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SI3 (Sea Ice modelling Integrated Initiative) is the sea ice engine of NEMO (Nucleus for European Modelling of the Ocean). It is adapted to regional and global sea ice and climate problems. It is intended to be a flexible tool for studying sea ice and its interactions with the other components of the Earth System over a wide range of space and time scales. For these applications, a 2+1D continuum approach is currently the most appropriate, SI3 is based on the Arctic Ice Dynamics Joint EXperiment (AIDJEX) framework, combining the ice thickness distribution framework, the conservation of horizontal momentum, an elastic-viscous plastic rheology, and energy-conserving halo-thermodynamics. Prognostic variables are the two-dimensional horizontal velocity field, ice volume, area, enthalpy, salt content, snow volume and enthalpy, and melt pond volume and area. In the horizontal direction, the model uses a curvilinear orthogonal grid. In the vertical direction, the model uses equally-spaced layers. In thickness space, the model uses thickness categories with prescribed boundaries. Various physical and numerical choices are available to describe sea ice physics. SI3 is interfaced with the NEMO ocean engine, and, via the OASIS coupler, with several atmospheric general circulation models. It also supports two-way grid embedding via the AGRIF software.

In theory, the same sea-ice models could be used for both research and operations, but in practice, differences in scientific\ud and software requirements and computational and human resources complicate the matter. Although sea-ice modeling tools\ud developed for climate studies and other research applications produce output of interest to operational forecast users, such as\ud ice motion, convergence, and internal ice pressure, the relevant spatial and temporal scales may not be sufficiently resolved.\ud For instance, sea-ice research codes are typically run with horizontal resolution of more than 3 km, while mariners need\ud information on scales less than 300 m. Certain sea-ice processes and coupled feedbacks that are critical to simulating the\ud Earth system may not be relevant on these scales; and therefore, the most important model upgrades for improving sea-ice\ud predictions might be made in the atmosphere and ocean components of coupled models or in their coupling mechanisms,\ud rather than in the sea-ice model itself. This paper discusses some of the challenges in applying sea-ice modeling tools\ud developed for research purposes for operational forecasting on short time scales, and highlights promising new directions in\ud sea-ice modeling.

The physical processes responsible for the formation in a large-scale ice ocean model of an offshore polynya near the Greenwich meridian in the Southern Ocean are analysed. In this area, the brine release during ice formation in autumn is sufficient to destabilise the water column and trigger convection. This incorporates relatively warm water into the surface layer which, in a first step, slows down ice formation. In a second step, it gives rise to ice melting until the total disappearance of the ice at the end of September. Two elements are crucial for the polynya opening. The first one is a strong ice-transport divergence in fall induced by southeasterly winds, which enhances the amount of local ice formation and thus of brine release. The second is an inflow of relatively warm water at depth originating from the Antarctic Circumpolar Current, that sustains the intense vertical heat flux in the ocean during convection. The simulated polynya occurs in a region where such features have been frequently observed. Nevertheless. the model polynya is too wide and persistent. In addition, it develops each year, contrary to observations. The use of a climatological forcing with no interannual variability is the major cause of these deficiencies, the simulated too low density in the deep Southern Ocean and the coarse resolution of the model playing also a role. a passive tracer released in the polynya area indicates that the water mass produced there contributes significantly to the renewal of deep water in the Weddell Gyre and that it is a major component of the Antarctic Bottom Water (AABW) inflow into the model Atlantic.

International audience; On the basis of long simulations performed with a three‐dimensional climate model, we propose an interhemispheric climate lag mechanism, involving the long‐term memory of deepwater masses. Warm anomalies, formed in the North Atlantic when warm conditions prevail at surface, are transported by the deep ocean circulation towards the Southern Ocean. There, the heat is released because of large scale upwelling, maintaining warm conditions and inducing a lagged response of about 150 years compared to the Northern Hemisphere. Model results and observations covering the first half of the second millenium suggest a delay between the temperature evolution in the Northern Hemisphere and in the Southern Ocean. The mechanism described here provides a reasonable hypothesis to explain such an interhemipsheric lag.

