William H. Peterson

A “simulation” of Mid‐Cretaceous climate

A series of general circulation model experiments utilizing GENESIS have been completed for the mid-Cretaceous based on geography, variable atmospheric carbon dioxide concentrations (2 to 6 times present-day concentrations), and variable poleward oceanic heat flux (.6 to 1.2 × 1015 W increased from present day). By combining all three major variables (CO2, geography, and oceanic heat flux), the distribution of mid-Cretaceous temperatures can be achieved. In the simulations, increased CO2 is required to promote global warmth, and increased oceanic heat flux is required to prevent the tropics from overheating with higher levels of CO2. Four times present-day CO2 with 1.2 × 1015 W provided the best match to the distribution of mid-Cretaceous data. The best match to the Cretaceous observations was achieved with a globally averaged surface temperature increase of 6.2°C, at the lower end of past estimates of mid-Cretaceous warmth. This value may be a better estimate of mid-Cretaceous global warming. Finally, the model experiments can be used to provide a “paleocalibration” of the global warming expected for a doubling of atmospheric carbon dioxide. The best estimates for the mid-Cretaceous appear to be a 2.5 to 4.0°C sensitivity, in the mid to upper range of the sensitivity of current climate models used to assess future global change.

Response of the Mid-Cretaceous global oceanic circulation to tectonic and CO2 forcings

The mid-Cretaceous was a period of unusually active tectonism that drove enhanced volcanic outgassing and high seafloor spreading rates. This intense tectonic activity is coincident with dramatic events in the marine environment, including oceanic anoxic events 1 (Aptian-Early Albian) and 2 (Cenomanian/Turonian boundary), high biological turnover rates, and a thermal maximum. In this study, a series of mid-Cretaceous ocean general circulation model experiments were completed using the Parallel Ocean Climate Model. These experiments demonstrate the effect of enhanced atmospheric CO2 concentrations and paleogeographic change on the global oceanic circulation. The experiments reveal that paleogeography, specifically the presence/absence of a marine connection between the North Atlantic and South Atlantic basins, may have governed the nature of the mid-Cretaceous global oceanic circulation. In the absence of this connection, an Albian simulation is characterized by extremely warm, saline conditions throughout the North Atlantic and northern South Atlantic Oceans. With a gateway in a Turonian simulation, Antarctic Bottom Water ventilates the Atlantic basins. In both Albian and Turonian simulations the Pacific-Indian basins are dominated by thermohaline circulation with deep water sources in the Southern Ocean. While atmospheric CO2 concentrations influence the global temperature and salinity, an increase from present-day to 4 times present-day levels alters the global circulation very little. Differences between the Albian and Turonian numerical simulations agree well with aspects of the marine record, supporting speculation that the climatic and oceanographic changes surrounding the Cenomanian-Turonian boundary were driven by the initiation of a connection between the Atlantic Oceans.

The Cenozoic ocean circulation based on ocean General Circulation Model results

Abstract A series of ocean General Circulation Model (GCM) experiments for Paleocene, Eocene, Miocene and present-day continental geography with atmospheric forcing prescribed from atmospheric GCM experiments is utilized to investigate the changes in surface and deep-water ocean circulation through the Cenozoic. The experiments illustrate a number of similarities with previous observation-based reconstructions, although the timing of the development of particular gyre systems and the nature of the high southern latitude circulation are generally different than previous interpretations. In addition, the model results suggest substantial changes in the sites of deep water formation, including a middle Eocene subtropical source. Because of a number of experimental limitations, the experiments should be considered largely as a sensitivity analysis of the role of changing geography in modifying the ocean circulation.

Past climate and the role of ocean heat transport: Model simulations for the Cretaceous

A series of general circulation model experiments using Global Environmental and Ecological Simulation of Interactive Systems (GENESIS) were executed to evaluate the sensitivity of simulated mid-Cretaceous climate to small perturbations in ocean heat transport. Three experiments were performed: (1) mixed layer ocean with no ocean heat transport, ZEROQ, (2) ocean heat transport specified as required for GENESIS to best match modern observations, ONEQ, and (3) doubled ocean heat transport, TWOQ. The ONEQ experiment represents an ocean heat transport which is actually about 15% of the values given by Carissimo et al. (1985) from modern observations. As a sensitivity experiment these model simulations represent a doubling of the role of the ocean. However, relative to the observations, they represent small perturbations to the total poleward heat transport in the model. With the exception of the tropics, no major changes in the structure of the general circulation of the atmosphere resulted from the modification of the ocean heat transport. However, relatively small increases in ocean heat transport resulted in a number of significant differences between simulations, including tropical cooling, polar warming, weakened equator-to-pole surface temperature gradients, weakened midlatitude jets, decreased land-sea pressure contrast, and decreased midlatitude storminess. The experiments indicate that changes in ocean heat transport which are well within the realm of possibility for Earth history can have significant climatic impact. Although ocean heat transport may be a significant factor in explaining Cretaceous polar warmth, the changes specified in these experiments alone are not sufficient to explain the polar warmth of the Cretaceous.

