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Wataru Yanase and Ayako Abe-Ouchi

Abstract

The dynamics of the atmospheric circulation change over the midlatitude North Pacific under the boundary conditions during the last glacial maximum (LGM) have been studied by atmospheric general circulation models (GCMs) with different ocean feedbacks. Three boundary conditions in the LGM were different from those of the present day (PD): ice sheet with elevated topography and high albedo, atmospheric CO2 concentration, and insolation. The ocean component was treated as follows: a full-circulation ocean with dynamical and thermal ocean feedback [coupled general circulation model (CGCM)]; a slab ocean only with thermal feedback used to calculate the surface heat balance [slab ocean GCM (SGCM)]; and no ocean feedback by fixing sea surface temperature (SST) with pure atmospheric dynamics (AGCM). Both CGCM and SGCM simulated a weakened Pacific high pressure system in boreal summer during the LGM compared to the PD and an intensified Aleutian low pressure system in winter. Both in summer and winter, therefore, the lower-tropospheric circulation during the LGM showed midlatitude North Pacific cyclonic anomalies (NPCAs).

To understand the dynamics determining the NPCAs, the sensitivity of the atmospheric response to the three boundary conditions were examined using the SGCM. It was shown that the high albedo of the ice sheet over North America was the dominant factor behind the NPCAs in both summer and winter. The ocean thermal feedback in winter played an essential role in the formation of the NPCA through SST change, while the ocean thermal feedback in summer and ocean dynamical feedback played secondary roles in the intensification of the NPCA. Possible mechanisms were inferred from the common features related to the NPCA formation in the experiments. In summer, the midlatitude NPCA was associated with the reduced land–ocean contrast of diabatic heating between the North Pacific and North America, which is consistent with theoretical studies on the mechanism for formation of subtropical high pressure systems. In winter, on the other hand, the anomaly of the SST gradient at midlatitude is thought to result in the NPCA through the modulation of heat and momentum transport in the storm track.

The small (large) sensitivity of the NPCA formation to the ocean feedbacks in summer (winter) explains the strong (weak) consistency among the previous GCM experiments. Since the NPCAs are consistent with some geological records, the present study should be informative in understanding the actual dynamics of the LGM climate change.

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Masakazu Yoshimori and Ayako Abe-Ouchi

Abstract

The many studies investigating the future change of the Greenland Ice Sheet surface mass balance from climate model output exhibit a wide range of projections. This study makes projections from the Coupled Model Intercomparison Project phase 3 models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report and explores the underlying physical processes behind their spread. The projections are made for three Special Report on Emissions Scenarios, B1, A1B, and A2, with a focused analysis on the A1B scenario. The estimate in the study suggests that about 60% of the intermodel difference in the twenty-first-century ablation rate change under the A1B scenario is accounted for by the global annual mean temperature change. In the current study, other processes responsible for the spread in model projections are investigated after excluding this global effect. A negative correlation (−0.60) was found between the simulated summer temperature bias over the Greenland Ice Sheet under present-day conditions and the ablation rate increase during the twenty-first century, partly because maximum warming over ice is approximately limited to the melting temperature. Models with relatively larger ablation rate increases during the twenty-first century exhibit greater warming with a greater reduction in sea ice cover. The authors found that these models also simulate relatively cooler summer conditions in high latitudes with more sea ice cover in the late twentieth century, suggesting the importance of sea ice feedbacks. Also, an anticorrelation (−0.75) is found between weakening of the Atlantic meridional overturning circulation and the ablation rate increase during the twenty-first century. The relation in the model spread between the twenty-first-century ablation change and the late twentieth-century climate conditions is then used to investigate the impact of model bias on the multimodel ensemble of projections. The result suggests that the models’ underestimation of present-day sea ice concentration near the coast of Greenland may cause an underestimation of future Greenland Ice Sheet ablation rate increase in the ensemble projection. These results emphasize the importance of correctly simulating present-day conditions and understanding the underlying multiple physical processes behind the intermodel difference to reduce the uncertainty in future projections.

