An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part III: Hydrography and fluxes

In this paper we compare the simulated Arctic Ocean in 15 global ocean–sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II). Most of these models are the ocean and sea-ice components of the coupled climate models used in the Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments. We mainly focus on the hydrography of the Arctic interior, the state of Atlantic Water layer and heat and volume transports at the gateways of the Davis Strait, the Bering Strait, the Fram Strait and the Barents Sea Opening. We found that there is a large spread in temperature in the Arctic Ocean between the models, and generally large differences compared to the observed temperature at intermediate depths. Warm bias models have a strong temperature anomaly of inflow of the Atlantic Water entering the Arctic Ocean through the Fram Strait. Another process that is not represented accurately in the CORE-II models is the formation of cold and dense water, originating on the eastern shelves. In the cold bias models, excessive cold water forms in the Barents Sea and spreads into the Arctic Ocean through the St. Anna Through. There is a large spread in the simulated mean heat and volume transports through the Fram Strait and the Barents Sea Opening. The models agree more on the decadal variability, to a large degree dictated by the common atmospheric forcing. We conclude that the CORE-II model study helps us to understand the crucial biases in the Arctic Ocean. The current coarse resolution state-of-the-art ocean models need to be improved in accurate representation of the Atlantic Water inflow into the Arctic and density currents coming from the shelves

North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part I: Mean states

Simulation characteristics from eighteen global ocean–sea-ice coupled models are presented with a focus on the mean Atlantic meridional overturning circulation (AMOC) and other related fields in the North Atlantic. These experiments use inter-annually varying atmospheric forcing data sets for the 60-year period from 1948 to 2007 and are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The protocol for conducting such CORE-II experiments is summarized. Despite using the same atmospheric forcing, the solutions show significant differences. As most models also differ from available observations, biases in the Labrador Sea region in upper-ocean potential temperature and salinity distributions, mixed layer depths, and sea-ice cover are identified as contributors to differences in AMOC. These differences in the solutions do not suggest an obvious grouping of the models based on their ocean model lineage, their vertical coordinate representations, or surface salinity restoring strengths. Thus, the solution differences among the models are attributed primarily to use of different subgrid scale parameterizations and parameter choices as well as to differences in vertical and horizontal grid resolutions in the ocean models. Use of a wide variety of sea-ice models with diverse snow and sea-ice albedo treatments also contributes to these differences. Based on the diagnostics considered, the majority of the models appear suitable for use in studies involving the North Atlantic, but some models require dedicated development effort

Large-scale features of Last Interglacial climate: results from evaluating the lig127k simulations for the Coupled Model Intercomparison Project (CMIP6)–Paleoclimate Modeling Intercomparison Project (PMIP4)

The modeling of paleoclimate, using physically based tools, is increasingly seen as a strong out-of-sample test of the models that are used for the projection of future climate changes. New to the Coupled Model Intercomparison Project (CMIP6) is the Tier 1 Last Interglacial experiment for 127 000 years ago (lig127k), designed to address the climate responses to stronger orbital forcing than the midHolocene experiment, using the same state-of-the-art models as for the future and following a common experimental protocol. Here we present a first analysis of a multi-model ensemble of 17 climate models, all of which have completed the CMIP6 DECK (Diagnostic, Evaluation and Characterization of Klima) experiments. The equilibrium climate sensitivity (ECS) of these models varies from 1.8 to 5.6 ∘C. The seasonal character of the insolation anomalies results in strong summer warming over the Northern Hemisphere continents in the lig127k ensemble as compared to the CMIP6 piControl and much-reduced minimum sea ice in the Arctic. The multi-model results indicate enhanced summer monsoonal precipitation in the Northern Hemisphere and reductions in the Southern Hemisphere. These responses are greater in the lig127k than the CMIP6 midHolocene simulations as expected from the larger insolation anomalies at 127 than 6 ka. New synthesis for surface temperature and precipitation, targeted for 127 ka, have been developed for comparison to the multi-model ensemble. The lig127k model ensemble and data reconstructions are in good agreement for summer temperature anomalies over Canada, Scandinavia, and the North Atlantic and for precipitation over the Northern Hemisphere continents. The model–data comparisons and mismatches point to further study of the sensitivity of the simulations to uncertainties in the boundary conditions and of the uncertainties and sparse coverage in current proxy reconstructions. The CMIP6–Paleoclimate Modeling Intercomparison Project (PMIP4) lig127k simulations, in combination with the proxy record, improve our confidence in future projections of monsoons, surface temperature, and Arctic sea ice, thus providing a key target for model evaluation and optimization

3. Besoins en modélisation numérique

Les modèles climatiques actuels permettent de reproduire de nombreuses caractéristiques générales du climat présent et de bien simuler le réchauffement observé depuis quelques décennies. Ils permettent également d’estimer, à un facteur deux près, l’amplitude du réchauffement global futur en réponse à un accroissement donné de la concentration des gaz à effet de serre*. Si l’incertitude sur l’amplitude de ce réchauffement paraît élevée, ce sont surtout les changements climatiques associés à ce..

5. Les scénarios climatiques futurs

Pour réaliser des simulations du climat futur, les climatologues reprennent les modèles climatiques utilisés pour les simulations historiques. En effet, ces modèles permettent de simuler correctement des climats bien plus froids (e.g. dernier âge glaciaire) ou bien plus chauds que celui dans lequel nous vivons. Evolution des facteurs anthropiques Il est extrêmement difficile de prévoir le climat futur car on ne peut pas anticiper l’évolution des forçages radiatifs liés aux éruptions volcaniqu..

7. Couplages entre les différents milieux

Rétroactions et couplages Le système climatique* est un ensemble couplé complexe au sein duquel l’océan, l’atmosphère, les surfaces et eaux continentales, la cryosphère* et la biosphère* interagissent continuellement, en échangeant notamment de l’eau, de la quantité de mouvement, de l’énergie et des composés chimiques. Ces échanges définissent de multiples couplages entre composantes du système climatique, c’est-à-dire des interactions pour lesquelles un changement d’une variable* caractérisa..

4. Les simulations historiques : xxe siècle et dernier millénaire

La température moyenne globale a augmenté de 0,74 ±0,18 °C sur la période 1906-2005. Pour distinguer l’action des forçages naturels (solaire, volcan, etc.) de celle des forçages anthropiques (GES, utilisation des sols, etc.), les observations directes du climat récent (depuis 1850 environ) apportent une réponse partielle. La modélisation climatique est un formidable recours pour comprendre les grandes tendances et/ou équilibres climatiques. Aujourd’hui, grâce à l’augmentation des capacités de..

Cyclones et changement climatique : un séminaire national

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Le niveau de la mer : variations passées, présentes et futures

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