The origin of the "European Medieval Warm Period"

Proxy records and results of a three dimensional climate model show that European summer temperatures roughly a millennium ago were comparable to those of the last 25 years of the 20th century, supporting the existence of a summer "Medieval Warm Period" in Europe. Those two relatively mild periods were separated by a rather cold era, often referred to as the "Little Ice Age". Our modelling results suggest that the warm summer conditions during the early second millennium compared to the climate background state of the 13th–18th century are due to a large extent to the long term cooling induced by changes in land-use in Europe. During the last 200 years, the effect of increasing greenhouse gas concentrations, which was partly levelled off by that of sulphate aerosols, has dominated the climate history over Europe in summer. This induces a clear warming during the last 200 years, allowing summer temperature during the last 25 years to reach back the values simulated for the early second millennium. Volcanic and solar forcing plays a weaker role in this comparison between the last 25 years of the 20th century and the early second millennium. Our hypothesis appears consistent with proxy records but modelling results have to be weighted against the existing uncertainties in the external forcing factors, in particular related to land-use changes, and against the uncertainty of the regional climate sensitivity. Evidence for winter is more equivocal than for summer. The forced response in the model displays a clear temperature maximum at the end of the 20th century. However, the uncertainties are too large to state that this period is the warmest of the past millennium in Europe during winter

The Impact of Wind Stress Feedback on the Stability of the Atlantic Meridional Overturning Circulation

Recent results based on models using prescribed surface wind stress forcing have suggested that the net freshwater transport Ó by the Atlantic meridional overturning circulation (MOC) into the Atlantic basin is a good indicator of the multiple-equilibria regime. By means of a coupled climate model of intermediate complexity, this study shows that this scalar Ó cannot capture the connection between the properties of the steady state and the impact of the wind stress feedback on the evolution of perturbations. This implies that, when interpreting the observed value of Ó, the position of the present-day climate is systematically biased toward the multiple-equilibria regime. The results show, however, that the stabilizing influence of the wind stress feedback on the MOC is restricted to a narrow window of freshwater fluxes, located in the vicinity of the state characterized by a zero freshwater flux divergence over the Atlantic basin. If the position of the present-day climate is farther away from this state, then wind stress feedbacks are unable to exert a persistent effect on the modern MOC. This is because the stabilizing influence of the shallow reverse cell situated south of the equator during the off state rapidly dominates over the destabilizing influence of the wind stress feedback when the freshwater forcing gets stronger. Under glacial climate conditions by contrast, a weaker sensitivity with an opposite effect is found. This is ultimately due to the relatively large sea ice extent of the glacial climate, which implies that, during the off state, the horizontal redistribution of fresh waters by the subpolar gyre does not favor the development of a thermally direct MOC as opposed to the modern case

Sea ice evolution over the 20th and 21st centuriesas simulated by current AOGCMs

Outputs from simulations performed with current atmosphere-ocean general circulation models for the Fourth Assess- ment Report of Intergovernmental Panel on Climate Change (IPCC AR4) are used to investigate the evolution of sea ice over the 20th and 21st centuries. We first use the results from the ‘‘Climate of the 20th Century Experiment’’ to assess the ability of these models to reproduce the observed sea ice cover changes over the periods 1981–2000 and 1951–2000. The projected sea ice changes over the 21st century in response to the IPCC Special Report on Emission Scenarios A1B are then examined. Overall, there is a large uncertainty in simulating the present-day sea ice coverage and thickness and in predict- ing sea ice changes in both hemispheres. Over the period 1981–2000, we find that the multimodel average sea ice extent agrees reasonably well with observations in both hemipsheres despite the wide differences between the models. The largest uncertainties appear in the Southern Hemisphere. The climate change projections over the 21st century reveal that the annual mean sea ice extent decreases at similar rates in both hemispheres, and that the reduction in annual mean sea ice volume is about twice that of sea ice extent reduction in the Northern Hemisphere, in agreement with earlier studies. We show that the amplitude of the seasonal cycle of sea ice extent increases in both hemispheres in a warming climate, with a larger magnitude in the Northern Hemisphere. Furthermore, it appears that the seasonal cycle of ice extent is more affected than the one of ice volume. By the end of the 21st century, half of the model population displays an ice-free Arctic Ocean in late summer