The hydrosphere is made up of a number media, such as the oceans, the shelf seas, the estuaries, the rivers, the land surface and ground water as well as the sea ice - which, for the sake of simplicity, is considered herein to be part of the hydrosphere. The processes taking place in these domains are vastly different in nature and are characterized by a wide range of spaceand time-scales. The components of the hydrosphere interact with each other. For instance, the shallow marine and estuarine regions, though accounting for less than 1% of the volume of the oceans, have a biomass far from negligible as compared to that of the oceans, implying that they play a significant role in global biogeochemical cycles. This is one of the reasons why models are now needed that deal with most, if not all, of the components of the hydrospheric system.Numerical models of each of the components of the hydrosphere already exist. However, an integrated model of the whole hydrosphere has yet to be developed. Building such a model is a daunting task, requiring the development of multi-scale/physics simulation tools. Numerical methods for dealing with multi-scale problems are developing rapidly. Unstructured meshes offer an almost infinite geometrical flexibility, allowing the space resolution to be increased when and where necessary. In addition, time steppings for dealing with a wide spectrum of timescales while retaining a high order of accuracy have been developed over recent years (e.g. multi-rate schemes).In this talk we will present the current status of SLIM, as well as developments planned in the near future. We will highlight the efficiency issues related to the use of unstructured meshes and present possible strategies to overcome them. Applications will be reported on, in particular those pertaining to the land-sea continuum of the Scheldt River (France, Belgium, The Netherlands), the Great Barrier Reef (Australia) and the Mahakam River (Indonesia).

The elastic-viscous-plastic (EVP) sea ice rheology has been introduced in the large-scale Louvain-la-Neuve sea-Ice Model, version 2 (LIM2), and its performance has been evaluated. Centred difference versions of the rheology have been implemented on both an Arakawa B grid and a C grid, and their performance have been intercompared in coupled simulations with the Nucleus for European Modelling of the Ocean (NEMO) model. Integrations with both implementations lead to fairly similar results which compare well with observations and with previous LIM simulations. The C grid version, however, offers a number of advantages: (a) easier ice coupling with NEMO, which is itself defined on a C grid; (b) possibility of representing ice transport across one-cell-wide straits and passages; (c) better representation of inertial-plastic compressive waves. For these reasons, we recommend the use of the C grid EVP formulation of the ice dynamics in future LIM applications.

International audience; no abstract

International audience; The fourth version of the atmosphere-ocean general circulation (AOGCM) model developed at the Institut Pierre-Simon Laplace (IPSL-CM4) is used to investigate the mechanisms influencing the Arctic freshwater balance in response to anthropogenic greenhouse gas forcing. The freshwater influence on the interannual variability of deep winter oceanic convection in the Nordic Seas is also studied on the basis of correlation and regression analyses of detrended variables. The model shows that the Fram Strait outflow, which is an important source of freshwater for the northern North Atlantic, experiences a rapid and strong transition from a weak state toward a relatively strong state during 1990\u20132010. The authors propose that this climate shift is triggered by the retreat of sea ice in the Barents Sea during the late twentieth century. This sea ice reduction initiates a positive feedback in the atmosphere-sea ice-ocean system that alters both the atmospheric and oceanic circulations in the Greenland-Iceland-Norwegian (GIN)-Barents Seas sector. Around year 2080, the model predicts a second transition threshold beyond which the Fram Strait outflow is restored toward its original weak value. The long-term freshening of the GIN Seas is invoked to explain this rapid transition. It is further found that the mechanism of interannual changes in deep mixing differ fundamentally between the twentieth and twenty-first centuries. This difference is caused by the dominant influence of freshwater over the twenty-first century. In the GIN Seas, the interannual changes in the liquid freshwater export out of the Arctic Ocean through Fram Strait combined with the interannual changes in the liquid freshwater import from the North Atlantic are shown to have a major influence in driving the interannual variability of the deep convection during the twenty-first century. South of Iceland, the other region of deep water renewal in the model, changes in freshwater import from the North Atlantic constitute the dominant forcing of deep convection on interannual time scales over the twenty-first century.

International audience; no abstract

A new Earth system model of intermediate complexity, LOVECLIM, has been developed in order to study long-term future climate changes. It includes an interactive Greenland and Antarctic ice sheet model (AGISM) as well as an oceanic carbon cycle model (LOCH). Those climatic components can have a great impact on future climate. The few studies in recent literature assessing the impact of polar ice sheets on future climate draw very different conclusions, which shows the need for developing such a model. A set of numerical experiments have been performed in order to study the possible perturbations of climate induced by human activities over the next millennia. A particular attention is given to the Greenland ice sheet. In most of the projections, the Greenland ice sheet undergoes a continuous reduction in volume, leading to an almost total disappearance in the most pessimistic scenarios. The impact of the Greenland deglaciation on climate has therefore been assessed through a sensitivity experiment using the scenario SRES A2. The removal of the Greenland ice sheet is responsible for a regional amplification of the global warming inducing a total melt of Arctic sea ice in summer. The freshwater flux from Greenland generates large salinity anomalies in the North Atlantic Ocean that reduce the rate of North Atlantic Deep Water formation, slowing down slightly the oceanic thermohaline circulation.

