H. Gallee

Simulation of the last glacial cycle by a coupled, sectorially averaged climate—ice sheet model: 1. The climate model

This paper describes a sectorially averaged seasonal model developed for simulating the long-term response of the climate system to the astronomical forcing. The model domain covers the northern hemisphere. The atmospheric dynamics is represented by an improved zonally averaged quasi-geostrophic model. It includes a new parameterization of the meridional transport of quasi-geostrophic potential vorticity and a parameterization of the Hadley sensible heat transport. The atmosphere interacts with the other components of the climate system (ocean, sea ice, and land surface covered or not by snow and ice) through vertical fluxes of momentum, heat and water vapor. The model explicity incorporates detailed radiative transfer, surface energy balances, and snow and sea ice budgets. The vertical profile of the upper ocean temperature is computed by an integral mixed-layer model which takes into account meridional convergence of heat. Sea ice is represented by a thermodynamic model including leads and a new parameterization for lateral accretion. This paper presents the model climate for present conditions and results of sensitivity experiments obtained by modifying some internal parameters or by deactivating certain parameterizations in the model. Simulation of the present climate shows that the model is able to reproduce the main characteristics of the general circulation and, in particular, the surface wind field. The seasonal cycles of oceanic mixed layer, sea ice, and snow cover are also well reproduced. Sensitivity experiments show the importance of the meridional sensible heat transport by the Hadley circulation in the tropics, the seasonal cycle of the oceanic mixed-layer depth and sea ice formation in latitude bands where the average water temperature is above the freezing point. In a forthcoming paper, this model will be coupled to an ice sheet model and applied to the simulation of the last glacial cycle in the northern hemisphere.

Simulation of the Last Glacial Cycle By a Coupled, Sectorially Averaged Climate-ice Sheet Model .2. Response To Insolation and Co2 Variations

A two-dimensional climate model which links the northern hemisphere atmosphere, ocean mixed layer, sea ice, and continents has been asynchronously coupled to a model of the three main northern ice sheets and their underlying bedrock. The coupled model has been used to test the influence of several factors, including snow surface albedo over the ice sheets, in producing plausible ice age simulations using astronomically derived insolation and CO2 data from the Vostok ice core. The impact of potentially important processes, such as the water vapor transport, clouds, and deep sea circulation, was not investigated in this study. After several sensitivity experiments designed to identify the main mechanism governing surface temperature and ice accumulation, the model is first run with ice sheet feedback by forcing it only with the astronomical insolation over the past 122 kyr. Large variations of ice volume are simulated between 122 and 55 kyr B.P., with a rapid latitudinal extension of the North American and Eurasian ice sheets starting at 120 kyr B.P. The simulated last glacial maximum is at 19 kyr B.P. The model is able to simulate deglaciation as well. The simulated evolution of the three northern ice sheets is generally in phase with geological reconstructions. The major discrepancy between the simulation and paleoclimate reconstructions lies in the underestimation of temperature variations (linked with an underestimation of the ice sheet extent and an excess in the prescribed CO2 concentration). Sensitivity experiments show that ablation is more important to the ice sheet response than snow precipitation variations. In the model a key mechanism in the deglaciation after the last glacial maximum appears to be the aging of snow, which decreases its albedo. The other factors which play an important role are, in decreasing level of importance, the ice sheet altitude, insolation, taiga cover, and ice sheet extent. A final set of experiments addresses the effects of CO2 on the simulated climate of the last glacial maximum and on a new long term experiment covering the last 122 kyr. This last experiment is made by forcing the model with both insolation and CO2 Variations. This additional forcing improves the temperature and ice volume results. Despite the limitations inherent to the present modeling approach, the sensitivity experiments performed can provide insight into the relative importance of possible mechanism responsible for the building and melting of huge ice sheets during the last glacial-interglacial cycle.

