J. P. van Ypersele

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.

The 1979–2005 Greenland Ice Sheet Melt Extent from Passive Microwave Data Using an Improved Version of the Melt Retrieval XPGR Algorithm

Analysis of passive microwave satellite observations over the Greenland ice sheet reveals a significant increase in surface melt over the period 1979-2005. Since 1979, the total melt area was found to have increased by +1.22 x 10(7) km(2). An improved version of the cross-polarized gradient ratio (XPGR) technique is used to identify the melt from the brightness temperatures. The improvements in the melt retrieval XPGR algorithm as well as the surface melt acceleration are discussed with results from a coupled atmosphere-snow regional climate model. From 1979 to 2005, the ablation period has been increasing everywhere over the melt zone except in the regions where the model simulates an increased summer snowfall. Indeed, more snowfall in summer decreases the liquid water content of the snowpack, raises the albedo and therefore reduces the melt. Finally, the observed melt acceleration over the Greenland ice sheet is highly correlated with both Greenland and global warming suggesting a continuing surface melt increase in the future.

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.

Coupled Ocean and Sea-Ice Models: Review and Perspectives

This article reviews the existing coupled models of ocean and sea ice used in climate studies, and describes the methods used to realize their coupling. The role of sea ice in the climate system is first described. Then a brief history of uncoupled sea-ice modelling is presented, following a hierarchical approach. The coupling of ocean and sea ice is divided into three areas: heat, momentum, and salt. Each area of coupling is reviewed in detail. Typical examples of coupled sea-ice/ocean models are discussed. An example application of a coupled model to the simulation of the Weddell Polynya is presented. It is shown that above-freezing sea-suface temperature in the polynya area is associated with intense convection, and that ice divergence helps to precondition the area for overturning. Perspectives on possible and needed progress in coupled ocean/sea-ice modelling are outlined.

Sea-Ice Interactions in Polar Regions

This chapter reviews the state of knowledge about ocean and sea-ice interactions, their role in the climate system and their modelling. It is meant as an introduction to sea-ice geophysics for researchers with an interest in polar phenomena, and not as an exhaustive review for specialists. Therefore, many details will be omitted, and replaced by appropriate references to the sea-ice literature. The reader who wants to start studying sea ice is also referred to the excellent books by Untersteiner (1986a), Washington and Parkinson (1986), and Smith (1990). Additional information on sea-ice modelling may also be found in Hibler (1988) and van Ypersele (1989, 1990). Remote sensing of sea ice is treated, e.g., in the book by Hall and Martinec (1985), and in Shuchman and Onstott (1990). The World Climate Research Programme (WCRP) publishes reports by its Working Group on Sea Ice and Climate (WCRP, 1983, 1984, 1987, 1989, 1990, 1992), where useful information on sea-ice research related to the WCRP objectives can be found.

Numerical Modelling of Arctic Sea Ice: Review and Preliminary Results

According to Untersteiner (1975), the total amount of water in all earthly forms is estimated to be 138 x 10 km. Of this, 97.4 % is sea water; 0.0009 % is atmospheric water vapour; 0.5 % is ground water, mostly at great depths; 0.1 % is contained in rivers and lakes, and 2.0 % is frozen. Today, perennial ice covers 11 % of the erath’s land surface and an average of 7 % of the world ocean.