Results from multiple model simulations are used to understand the tropical sea surface temperature (SST) response to the reduced greenhouse gas concentrations and large continental ice sheets of the ...last glacial maximum (LGM). We present LGM simulations from the Paleoclimate Modelling Intercomparison Project, Phase 2 (PMIP2) and compare these simulations to proxy data collated and harmonized within the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface Project (MARGO). Five atmosphere–ocean coupled climate models (AOGCMs) and one coupled model of intermediate complexity have PMIP2 ocean results available for LGM. The models give a range of tropical (defined for this paper as 15°S–15°N) SST cooling of 1.0–2.4°C, comparable to the MARGO estimate of annual cooling of 1.7 ± 1°C. The models simulate greater SST cooling in the tropical Atlantic than tropical Pacific, but interbasin and intrabasin variations of cooling are much smaller than those found in the MARGO reconstruction. The simulated tropical coolings are relatively insensitive to season, a feature also present in the MARGO transferred-based estimates calculated from planktonic foraminiferal assemblages for the Indian and Pacific Oceans. These assemblages indicate seasonality in cooling in the Atlantic basin, with greater cooling in northern summer than northern winter, not captured by the model simulations. Biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain for the LGM simulations and are partly responsible for the more homogeneous spatial and temporal LGM tropical cooling simulated by the models. The PMIP2 LGM simulations give estimates for the climate sensitivity parameter of 0.67°–0.83°C per Wm
−2
, which translates to equilibrium climate sensitivity for doubling of atmospheric CO
2
of 2.6–3.1°C.
We analyzed the chemical compositions of dust and sea‐salt particles in the EPICA Dome C (EDC) ice core during 26–7 kyr BP using an ice‐sublimation technique and compared the results with existing ...data of the Dome Fuji (DF) ice core. Combined with ion concentration data, our data suggested similar sea‐salt fluxes in both cores and significantly lower dust flux in the EDC core. The differences in modal size and aspect ratio of dust particles between the two cores support the dominance of Patagonian source suggested by earlier works. The compositions of calcic dust showed major change at ~17 kyr BP, possibly reflecting a relative increase in dust transported via the upper troposphere. The calcium sulfate fraction was higher in the DF core than in the EDC core after ~17 kyr BP, suggesting that higher Patagonian dust contribution to the DF region. Abundant NaCl particles were found in the DF core in comparison with the EDC core from the LGM to early Holocene, possibly because of the high concentration of terrestrial dust in the DF core that reduced acid availability for sea‐salt modification. During the Holocene, the lower NaCl fraction and Cl−/Na+ ratio in the EDC core suggested that most Cl− was lost to the atmosphere from snow at Dome C, while it was preserved at Dome Fuji as NaCl and solid solution.
Key Points
Dust flux in the Dome C core is significantly lower than in the Dome Fuji core from the LGM to the early Holocene
Comparison of dust size and aspect ratio between the two cores suggest dominance of Patagonian dust source during the LGM
Most Cl was lost to the atmosphere from snow at Dome C, while it was preserved at Dome Fuji as NaCl and in solid solution in the Holocene
Proxy records indicate that export production was enhanced at the onset of the last glaciation. We examine how glacial cooling affects marine export production through temperature‐dependent ...phytoplankton productivity and organic carbon remineralization using glacial climate simulation. Compared with the standard assumption of no temperature dependence as applied in existing paleoclimate models, including temperature‐dependence enhances export production globally under glacial climate conditions. The near freezing temperatures of Antarctic seawater significantly reduce organic carbon remineralization. Less remineralization than production results in increasing carbon export to the deep ocean. Nutrients remaining near the surface at high latitudes are advected to lower latitudes via Antarctic Intermediate Water, enhancing primary production, and hence export, in temperate and tropical regions as well. Including temperature‐dependence improves the model's agreement with the glacial proxy records of export production and stable carbon isotopes even under stably stratified conditions with a weakening of North Atlantic Deep Water.
