Explaining Extreme Events of 2012 from a Climate Perspective

Attribution of extreme events is a challenging science and one that is currently undergoing considerable evolution. In this paper are 19 analyses by 18 different research groups, often using quite different methodologies, of 12 extreme events that occurred in 2012. In addition to investigating the causes of these extreme events, the multiple analyses of four of the events, the high temperatures in the United States, the record low levels of Arctic sea ice, and the heavy rain in northern Europe and eastern Australia, provide an opportunity to compare and contrast the strengths and weaknesses of the various methodologies. The differences also provide insights into the structural uncertainty of event attribution, that is, the uncertainty that arises directly from the differences in analysis methodology. In these cases, there was considerable agreement between the different assessments of the same event. However, different events had very different causes. Approximately half the analyses found some evidence that anthropogenically caused climate change was a contributing factor to the extreme event examined, though the effects of natural fluctuations of weather and climate on the evolution of many of the extreme events played key roles as well.Peer Reviewe

A multi-model CMIP6-PMIP4 study of Arctic sea ice at 127 ka: sea ice data compilation and model differences

The Last Interglacial period (LIG) is a period with increased summer insolation at high northern latitudes, which results in strong changes in the terrestrial and marine cryosphere. Understanding the mechanisms for this response via climate modelling and comparing the models' representation of climate reconstructions is one of the objectives set up by the Paleoclimate Modelling Intercomparison Project for its contribution to the sixth phase of the Coupled Model Intercomparison Project. Here we analyse the results from 16 climate models in terms of Arctic sea ice. The multi-model mean reduction in minimum sea ice area from the pre industrial period (PI) to the LIG reaches 50 % (multi-model mean LIG area is 3.20×106 km2, compared to 6.46×106 km2 for the PI). On the other hand, there is little change for the maximum sea ice area (which is 15–16×106 km2 for both the PI and the LIG. To evaluate the model results we synthesise LIG sea ice data from marine cores collected in the Arctic Ocean, Nordic Seas and northern North Atlantic. The reconstructions for the northern North Atlantic show year-round ice-free conditions, and most models yield results in agreement with these reconstructions. Model–data disagreement appear for the sites in the Nordic Seas close to Greenland and at the edge of the Arctic Ocean. The northernmost site with good chronology, for which a sea ice concentration larger than 75 % is reconstructed even in summer, discriminates those models which simulate too little sea ice. However, the remaining models appear to simulate too much sea ice over the two sites south of the northernmost one, for which the reconstructed sea ice cover is seasonal. Hence models either underestimate or overestimate sea ice cover for the LIG, and their bias does not appear to be related to their bias for the pre-industrial period. Drivers for the inter-model differences are different phasing of the up and down short-wave anomalies over the Arctic Ocean, which are associated with differences in model albedo; possible cloud property differences, in terms of optical depth; and LIG ocean circulation changes which occur for some, but not all, LIG simulations. Finally, we note that inter-comparisons between the LIG simulations and simulations for future climate with moderate (1 % yr−1) CO2 increase show a relationship between LIG sea ice and sea ice simulated under CO2 increase around the years of doubling CO2. The LIG may therefore yield insight into likely 21st century Arctic sea ice changes using these LIG simulations

Retrait de la banquise arctique : retour sur les dernières décennies

La fonte des glaces en général et de la banquise arctique en particulier est probablement l'une des images les plus concrètes du réchauffement climatique en cours. La surface couverte de glace en Arctique est en effet une grandeur mesurée en temps réel, plus simple à appréhender, notamment pour le grand public, que des changements à long terme de température ou de précipitations. Cependant, l'interprétation scientifique de ce retrait de la banquise reste délicate. Cet article propose de faire le point sur les enseignements à tirer des observations de la glace de mer par satellite et des progrès de la modélisation.The melting of the world's ice and snow, including Arctic sea ice is probably one of the most striking pictures of ongoing climate change. Arctic sea ice cover is now observed in real-time by satellite, and its changes in time are probably more visible to the general public than long term temperature or precipitation changes. However, the interpretation of the current retreat of Arctic sea ice is not straightforward. This article reviews progress in the scientific understanding of recent trends in sea ice due to recent observations and breakthroughs in sea ice modelling

Développement et validation d'un modèle couplé océan-glace de mer pour l'étude du climat des hautes latitudes

The goal of this work is to set up a coupled ocean-sea ice system to improve the simulation of climate in high latitudesL'objectif de la thèse consiste à mettre en place un système couplé océan-glace de mer pour simuler le climat dans les hautes latitude

Simulation of the Atlantic meridional overturning circulation in an atmosphere-ocean global coupled model. Part II: weakening in a climate change experiment: a feedback mechanism. Climate Dynamics.

