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G. Vettoretti and W. R. Peltier

Abstract

Post-Eemian glacial inception [the transition between marine oxygen isotopic stage (MOIS) 5e and MOIS 5d] began approximately 117 000 years before present (117 kyr BP) and led to significant Northern Hemisphere glaciation within the ensuing 5000 yr. Previous sensitivity studies with atmospheric general circulation models (AGCMs) have had difficulty producing glacial nucleation in high northern latitude regions of the globe. A base simulation of this process has been conducted using the Canadian Centre for Climate Modelling and Analysis (CCCma) GCMII with mixed layer slab ocean model constrained so as to ensure that the model reproduces the set of Atmospheric Model Intercomparison Project 2 (AMIP2) modern sea surface temperatures (SSTs) under conditions of modern radiative forcing. This simulation demonstrates that entry into glacial conditions at 116 kyr BP requires only the introduction of post-Eemian orbital insolation and standard preindustrial CO2 concentrations. Two additional sensitivity experiments are also described herein in which the associated modern control climates have modified oceanic heat transports and solar radiation parameterizations. These simulations produce modern Northern Hemisphere sea surface temperatures that are either cold biased or warm biased with respect to the AMIP2 SSTs. Three modern control and three post-Eemian simulations are therefore employed to investigate the sensitivity of the onset of large-scale glaciation at high northern latitudes to the summer seasonal temperature bias in the model. Also discussed are the elements of the hydrological cycle at 116 kyr BP in order to more precisely isolate the primary causes of the onset of perennial snow cover. In particular, one novel feature is described that is characteristic of the two post-Eemian simulations that do initiate glaciation, namely, the absence of perennial snow cover in the Alaskan region, a result that is in accord with geological evidence.

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G. Vettoretti and W. R. Peltier

Abstract

This paper extends the analyses of the glacial inception process described in a previous paper (“Part I: The Impact of Summer Seasonal Temperature Bias”). The analyses described therein were based upon the use of the Canadian Centre for Climate Modelling and Analysis (CCCma) GCMII. Three simulations of the modern climate system were described that were, respectively, warm biased, unbiased, and cold biased with respect to the set of Atmospheric Model Intercomparison Project 2 SSTs and land surface temperatures in summer. These three control models were perturbed by the modification of the orbital insolation regime appropriate to the time 116 000 years before present (116 kyr BP) during which the most recent period of continental glaciation began. Two of the three simulations do deliver perennial snow cover in polar latitudes. Analyses of the land surface energy balance, hydrological cycle, and energetics of the atmosphere in the Northern Hemisphere polar region at 116 kyr BP discussed in greater detail herein reveal a set of positive feedback mechanisms favoring glaciation. It is proposed that these feedbacks are coupled to the main Milankovitch ice–albedo feedback that has heretofore been assumed to be the key to understanding the initiation of widespread continental glaciation. In particular, it is demonstrated that the polar surface energy balance plays an important role in summer snowmelt and in the annual maintenance of perennial snow cover. Furthermore, increased water vapor transport into the Northern Hemisphere summer polar regions at 116 kyr BP increases the net annual snow accumulation in these post-Eemian climate simulations through the action of an atmospheric–cryospheric feedback mechanism. An explanation for the absence of perennial snow cover in Alaska during the post-Eemian period is proposed. It is suggested that the transport of latent and sensible heat into this region is increased under 116 kyr BP orbital forcing, which therefore acts to maintain sufficient summer snowmelt that is vitally important in preventing glacial initiation.

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G. Vettoretti, W. R. Peltier, and N. A. McFarlane

Abstract

The authors describe a first paleoclimatological application of the Canadian Centre for Climate Modelling and Analysis atmospheric general circulation model (AGCM) to simulate the climate state 6000 calendar years before present (6 kyr BP). Climate reconstructions for this period are performed with both fixed SSTs and with the AGCM coupled to mixed layer ocean and thermodynamic sea–ice modules. The most important difference between this epoch and the present involves the increased surface heating and cooling of the continental land masses in the Northern Hemisphere during summer and winter, respectively, which are a consequence of the modified orbital configuration. A comparison of a fixed SST experiment with a calculated SST experiment, incorporating a thermodynamic representation of oceanic response, is performed to assess the impact on the mid-Holocene climate. The results are also contrasted with those obtained on the basis of proxy climate reconstructions during this mid-Holocene optimum period. Of interest in this calculated SST experiment is the impact on the seasonal cycle of sea–ice distribution due to the increased insolation at high latitudes during Northern Hemisphere summer. Also important is the fact that the mixed layer ocean in the simulation is found to further enhance the monsoon circulation beyond the enhancement found to occur due to the influence of modified orbital forcing alone. This increased response is found to be a consequence of the sensitivity of tropical SST to the amplification of the seasonal cycle due to the change in insolation forcing that was characteristic of the mid-Holocene period.

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R. J. Stouffer, J. Yin, J. M. Gregory, K. W. Dixon, M. J. Spelman, W. Hurlin, A. J. Weaver, M. Eby, G. M. Flato, H. Hasumi, A. Hu, J. H. Jungclaus, I. V. Kamenkovich, A. Levermann, M. Montoya, S. Murakami, S. Nawrath, A. Oka, W. R. Peltier, D. Y. Robitaille, A. Sokolov, G. Vettoretti, and S. L. Weber

Abstract

The Atlantic thermohaline circulation (THC) is an important part of the earth's climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-Sv (1 Sv ≡ 106 m3 s−1) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.

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