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Masakazu Yoshimori, Tokuta Yokohata, and Ayako Abe-Ouchi

Abstract

Studies of the climate in the past potentially provide a constraint on the uncertainty of climate sensitivity, but previous studies warn against a simple scaling to the future. Climate sensitivity is determined by a number of feedback processes, and they may vary according to climate states and forcings. In this study, the similarities and differences in feedbacks for CO2 doubling, a Last Glacial Maximum (LGM), and LGM greenhouse gas (GHG) forcing experiments are investigated using an atmospheric general circulation model coupled to a slab ocean model. After computing the radiative forcing, the individual feedback strengths of water vapor, lapse-rate, albedo, and cloud feedbacks are evaluated explicitly. For this particular model, the difference in the climate sensitivity between the experiments is attributed to the shortwave cloud feedback, in which there is a tendency for it to become weaker or even negative in cooling experiments. No significant difference is found in the water vapor feedback between warming and cooling experiments by GHGs. The weaker positive water vapor feedback in the LGM experiment resulting from a relatively weaker tropical forcing is compensated for by the stronger positive lapse-rate feedback resulting from a relatively stronger extratropical forcing. A hypothesis is proposed that explains the asymmetric cloud response between the warming and cooling experiments associated with a displacement of the region of mixed-phase clouds. The difference in the total feedback strength between the experiments is, however, relatively small compared to the current intermodel spread, and does not necessarily preclude the use of LGM climate as a future constraint.

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Alexandre Laîné, Masakazu Yoshimori, and Ayako Abe-Ouchi

Abstract

Arctic amplification (AA) is a major characteristic of observed global warming, yet the different mechanisms responsible for it and their quantification are still under investigation. In this study, the roles of different factors contributing to local surface warming are quantified using the radiative kernel method applied at the surface after 100 years of global warming under a representative concentration pathway 4.5 (RCP4.5) scenario simulated by 32 climate models from phase 5 of the Coupled Model Intercomparison Project. The warming factors and their seasonality for land and oceanic surfaces were investigated separately and for different domains within each surface type where mechanisms differ. Common factors contribute to both land and oceanic surface warming: tropospheric-mean atmospheric warming and greenhouse gas increases (mostly through water vapor feedback) for both tropical and Arctic regions, nonbarotropic warming and surface warming sensitivity effects (negative in the tropics, positive in the Arctic), and warming cloud feedback in the Arctic in winter. Some mechanisms differ between land and oceanic surfaces: sensible and latent heat flux in the tropics, albedo feedback peaking at different times of the year in the Arctic due to different mean latitudes, a very large summer energy uptake and winter release by the Arctic Ocean, and a large evaporation enhancement in winter over the Arctic Ocean, whereas the peak occurs in summer over the ice-free Arctic land. The oceanic anomalous energy uptake and release is further studied, suggesting the primary role of seasonal variation of oceanic mixed layer temperature changes.

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Sam Sherriff-Tadano and Ayako Abe-Ouchi

Abstract

During glacial periods, climate varies between two contrasting modes, the interstadials and stadials. These climate changes are often associated with drastic reorganizations of the Atlantic meridional overturning circulation (AMOC). Previous studies highlight the important role of sea ice in maintaining contrasting modes of the AMOC through its insulating effect on the oceanic heat flux and the buoyancy flux (sea ice–buoyancy flux feedback); however, the effect of feedback from the atmosphere remains unclear. Here, the effect of sea ice–surface wind interactions over the North Atlantic Ocean on the AMOC is explored. For this purpose, results from comprehensive atmosphere–ocean coupled general circulation models (AOGCMs) are analyzed. Then, sensitivity experiments are conducted with the atmospheric component of the AOGCM. Last, to explore the impact of modifications in surface winds induced by sea ice on the maintenance of the AMOC, partially coupled experiments are conducted with the AOGCMs. Experiments show that the expansion of sea ice associated with a weakening of the AMOC reduces surface winds by suppressing the oceanic heat flux and increasing the atmospheric static stability. This wind anomaly then causes a weakening of the wind-driven ocean salt transport to the northern North Atlantic and maintains the weak AMOC, therefore working as a positive feedback. It is shown that, together with the sea ice–buoyancy flux and sea ice–surface wind feedback, changes in sea ice and oceanic heat flux sustain the contrasting modes of the AMOC. These results may provide useful information for interpreting the differences in the self-sustained internal oscillations of the AMOC produced by recent AOGCMs.