Testing the Astronomical Theory With a Coupled Climate Ice-sheet Model

Ice, land and deep-sea cores have provided strong evidence that orbital perturbations are of primary importance in the timing of the waxing and waning of Pleistocene continental ice sheets. Spectral analyses of climatic records have shown that, at least near the frequencies of variation in obliquity and precession, a considerable fraction of the climate variance is driven in some way by insolation changes accompanying changes in the Earths orbit. The exceptional strength of the dominating 100,000-year cycle needs a non-linear amplification of the eccentricity-precession cycles from the ice sheets themselves and related mechanisms. n nNow that the astronomical model has passed both severe statistical tests and some tests of physical plausibility, the influence of orbital variations is thought to be real enough to be tested with numerical climate models. In addition to the calculation of the Earths climate which is in equilibrium with a particular insolation pattern and boundary conditions, the simulation of the transient response of the climate system to orbital variations is important since it will lead to better understanding of the physical mechanisms involved in the relationship between the astronomical forcing and climate. It has been suggested that the time evolution of the latitudinal distribution of the seasonal pattern of insolation is the key factor driving the climate system on the 1000-year time scale. n nIn this paper, a 2.5-D time-dependent climate model taking into account the feedbacks between the atmosphere, the upper ocean, the sea ice, the ice sheets and the lithosphere, is used to confirm this thesis. The model results produce long-term variations of the global ice volume over the past 125,000 years that are in general agreement with the most recent geological reconstructions. A comparison with the sea-level curve of Chappell and Shackleton and with the δ18O curve of Duplessy-Labeyrie-Blanc shows that our early stage 3 is a few thousands of years younger than dated by Chappell and Shackleton, and our stage 3 high-sea level itself is 20 meters lower than their value (i.e., 50 m below present sea level instead of 30 m).

Modelling of large-scale melt parameters with a regional climate model in south Greenland during the 1991 melt season

Abstract Large-scale positive degree-day based melt parameterizations for the Greenland ice sheet are highly sensitive to their parameters (standard temperature deviation, snow and ice degree-day factors). In this paper, these parameters are simulated with a coupled atmosphere–snow regional climate model for southern Greenland during summer 1991, forced at the lateral boundaries with European Centre for Medium-Range Weather Forecasts re-analyses at a high horizontal resolution of 20 km. the calculated (from net ablation, i.e. melt minus refreezing) snow and ice positive degree-day factors vary considerably over the ice sheet. At low elevations, the modelled snow degree-day factor closely approaches the generally accepted value of 3 mm w.e. d–1 ˚C–1.Higher up the ice sheet, large values up to 15 mm w.e. d– 1 ˚C– 1 are simulated. for ice melt, maximum values of 40 mm w.e. d–1 ˚C– 1 are found. the snow and ice positive degree-day factor distributions peak, respectively, at 3 and 8mm w.e. d–1 ˚C–1. Refreezing is of small importance close to the ice-sheet margin. Higher up the ice sheet, refreezing considerably lowers the amount of net ablation. the monthly simulated 2 m air-temperature standard deviation exhibits a strong seasonal cycle, with the highest (3.0–5.0˚C) values in May and June. July shows the lowest temperature fluctuations, due to the melting of the surface.

Entering the Glaciation With a 2-d Coupled Climate Model

A 2-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea-ice, the continents, the ice sheets and their underlying lithosphere has been used to test the Milankovitch theory over the last glacial-interglacial cycle. Sensitivity tests have shown that the orbital variations can induce, in such a system, feedbacks sufficient to generate the low frequency part of the climatic variations over the last 122 ka BP. These variations at the astronomical time scale are broadly in agreement with ice volume and sea level reconstruction independently obtained from geological data. At the beginning of the integration during isotopic Substage 5e when there was no northern american nor eurasian ice sheets, June insolation at high latitudes seemed to correlate well with summer temperature at the surface of the ice sheets and with the ablation rate. The relation is more complicated when large ice sheets exist. Broadly, the June-July insolation from 60 to 70-degrees-N. leads the ice volume by roughly 5000 years over the last glacial-interglacial cycle. The simulated climate was shown to be sensitive to the ice albedo-temperature feedback, to the precipitation-altitude negative feedback over the ice sheets, to the ice sheet slope and continentality effects, and to the albedo of the ice sheets as a function of the snowfall frequency at their surface. The formation and waxing of the ice sheets are particularly sensitive to ablation, more than to snowfalls. Imperfections in the simulated climate were recognized, in particular at the last glacial maximum when the cooling was not large enough and the northern hemisphere ice sheets did not extend far enough to the south, weaknesses which are partly solved in a further experiment (Gallee et al., 1989) by using the proper CO2 atmospheric concentration given by the Vostok ice core in addition to the astronomical forcing.