Abrupt millennial variability and interdecadal-interstadial oscillations in a global coupled model: sensitivity to the background climate state

The origin and bifurcation structure of abrupt millennial-scale climate transitions under steady external solar forcing and in the absence of atmospheric synoptic variability is studied by means of a global coupled model of intermediate complexity. We show that the origin of Dansgaard-Oeschger type oscillations in the model is caused by the weaker northward oceanic heat transport in the Atlantic basin. This is in agreement with previous studies realized with much simpler models, based on highly idealized geometries and simplified physics. The existence of abrupt millennial-scale climate transitions during glacial times can therefore be interpreted as a consequence of the weakening of the negative temperature-advection feedback. This is confirmed through a series of numerical experiments designed to explore the sensitivity of the bifurcation structure of the Atlantic meridional overturning circulation to increased atmospheric CO2 levels under glacial boundary conditions. Contrasting with the cold, stadial, phases of millennial oscillations, we also show the emergence of strong interdecadal variability in the North Atlantic sector during warm interstadials. The instability driving these interdecadal-interstadial oscillations is shown to be identical to that found in ocean-only models forced by fixed surface buoyancy fluxes, that is, a large-scale baroclinic instability developing in the vicinity of the western boundary current in the North Atlantic. Comparisons with modern observations further suggest a physical mechanism similar to that driving the 30–40 years time scale associated with the Atlantic multidecadal oscillation

The different nature of the interdecadal variability of the thermohaline circulation under mixed and flux boundary conditions

The differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse-resolution ocean model in an idealized flat-bottom single-hemisphere basin. Objective features are determined that allow one type of oscillation to be distinguished versus the other. First, by performing a linear stability analysis of the steady state obtained under restoring boundary conditions, it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises, respectively, from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state. Based on the budgets of density variance, it is shown next that the two types of oscillations have different energy sources: Under the constant-flux boundary condition (the thermal mode), the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability, whereas under mixed boundary conditions (the salinity mode), a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation. Furthermore, the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance. In addition, the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux, while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure. The authors propose that these differences will allow one to identify which type of oscillation, if any, is at play in the more exhaustive climate models

Régions polaires, cryosphère, circulation thermohaline

The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report recently appeared. It proposes an updated assessment of the scientific, technical and socio-economic information relevant for the understanding of climate change, its potential impacts and options for adaptation and mitigation. It is based on numerical simulations of preindustrial, 20th and 21st centuries climates, performed in a coordinated way by modelling groups world-wide. This paper presents key results obtained by the French modelling groups by analysing the IPCC simulations in the framework of the French national project Escrime. More specifically, conclusions about how the cryosphere, the thermohaline circulation and the climate of polar regions could be affected during the 21st century by the ongoing global warming are given.Le 4e rapport du Groupe intergouvernemental d'experts sur l'évolution du climat (Giec), récemment paru, propose une évaluation remise à jour des éléments scientifiques, techniques et socio-économiques permettant une meilleure compréhension du changement climatique, de ses impacts potentiels et des possibilités d'adaptation et de mitigation. Il s'appuie sur des simulations numériques des climats préindustriels du XXe et du XXIe siècles, réalisées de manière concertée par les principaux groupes de recherche sur le climat de par le monde. Nous proposons ici une synthèse du travail réalisé sur ces simulations par la communauté de recherche française dans le cadre du projet national Escrime, en montrant en particulier comment, selon ces études, la cryosphère, la circulation thermohaline et le climat des régions polaires pourraient évoluer au cours du XXIe siècle dans le cadre du réchauffement climatique