Key Points
Phytoplankton growth and remineralization are sensitive to glacial cooling
Less remineralization than production in polar region enhances export production
High export production can be simulated even under glacial stratified conditions
The effect of bias on control simulation is a significant issue for climate change modeling studies. We investigated the effect of the sea surface temperature (SST) bias in present day (0 ka) ...Atmosphere–Ocean Coupled General Circulation Model (AOGCM) simulations on simulations of the mid-Holocene (6 ka, i.e., 6,000 years before present) Asian monsoon enhancement. Because changes in ocean heat transport have a negligible effect on the 6 ka Asian monsoon (Ohgaito and Abe-Ouchi in Clim Dyn 29(1):39–50, 2007), we used an Atmospheric General Circulation Model (AGCM) rather than an AOGCM. Simulations using the AGCM coupled to a mixed layer ocean model (MLM) were conducted for 0 and for 6 ka with different ocean heat transport estimated from the climatological SST of the 0 ka simulations from nine Paleoclimate Modeling Intercomparison Project (PMIP) phase 2 (PMIP2) AOGCMs (henceforth “MA” is used to refer to experiments using the AGCM coupled with the MLM). No correlation between MA and the corresponding PMIP2 was seen in the 0 ka precipitation and it was not very strong for the 6 ka precipitation enhancement. Thus, the influences from the different AGCMs play a substantial role on the 0 ka precipitation and the 6 ka precipitation enhancement. The sensitivity experiments indicated that it was the pattern of the 0 ka SST bias which played a dominant role in the 0 ka precipitation and the 6 ka precipitation enhancement, not the difference in the mean value of the SST bias. The distributions of the 6 ka precipitation enhancements for the nine PMIP2 AOGCMs and nine MA experiments were compared. These showed that the effects of SST bias on 6 ka precipitation enhancement among the AOGCMs were not negligible. The effects of biases among the AGCMs were not negligible either, but of comparable size. That is, improvements in both the SST bias and the AGCM contribute to simulate better 6 ka monsoon.
The mid‐Holocene (6000 years before present) North Atlantic Oscillation (NAO) from nine models in the Paleoclimate Modeling Intercomparison Project Phase 2 is studied, primarily through principal ...component analysis of winter time North Atlantic sea level pressure (SLP). Modeled mid‐Holocene NAO and mean SLP show small changes compared to pre‐industrial control runs, with a shift in mean state towards a more positive NAO regime for three of the models. Modeled NAO variability shows little change, with a small increase for some models in the fraction of time spent in the NAO‐negative phase during the mid‐Holocene. Proxy based reconstructions of the NAO indicate a more positive NAO regime compared to present day during the mid‐ Holocene. We hypothesise that there was a small NAO+ like shift in mean state during the mid‐Holocene.
The present paper revisits the future surface-climate experiments on the Greenland ice sheet proposed by the Sea-level Response to Ice Sheet Evolution (SeaRISE; Bindschadler et al., 2013) study. The ...projections of the different SeaRISE participants show dispersion, which has not been examined in detail to date. A series of sensitivity experiments are conducted and analyzed using the ice-sheet model for integrated Earth-system studies (IcIES) by replacing one or more formulations of the model parameters with those adopted in other model(s). The results show that large potential sources of the dispersion among the projections of the different SeaRISE participants are differences in the initialization methods and in the surface mass balance methods, and both aspects have almost equal impact on the results. The treatment of ice-sheet margins in the simulation has a secondary impact on the dispersion. We conclude that spinning up the model using fixed topography through the spin-up period while the temperature is allowed to evolve according to the surface temperature history is the preferred representation, at least for the experiment configuration examined in the present paper. A benchmark model experimental setup that most of the numerical models can perform is proposed for future intercomparison projects, in order to evaluate the uncertainties relating to pure ice-sheet model flow characteristics.