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Shigenori Murakami, Rumi Ohgaito, and Ayako Abe-Ouchi

Abstract

The atmospheric local energy cycle in the Last Glacial Maximum (LGM) climate simulated by an atmosphere–ocean GCM (AOGCM) is investigated using a new diagnostic scheme. In contrast to existing ones, this scheme can represent the local features of the Lorenz energy cycle correctly, and it provides the complete information about the three-dimensional structure of the energy interactions between mean and eddy fields. The diagnosis reveals a significant enhancement of the energy interactions through the barotropic processes in the Atlantic sector at the LGM. Energy interactions through the baroclinic processes are also enhanced in the Atlantic sector, although those in the Pacific sector are rather weakened. These LGM responses, however, are not evident in the global energy cycle except for an enhancement of the energy flow through the stationary eddies.

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Tomoko Nitta, Kei Yoshimura, and Ayako Abe-Ouchi

Abstract

Wetlands cover large areas of the middle and high latitudes and influence the surface water and energy budget, surface hydrology, and the climate system. In this study, a scheme implicitly representing a snow-fed wetland, in which snowmelt can be stored with consideration of subgrid terrain complexity, was implemented in the Minimal Advanced Treatments of Surface Interaction and Runoff (MATSIRO) land surface model. An atmospheric general circulation model (AGCM) experiment was conducted using the Model for Interdisciplinary Research on Climate, version 5 (MIROC5), with and without the wetland scheme, with the main aim of reducing the model bias of warm and dry boreal summer at mid- to high latitudes. The experiment showed not only a better surface hydrology but also a weaker land–atmosphere coupling strength and larger (smaller) latent (sensible) heat flux due to the delayed snowmelt runoff. The summer warm and dry bias was partially improved over snowy and flat areas, particularly over much of western Eurasia and North America, without an apparent deterioration of simulated surface hydrology and climate over the rest of the land in the other seasons; the mean absolute error of 2-m air temperature and precipitation over land at 45°–90°N in summer decreased by 19% and 4%, respectively. The next step of model development will involve implementing an explicit representation of subgrid-scale surface water and related processes.

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Jun-Mei Lü, Seong-Joong Kim, Ayako Abe-Ouchi, Yongqiang Yu, and Rumi Ohgaito

Abstract

Changes in the Arctic Oscillation (AO) during the mid-Holocene and the Last Glacial Maximum were compared to preindustrial (PI) simulations using four coupled ocean–atmosphere models [i.e., Community Climate System Model (CCSM), third climate configuration of the Met Office Unified Model (HadCM3) Met Office Surface Exchanges Scheme, version 2 (MOSES2), L’Institut Pierre-Simon Laplace (IPSL), and Model for Interdisciplinary Research on Climate 3.2 (MIROC3.2)] from the second phase of the Paleoclimate Modeling Intercomparison Project. Results show that the amplitude of the simulated AO during the mid-Holocene is a little smaller than that of the preindustrial simulation. Although the AO pattern and vertical structures are similar to those in the preindustrial simulation, the polar westerlies are slightly weakened and displaced downward to the lower stratosphere, accompanied by weakening of the polar vortex and warming of the cold polar cap region. During the Last Glacial Maximum, when the Northern Hemisphere experiences severe cooling, the intensity of the AO decreases substantially compared to the mid-Holocene, with smaller standard deviations of the AO indices in all models. Furthermore, the magnitude of positive and negative centers of the AO spatial pattern decreases and the strength of the polar vortex and westerlies weakens further with the center of westerlies displaced into the midlatitude upper troposphere. The polar cap region becomes anomalously warm in the stratosphere, whereas it remains cold in the troposphere. The AO appears to be sensitive to background climate state.