Model Simulation of the Cretaceous Ocean Circulation

Three-dimensional numerical ocean circulation model experiments that were designed to evaluate the circulation characteristics for the mid-Cretaceous (∼100 million years ago) show that the primary direction of flow through the Tethys Ocean was eastward; in contrast, a westward flowing circumglobal Tethys current has been a consistent feature of earlier reconstructions of Cretaceous ocean circulation. The model studies demonstrate that (i) ocean circulation is sufficiently sensitive to the role of continental positions, sea level, and climate to limit the application of modern analogs to past circulations, and (ii) reconstructions based on limited biogeographic data may not provide unique surface circulation patterns.

The impact of paleogeographic evolution on the surface oceanic circulation and the marine environment within the Mid‐Cretaceous tethys

The concept of a stable, westward flowing circumglobal current throughout the Cretaceous Tethys has become the subject of extensive debate. Results from a series of oceanic general circulation model experiments, using the NCAR Parallel Ocean Climate Model, are presented which oppose this concept and suggest a more complicated circulation pattern dominated by a clockwise gyre in the Mediterranean Tethys. Although a narrow westward flowing current is simulated hugging the southern margin of Tethys, ocean model experiments reveal the large sensitivity of this current to paleogeographic evolution. Small modifications to continental geometry, representing sea level change or tectonic plate movement during the Cretaceous, alter the direction and strength of the Tethyan circulation and, consequently, lead to regional changes in seawater properties (i.e., temperature and salinity). These oceanic general circulation model experiments indicate that geographic evolution is an important mechanism of climatic and environmental change. Since the Cretaceous experienced rapid tectonic changes these results imply large surface circulation variations in the Tethys Ocean, supporting the idea of a dynamic tropical oceanic environment. In fact, a simulation of sea-level rise during the mid-Cretaceous reproduces the fluctuations in seawater properties recorded by fauna in the Caribbean Sea.

Simulation of ocean temperature change due to the opening of Drake Passage

An ocean general circulation model (GCM) is used to test the hypothesis that the opening of Drake Passage in the Oligocene may have lead to cooling in southern high latitudes. Results of previous studies show that no ocean models using restoring boundary conditions for the surface heat flux could reveal substantial southern high latitude cooling without prescribing atmospheric cooling a priori. To compute the surface heat flux that would be free from the restoring constraints we use another method, which is derived from an atmospheric energy balance model with lateral heat transport. In experiments conducted with both idealized and realistic topography the energy balance method produces substantial cooling not only in high latitudes but in deep waters as well.

A reinterpretation of Mid-Cretaceous shallow marine temperatures through model-data comparison

Mid-Cretaceous δ18O paleotemperatures are compared with temperatures predicted by an ocean general circulation model. The δ18O paleotemperatures, calculated assuming a δw of −1.0‰ (SMOW), are 5.6°C lower at low latitudes and 2.4°C higher at middle and high latitudes than the model-predicted, mixed-layer temperatures. The model-data comparison is improved after accounting for the spatial variability of δw and after considering the paleohabitats of the marine organisms from which δ18O values were measured. Paleotemperatures adjusted using the Broecker [1989] δw-S relationship and the model-predicted salinity are 3.4°C higher at low-latitude sites and 1.9°C lower at higher-latitude sites. Moreover, seven of nine low-latitude paleotemperatures intersect the model-predicted temperature within the upper 100 m of the water column, a depth consistent with the interpreted paleohabitat of the analyzed organisms. However, adjusted paleotemperatures for three of five Southern Hemisphere high-latitude locations are at least 9°C higher than the model-predicted temperatures for these locations. The model-data comparison suggests that mid-Cretaceous δ18O values are compatible with sea surface temperatures as high or higher than those simulated by an ocean general circulation model of the mid-Cretaceous with 4 × present-day atmospheric pCO2.

Continental runoff and early Cenozoic bottom-water sources

The dominance of warm saline bottom water during the mid-Cretaceous and the early Cenozoic has been inferred from sea-floor sediment records, an interpretation supported by early ocean general circulation model experiments. Thermohaline circulation depends in part on upper ocean salinities; however, early ocean models neglected continental runoff, a potentially critical factor in the salinity budget of the surface ocean. Our early Eocene ocean model sensitivity tests show that model deep-water sources can be enhanced, diminished, or turned off by varying the treatment of continental runoff in the atmosphere-ocean moisture flux calculation. Failure to treat surface runoff adequately thus has important implications for the simulation of thermohaline flow and formation of warm saline bottom water. Variations in runoff could have led to rapid changes in the relative importance of high-latitude versus subtropical deep water, such as may have occurred during the late Paleocene–early Eocene boundary interval (≈ 53.6–56.2 Ma).

Ekman transport and upwelling during Younger Dryas estimated from wind stress from GENESIS Climate model experiments with variable North Atlantic heat convergence

Reconstructed climatic changes during the Younger Dryas interval are similar to, but somewhat larger and more wide-spread than, those expected based on the direct atmospheric effects of reduced North Atlantic oceanic heat transport. The paleoclimatic data show that North Atlantic cooling during the Younger Dryas was accompanied by stronger winds in many regions, suggesting that enhanced wind-driven ocean upwelling may have served as a positive feedback on the cooling. We test this hypothesis using the GENESIS atmospheric general circulation model, and find that Younger Dryas-age specified reduction in North Atlantic oceanic heat transport increases tropical Ekman divergence by ≈10% in regions and at times of prominent upwelling, sufficient to affect tropical temperatures.