) and zonal (u10) wind speeds as well as JJA shortwave net surface radiation (srads), net clear sky surface radiation (srafs), longwave net surface radiation (trads) and clear sky surface radiation (trafs)

COSMOS simulated basic surface ocean characteristics for comparison with palynological data

Here we provide simulated seasonal (DJF and JJA) sea surface temperature (THO), sea surface salinity (SAO) as well as surface zonal (UKO) and meridional (VKE) ocean currents velocities. THO and SAO are provided on the regular grid in 360x180 resolution while UKO and VKE on the curvilinear grid in 122x101 resolution (which is the resolution of the ocean component MPIOM). The simulations were produced with COSMOS in a coupled atmosphere-ocean configuration. All of the simulations were run with 4 x pre-industrial (PI) CO2 level (1120 ppm). The experiments differ by alterations in gateway configurations between the Arctic Ocean and North proto-Atlantic Basin. Our spin-up experiment (C-1120) employs Maastrichtian (~70 Ma) paleogeography of Markwick and Valdes [2004, doi:10.1016/j.palaeo.2004.06.015]. The experiment was run to the model year 10 600. Two other experiments were branched off from C-1120 from the model year 9200. One of them differs from C-1120 by the closure of Hudson Seaway (HUD-0) and was run for the next 1400 years. The other one (GNS-47) has each gateway between the Arctic Ocean and North proto-Atlantic basin closed apart Greenland-Norwegian Sea and was run for the next 2100 years. More information about the geological background and COSMOS configuration can be found in Radmacher et al. [2020, doi:10.1127/nos/2019/0527]