Upward-propagating stationary Rossby waves are found to be stronger during the mid-Holocene and Last Glacial Maximum than in the preindustrial simulation. This increase in planetary wave activity might be responsible for the simulated weakening of the AO during the mid-Holocene and Last Glacial Maximum. Recent studies have shown that there is a significant correlation between Eurasian fall snow cover and the winter AO. The upward propagation of Rossby waves was further proposed to explain the physical process linking the AO with the snow depth. It is suggested that a large increase in fall snow depth during the Last Glacial Maximum strengthens the upward-propagating stationary Rossby waves relative to the PI.

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Shigenori Murakami, Rumi Ohgaito, Ayako Abe-Ouchi, Michel Crucifix, and Bette L. Otto-Bliesner

Abstract

Three coupled atmosphere–ocean general circulation model (AOGCM) simulations of the Last Glacial Maximum (LGM: about 21 000 yr before present), conducted under the protocol of the second phase of the Paleoclimate Modelling Intercomparison Project (PMIP2), have been analyzed from a viewpoint of large-scale energy and freshwater balance. Atmospheric latent heat (LH) transport decreases at most latitudes due to reduced water vapor content in the lower troposphere, and dry static energy (DSE) transport in northern midlatitudes increases and changes the intensity contrast between the Pacific and Atlantic regions due to enhanced stationary waves over the North American ice sheets. In low latitudes, even with an intensified Hadley circulation in the Northern Hemisphere (NH), reduced DSE transport by the mean zonal circulation as well as a reduced equatorward LH transport is observed. The oceanic heat transport at NH midlatitudes increases owing to intensified subpolar gyres, and the Atlantic heat transport at low latitudes increases in all models whether or not meridional overturning circulation (MOC) intensifies. As a result, total poleward energy transport at the LGM increases in NH mid- and low latitudes in all models. Oceanic freshwater transport decreases, compensating for the response of the atmospheric water vapor transport. These responses in the atmosphere and ocean make the northern North Atlantic Ocean cold and relatively fresh, and the Southern Ocean relatively warm and saline. This is a common and robust feature in all models. The resultant ocean densities and ocean MOC response, however, show model dependency.

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Masakazu Yoshimori, Julia C. Hargreaves, James D. Annan, Tokuta Yokohata, and Ayako Abe-Ouchi

Abstract

Climate sensitivity is one of the most important metrics for future climate projections. In previous studies the climate of the last glacial maximum has been used to constrain the range of climate sensitivity, and similarities and differences of temperature response to the forcing of the last glacial maximum and to idealized future forcing have been investigated. The feedback processes behind the response have not, however, been fully explored in a large model parameter space. In this study, the authors first examine the performance of various feedback analysis methods that identify important feedbacks for a physics parameter ensemble in experiments simulating both past and future climates. The selected methods are then used to reveal the relationship between the different ensemble experiments in terms of individual feedback processes. For the first time, all of the major feedback processes for an ensemble of paleoclimate simulations are evaluated. It is shown that the feedback and climate sensitivity parameters depend on the nature of the forcing and background climate state. The forcing dependency arises through the shortwave cloud feedback while the state dependency arises through the combined water vapor and lapse-rate feedback. The forcing dependency is, however, weakened when the feedback is estimated from the forcing that includes tropospheric adjustments. Despite these dependencies, past climate can still be used to provide a useful constraint on climate sensitivity as long as the limitation is properly taken into account because the strength of each feedback correlates reasonably well between the ensembles. It is, however, shown that the physics parameter ensemble does not cover the range of results simulated by structurally different models, which suggests the need for further study exploring both structural and parameter uncertainties.

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