International audienceMost state-of-the art global coupled models simulate a weakening of the Atlantic Meridional Overturning Circulation (MOC) in climate change scenarios but the mechanisms leading to this weakening are still being debated. The third version of the CNRM (Centre National de Recherches Météorologiques) global atmosphere-ocean-sea ice coupled model (CNRM-CM3) was used to conduct climate change experiments for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4). The analysis of the A1B scenario experiment shows that global warming leads to a slowdown of North Atlantic deep ocean convection and thermohaline circulation south of Iceland. This slowdown is triggered by a freshening of the Arctic Ocean and an increase in freshwater outflow through Fram Strait. Sea ice melting in the Barents Sea induces a local amplification of the surface warming, which enhances the cyclonic atmospheric circulation around Spitzberg. This anti-clockwise circulation forces an increase in Fram Strait outflow and a simultaneous increase in ocean transport of warm waters toward the Barents Sea, favouring further sea ice melting and surface warming in the Barents Sea. Additionally, the retreat of sea ice allows more deep water formation north of Iceland and the thermohaline circulation strengthens there. The transport of warm and saline waters toward the Barents Sea is further enhanced, which constitutes a second positive feedback

DEVELOPPEMENT ET VALIDATION D'UN MODELE COUPLE OCEAN-GLACE DE MER POUR L'ETUDE DU CLIMAT DES HAUTES LATITUDES

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Quel est le rôle de la cryosphère dans l'équilibre climatique et sa modélisation ?

International audienceLe programme ARGO a révolutionné l'observation de l'océan. Depuis les années 2000, l'océan mondial est parsemé de flotteurs, des tubes d'acier d'environ 1,5 m de long qui dérivent à 1000 m de profondeur pendant 10 jours, puis remontent à la surface, à la manière de sous marins miniatures, pour envoyer par satellite les observations de température et de salinité de l'océan qu'ils ont collectées le long de leur chemin, avant de replonger et recommencer un cycle. Le réseau de 3500 flotteurs, maintenu en partenariat par 30 nations, permet de suivre en temps réel le contenu de chaleur et la salinité de l'océan mondial

Simulation of the Atlantic meridional overturning circulation in an atmosphere-ocean global coupled model. Part I: a mechanism governing the variability of ocean convection in a preindustrial experiment.

International audienceThe thermohaline circulation (THC) is a large scale oceanic circulation driven by density gradients. Its Atlantic component is responsible for a significant part of the northward heat transport of the climate system (Broecker, 1991; Lavin et al., 2003), roughly 1PW over the 6PW total energy transported by the entire system (Ganachaud and Wunsch, 2000, 2003; Trenberth and Caron, 2001). Although some results question the impact of THC on climate (Seager et al., 2002), a number of studies showed that its variability can cause large changes in regional surface temperatures and precipitation (Manabe and Stouffer, 1999, 2000; Dong and Sutton, 2002; Vellinga and Wood, 2002; Swingedouw et al., 2006). It is thought to interact with the main atmospheric modes of variability, namely the El Niño Southern Oscillation (ENSO, Dong and Sutton, 2002) and the North Atlantic Oscillation (NAO, Wu and Gordon, 2002). Some variations in its strength may have played an important role in paleoclimate fluctuations (Clark et al., 2002; Rahmstorf, 2002). Shaffrey and Sutton (2006) have also suggested, following Bjerknes' hypothesis (1964), that on decadal timescales, an increase in THC heat transport in the northern extratropics (20°N-70°N latitude band) is compensated by a decrease in midlatitude heat and moisture transport by the storm tracks and vice-versa. When the ocean heat transport rises, the induced decrease in the equator-to-pole temperature gradient weakens the atmospheric baroclinicity which causes a reduction in the atmospheric transient energy transport. Given this potential climatic role of the THC, there is an obvious need to better understand this component of the climate system

Rôle de la surface marine sur la variabilité intrasaisonnière estivale de l'atmosphère dans la région Nord Atlantique Europe

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Rôle de la surface marine sur la variabilité intrasaisonnière estivale de l'atmosphère dans la région Nord Atlantique Europe

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF