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Christophe Cassou

Bio: Christophe Cassou is an academic researcher from University of Toulouse. The author has contributed to research in topics: Climate model & Teleconnection. The author has an hindex of 10, co-authored 24 publications receiving 736 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors describe the main characteristics of CNRM-CM6-1, the fully coupled atmosphere-ocean general circulation model of sixth generation jointly developed by Centre National de Recherches Meteorologiques (CNRM) and Cerfacs for the sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6).
Abstract: This paper describes the main characteristics of CNRM-CM6-1, the fully coupled atmosphere-ocean general circulation model of sixth generation jointly developed by Centre National de Recherches Meteorologiques (CNRM) and Cerfacs for the sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6). The paper provides a description of each component of CNRM-CM6-1, including the coupling method and the new online output software. We emphasize where model's components have been updated with respect to the former model version, CNRM-CM5.1. In particular, we highlight major improvements in the representation of atmospheric and land processes. A particular attention has also been devoted to mass and energy conservation in the simulated climate system to limit long-term drifts. The climate simulated by CNRM-CM6-1 is then evaluated using CMIP6 historical and Diagnostic, Evaluation and Characterization of Klima (DECK) experiments in comparison with CMIP5 CNRM-CM5.1 equivalent experiments. Overall, the mean surface biases are of similar magnitude but with different spatial patterns. Deep ocean biases are generally reduced, whereas sea ice is too thin in the Arctic. Although the simulated climate variability remains roughly consistent with CNRM-CM5.1, its sensitivity to rising CO 2 has increased: the equilibrium climate sensitivity is 4.9 K, which is now close to the upper bound of the range estimated from CMIP5 models.

441 citations

Journal ArticleDOI
TL;DR: It is shown that an El Niño tends to peak during the year following large eruptions in simulations of the Fifth Coupled Model Intercomparison Project (CMIP5), and that a Pinatubo-like eruption cools tropical Africa and drives westerly wind anomalies in the Pacific favouring anEl Niño response.
Abstract: Stratospheric aerosols from large tropical explosive volcanic eruptions backscatter shortwave radiation and reduce the global mean surface temperature. Observations suggest that they also favour an El Nino within 2 years following the eruption. Modelling studies have, however, so far reached no consensus on either the sign or physical mechanism of El Nino response to volcanism. Here we show that an El Nino tends to peak during the year following large eruptions in simulations of the Fifth Coupled Model Intercomparison Project (CMIP5). Targeted climate model simulations further emphasize that Pinatubo-like eruptions tend to shorten La Ninas, lengthen El Ninos and induce anomalous warming when occurring during neutral states. Volcanically induced cooling in tropical Africa weakens the West African monsoon, and the resulting atmospheric Kelvin wave drives equatorial westerly wind anomalies over the western Pacific. This wind anomaly is further amplified by air-sea interactions in the Pacific, favouring an El Nino-like response.El Nino tends to follow 2 years after volcanic eruptions, but the physical mechanism behind this phenomenon is unclear. Here the authors use model simulations to show that a Pinatubo-like eruption cools tropical Africa and drives westerly wind anomalies in the Pacific favouring an El Nino response.

150 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used a state-of-the-art climate model to simulate the last millennium and found that modelled variations of surface temperature in the Northern Hemisphere are coherent with existing reconstructions.
Abstract: The variability of the climate during the last millennium is partly forced by changes in total solar irradiance (TSI). Nevertheless, the amplitude of these TSI changes is very small so that recent reconstruction data suggest that low frequency variations in the North Atlantic Oscillation (NAO) and in the thermohaline circulation may have amplified, in the North Atlantic sector and mostly in winter, the radiative changes due to TSI variations. In this study we use a state-of-the-art climate model to simulate the last millennium. We find that modelled variations of surface temperature in the Northern Hemisphere are coherent with existing reconstructions. Moreover, in the model, the low frequency variability of this mean hemispheric temperature is found to be correlated at 0.74 with the solar forcing for the period 1001–1860. Then, we focus on the regional climatic fingerprint of solar forcing in winter and find a significant relationship between the low frequency TSI forcing and the NAO with a time lag of more than 40 years for the response of the NAO. Such a lag is larger than the around 20-year lag suggested in other studies. We argue that this lag is due, in the model, to a northward shift of the tropical atmospheric convection in the Pacific Ocean, which is maximum more than four decades after the solar forcing increase. This shift then forces a positive NAO through an atmospheric wave connection related to the jet-stream wave guide. The shift of the tropical convection is due to the persistence of anomalous warm SST forcing the anomalous precipitation, associated with the advection of warm SST by the North Pacific subtropical gyre in a few decades. Finally, we analyse the response of the Atlantic meridional overturning circulation to solar forcing and find that the former is weakened when the latter increases. Changes in wind stress, notably due to the NAO, modify the barotropic streamfunction in the Atlantic 50 years after solar variations. This implies a wind-driven modification of the oceanic circulation in the Atlantic sector in response to changes in solar forcing, in addition to the variations of the thermohaline circulation.