North America warms more than the zonal mean, while Europe warms less than the zonal mean during the next millennia. This asymmetry can be explained by both regional and larger-scale processes. The amplification of the warming in NAM is due in winter and spring to the snow cover feedback and, in addition, to the sea ice feedback in autumn. The poleward heat transport in the Atlantic is reduced by 10%. This weakens the sst increase along European coasts in winter and therefore reduces the warming over the continent during this season.

As part of the Coupled Model Intercomparison Project, integrations with a common design have been undertaken with eleven different climate models to compare the response of the Atlantic thermohaline circulation (THC) to time-dependent climate change caused by increasing atmospheric CO2 concentration. Over 140 years, during which the CO2 concentration quadruples, the circulation strength declines gradually in all models, by between 10 and 50%. No model shows a rapid or complete collapse, despite the fairly rapid increase and high final concentration of CO2. The models having the strongest overturning in the control climate tend to show the largest THC reductions. In all models, the THC weakening is caused more by changes in surface heat flux than by changes in surface water flux. No model shows a cooling anywhere, because the greenhouse warming is dominant.

International audience; The estimate of future climate change and of its impact on the environment requires to increase our knowledge of the complex interactions between the atmosphere, the ocean, sea-ice, land surfaces and glaciers. These components are coupled through the cycles of energy and water, but also through biogeochemical cycles such as the carbon or the ozone cycles. One of the goals of the IPSL modeling community is to study how these different couplings can modulate climate and climate variability, and to determine how feedbacks in the Earth system control the response of climate to a perturbation such as the anthropogenic emissions of greenhouse gases. For this purpose, the Earth system model of the IPSL is developed as a modular suite of model components of the Earth system that can be use either as stand alone models or coupled to each other. This note presents the new features and results of the last version of the global IPSL coupled model that will be used to run the set of simulations planned for the next IPCC assessment. In particular, chapter 2 presents the model components of the coupled system, highlighting important features for the coupling or the quality of model simulations. Chapter 3 synthesise all the coupling procedures and the coupling environment, and chapter 4 discussed the major characteristics of the model climatology.

[1] Simulated sea ice thickness in the ORCA2-LIM coupled sea ice - ocean model is compared with thicknesses from the Antarctic Sea Ice Processes and Climate (ASPeCt) database. We find a qualitative agreement of the large-scale patterns of ice thickness distribution. Regional averages for the various sectors of the Southern Ocean yield a very good correspondence between observations and model data. Exceptions are the eastern Bellingshausen and northwestern Weddell Seas. A poor representation of the Antarctic Peninsula in the atmospheric forcing data and the related overestimation of westerly winds in this region lead to a spurious accumulation of sea ice on the western side of the peninsula and to an underestimation of sea ice coverage on the eastern side. Since the spatial scale of observations is not comparable to the size of a model grid cell, there is little agreement between individual observations and the corresponding model ice thicknesses. A model analysis of the seasonal and interannual variability indicates that the ASPeCt data underestimate the climatological ice thickness in the central and southern Weddell Sea and the eastern Ross Sea by up to 1 m. Because of a winter bias in the observations an overestimation of similar magnitude is expected in the Bellingshausen Sea. Ice thickness data in most of the Indo-Pacific sector appear to be representative for the long-term climatology. A model estimate of the bias is used to compute a revised distribution of climatological sea ice thickness.

[1] The climate response to a massive release of methane from gas hydrates is simulated in two 2500-year-long numerical experiments performed with a three-dimensional, global coupled atmosphere-sea ice-ocean model of intermediate complexity. Two different equilibrium states were used as reference climates; the first state with preindustrial forcing conditions and the second state with a four times higher atmospheric CO2 concentration. These climates were perturbed by prescribing a methane emission scenario equivalent to that computed for the Paleocene/Eocene thermal maximum (PETM; similar to 55.5 Ma), involving a sudden release of 1500 Gt of carbon into the atmosphere in 1000 years. In both cases, this produced rapid atmospheric warming (up to 10°C at high latitudes) and a reorganization of the global overturning ocean circulation. In the ocean, maximum warming (2-4°C) occurred at intermediate depths where methane hydrates are stored in the upper slope sediments, suggesting that further hydrate instability could result from the prescribed scenario.