Physical Interactions Within a Coupled Climate Model Over the Last Glacial Interglacial Cycle

A two-dimensional (2-D) seasonal model has been developed for stimulating the transient response of the climate system to the astronomical forcing. The atmosphere is represented by a zonally averaged quasi-geostrophic model which includes accurate treatment of radiative transfer. The atmospheric model interacts with the other components of the climate system (ocean, sea-ice and land surface covered or not by snow and ice) through vertical fluxes of momentum, heat and humidity. The model explicitly incorporates surface energy balances and has snow and sea-ice mass budgets. The vertical profile of the upper-ocean temperature is computed by an interactive mixed-layer model which takes into account the meridional turbulent diffusion of heat. This model is asynchronously coupled to a model which simulates the dynamics of the Greenland, the northern American and the Eurasian ice sheets. Over the last glacial-interglacial cycle, the coupled model simulates climatic changes which are in general agreement with the low frequency part of the deep-sea, ice and sea-level records. However, after 6000 yBP, the remaining ice volume of the Greenland and northern American ice sheets is overestimated in the simulation. The simulated climate is sensitive to the initial size of the Greenland ice sheet, to the ice-albedo positive feedback, to the precipitation-altitude negative feedback over the ice sheets, to the albedo of the aging snow and to the insolation increase, particularly at the southern edge of the ice sheets, which is important for their collapse or surge.

Ice-sheet growth and high-latitudes sea-surface temperature

The response of the LLN 2-D climate model to the insolation and CO2 forcings during the Eemian interglacial is compared to reconstructions obtained from deep-sea cores drilled in the Norwegian Sea and in the North Atlantic. Both reconstructions and modeling results show a decrease of sea-surface temperature (SST) in the higher latitudes (70–75°N zonal belt for the model and the Norwegian Sea for the proxy records), associated with a more moderate cooling at lower latitudes (50–55°N and North Atlantic), at the middle of isotopic substage 5e, several millenia before the beginning of continental ice-sheet growth. Such a comparison between the simulated SST and ice volume of the Northern Hemisphere has been extended to the whole last glacial-interglacial cycle. The influence of the insolation forcing on SST and the shortcomings of the model due to its zonal character are discussed.

The last two glacial-interglacial cycles simulated by the LLN model

A 2-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea ice, the continents, the ice sheets and their underlying bedrock has been used to test the Milankovitch theory over the last two glacial-interglacial cycles. A series of sensitivity analyses have allowed us to better understand the internal mechanisms which drive the simulated climate system and in particular the feedbacks related to surface albedo and water vapour.

Simulation of the Climate of the Last 200 Kyr with the LLN 2D-Model

A sectorially averaged model of the northern hemisphere has been developed, taking into account the coupling between the atmosphere, the upper ocean, sea-ice, the ice sheets and the underlying bedrock. It has been used to simulate the last glacial-interglacial cycle (last 122 kyr) as a response to the insolation and CO2 forcings (Gallee et al., 1992). In this paper a simulation of the climate of the last 200 kyr is presented. For this simulation, both the insolation forcing and the CO2 variations reconstructed from deep sea cores are taken into account. Except for variations with time scales shorter than 5 kyr, the simulated long term variation of the total ice volume may be compared with that reconstructed from deep sea cores. For example, the model simulates glacial maxima of similar amplitudes at 134 kyr B.P. and 15 kyr B.P., followed by abrupt deglaciations. The complete deglaciation of the three main northern hemisphere ice sheets, which is simulated around 122 kyr B.P., is in partial disagreement with the reconstructions, which indicate that the Greenland ice sheet survived during the Eemian interglacial. The continental ice volume variations during the last 122 kyr of the 200 kyr simulation are not significantly affected by this shortcoming.

Climate Studies with a Coupled Atmosphere--Upper-Ocean--Ice-Sheet Model

A two-dimensional zonally averaged model has been developed for simulating the seasonal cycle of the climate of the Northern Hemisphere. The atmospheric component of the model is based on the two-level quasi-geostrophic potential vorticity system of equations. At the surface, the model has land—sea resolution and incorporates detailed snow and sea-ice mass budgets. The upper ocean is represented by an integral mixed-layer model that takes into account the meridional advection and turbulent diffusion of heat. Comparisons between the computed present-day climate and climatological data show that the model does reasonably well in simulating the seasonal cycle of the temperature field. In response to a projected CO2 trend based on the scenario of Wuebbles et al. (DOE/ NBB-0066 Technical Report 15 (1984)), the modelled annual hemispheric mean surface temperature increases by 2 °C between 1983 and 2063. In the high latitudes, the response undergoes significant seasonal variations. The largest surface warmings occur during autumn and spring. The model is then asynchronously coupled to a model that simulates the dynamics of the Greenland, the Eurasian and the North American ice sheets in order to investigate the transient response of the climate to the long-term insolation anomalies caused by orbital perturbations. Over the last interglacial-glacial cycle, the coupled model produces continental ice-volume changes that are in general agreement with the low-frequency part of palaeoclimatic records.