Studies of climate change 6,000 years before present using atmospheric general circulation models (AGCMs) suggest the enhancement and northward shift of the summer Asian and African monsoons in the ...Northern Hemisphere. Although enhancement of the African monsoonal precipitation by ocean coupling is a common and robust feature, contradictions exist between analyses of the role of the ocean in the strength of the Asian monsoon. We investigated the role of the ocean in the Asian monsoon and sought to clarify which oceanic mechanisms played an important role using three ocean coupling schemes: MIROC, an atmosphere-ocean coupled general circulation model C; an AGCM extracted from MIROC coupled with a mixed-layer ocean model M; and the same AGCM, but with prescribed sea surface temperatures A. The effect of “ocean dynamics” is quantified through differences between experiments C and M. The effect of “ocean thermodynamics” is quantified through differences between experiments M and A. The precipitation change for the African and Asian monsoon area suggested that the ocean thermodynamics played an important role. In particular, the enhancement of the Asian monsoonal precipitation was most vigorous in the AGCM simulations, but mitigated in early summer in ocean coupled cases, which were not significantly different from each other. The ocean feedbacks were not significant for the precipitation change in late summer. On the other hand, in Africa, ocean thermodynamics contributed to the further enhancement of the precipitation from spring to autumn, and the ocean dynamics had a modest impact in enhancing precipitation in late summer.
A set of coupled ocean-atmosphere simulations using state of the art climate models is now available for the Last Glacial Maximum and the Mid-Holocene through the second phase of the Paleoclimate ...Modeling Intercomparison Project (PMIP2). This study presents the large-scale features of the simulated climates and compares the new model results to those of the atmospheric models from the first phase of the PMIP, for which sea surface temperature was prescribed or computed using simple slab ocean formulations. We consider the large-scale features of the climate change, pointing out some of the major differences between the different sets of experiments. We show in particular that systematic differences between PMIP1 and PMIP2 simulations are due to the interactive ocean, such as the amplification of the African monsoon at the Mid-Holocene or the change in precipitation in mid-latitudes at the LGM. Also the PMIP2 simulations are in general in better agreement with data than PMIP1 simulations.
The understanding of the nature and behavior of ice sheets in past warm periods is important for constraining the potential impacts of future climate change. The Pliocene warm period (between 3.264 ...and 3.025 Ma) saw global temperatures similar to those projected for future climates; nevertheless, Pliocene ice locations and extents are still poorly constrained. We present results from the efforts to simulate mid-Pliocene Greenland Ice Sheets by means of the international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP). We compare the performance of existing numerical ice sheet models in simulating modern control and mid-Pliocene ice sheets with a suite of sensitivity experiments guided by available proxy records. We quantify equilibrated ice sheet volume on Greenland, identifying a potential range in sea level contributions from warm Pliocene scenarios. A series of statistical measures are performed to quantify the confidence of simulations with focus on inter-model and inter-scenario differences. We find that Pliocene Greenland Ice Sheets are less sensitive to differences in ice sheet model configurations and internal physical quantities than to changes in imposed climate forcing. We conclude that Pliocene ice was most likely to be limited to the highest elevations in eastern and southern Greenland as simulated with the highest confidence and by synthesizing available regional proxies; however, the extent of those ice caps needs to be further constrained by using a range of general circulation model (GCM) climate forcings.
Compared to the rapid glacial terminations, the buildup of glacial ice sheets in the Northern Hemisphere took tens of thousands of years. During the buildup phase, the growing ice sheets were subject ...to major orbitally induced summer insolation changes, without experiencing complete disintegration. The reason for this behavior still remains elusive. Here we propose that between 110 and 60 kyr B.P., every ∼20 kyr increased summer insolation in high northern latitudes triggered massive instabilities of the Northern Hemispheric ice sheets, leading to glacial meltwater pulses and subsequent disruptions of the Atlantic Meridional Overturning Circulation (AMOC). Associated severe cooling of the northern extratropics may have offset the warming trends driven by increased precessional summer insolation. This temperature response diminished the melting trend and stabilized the ice sheets. Our results suggest that the competition between the direct insolation changes and the indirect climate responseto AMOC disturbances may be an important negative feedback that supports the buildup of glacial ice sheets.
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