116 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a review paper about Decadal Climate Variability and Predictability (DCV) and propose a unifying theme referred to as drivers of teleconnectivity as a backbone to address and structure the core DCV research challenge.
Abstract: The study of Decadal Climate Variability (DCV) and Predictability is the interdisciplinary endeavor to characterize, understand, attribute, simulate, and predict the slow, multiyear variations of climate at global (e.g., the recent slowdown of global mean temperature rise in the early 2000s) and regional (e.g., decadal modulation of hurricane activity in the Atlantic, ongoing drought in California or in the Sahel in the 1970s–80s, etc.) scales. This study remains very challenging despite decades of research, extensive progress in climate system modeling, and improvements in the availability and coverage of a wide variety of observations. Considerable obstacles in applying this knowledge to actual predictions remain.This short article is a succint review paper about DCV and predictability. Based on listed issues and priorities, it also proposes a unifying theme referred to as “drivers of teleconnectivity” as a backbone to address and structure the core DCV research challenge. This framework goes be...

90 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a record of these extreme climatic events that have occurred in the French Mediterrannean coast over the past 1500 years, based on the analysis of sediment cores from Gulf of Aigues-Mortes lagoons.

77 citations


Cited by
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01 Dec 2010
TL;DR: In this paper, spatial variations in sea surface temperature (SST) and rainfall changes over the tropics are investigated based on ensemble simulations for the first half of the twenty-first century under the greenhouse gas emission scenario A1B with coupled ocean-atmosphere general circulation models of the Geophysical Fluid Dynamics Laboratory (GFDL) and National Center for Atmospheric Research (NCAR).
Abstract: Spatial variations in sea surface temperature (SST) and rainfall changes over the tropics are investigated based on ensemble simulations for the first half of the twenty-first century under the greenhouse gas (GHG) emission scenario A1B with coupled ocean–atmosphere general circulation models of the Geophysical Fluid Dynamics Laboratory (GFDL) and National Center for Atmospheric Research (NCAR). Despite a GHG increase that is nearly uniform in space, pronounced patterns emerge in both SST and precipitation. Regional differences in SST warming can be as large as the tropical-mean warming. Specifically, the tropical Pacific warming features a conspicuous maximum along the equator and a minimum in the southeast subtropics. The former is associated with westerly wind anomalies whereas the latter is linked to intensified southeast trade winds, suggestive of wind–evaporation–SST feedback. There is a tendency for a greater warming in the northern subtropics than in the southern subtropics in accordance ...

782 citations

Journal ArticleDOI
TL;DR: In this article, the authors show that the global surface temperature response to CO2 doubling has increased substantially in the Coupled Model Intercomparison Project phase 6 (CMIP6), with values spanning 1.8-5.6k across 27 GCMs and exceeding 4.5K in 10 of them.
Abstract: 15 Equilibrium climate sensitivity, the global surface temperature response to CO2 16 doubling, has been persistently uncertain. Recent consensus places it likely within 1.517 4.5K. Global climate models (GCMs), which attempt to represent all relevant physical 18 processes, provide the most direct means of estimating climate sensitivity via CO2 qua19 drupling experiments. Here we show that the closely related effective climate sensitiv20 ity has increased substantially in Coupled Model Intercomparison Project phase 6 (CMIP6), 21 with values spanning 1.8-5.6K across 27 GCMs and exceeding 4.5K in 10 of them. This 22 (statistically insignificant) increase is primarily due to stronger positive cloud feedbacks 23 from decreasing extratropical low cloud coverage and albedo. Both of these are tied to 24 the physical representation of clouds which in CMIP6 models lead to weaker responses 25 of extratropical low cloud cover and water content to unforced variations in surface tem26 perature. Establishing the plausibility of these higher sensitivity models is imperative 27 given their implied societal ramifications. 28 Plain Language Summary 29 The severity of climate change is closely related to how much the Earth warms in 30 response to greenhouse gas increases. Here we find that the temperature response to an 31 abrupt quadrupling of atmospheric carbon dioxide has increased substantially in the lat32 est generation of global climate models. This is primarily because low cloud water con33 tent and coverage decrease more strongly with global warming, causing enhanced plan34 etary absorption of sunlight – an amplifying feedback that ultimately results in more warm35 ing. Differences in the physical representation of clouds in models drive this enhanced 36 sensitivity relative to the previous generation of models. It is crucial to establish whether 37 the latest models, which presumably represent the climate system better than their pre38 decessors, are also providing a more realistic picture of future climate warming. 39

719 citations

Journal ArticleDOI
TL;DR: The High-ResMIP (High-resolution Model Intercomparison Project) as mentioned in this paper is a multi-model approach to the systematic investigation of the impact of horizontal resolution on the simulated mean climate and its variability.
Abstract: . Robust projections and predictions of climate variability and change, particularly at regional scales, rely on the driving processes being represented with fidelity in model simulations. The role of enhanced horizontal resolution in improved process representation in all components of the climate system is of growing interest, particularly as some recent simulations suggest both the possibility of significant changes in large-scale aspects of circulation as well as improvements in small-scale processes and extremes. However, such high-resolution global simulations at climate timescales, with resolutions of at least 50 km in the atmosphere and 0.25° in the ocean, have been performed at relatively few research centres and generally without overall coordination, primarily due to their computational cost. Assessing the robustness of the response of simulated climate to model resolution requires a large multi-model ensemble using a coordinated set of experiments. The Coupled Model Intercomparison Project 6 (CMIP6) is the ideal framework within which to conduct such a study, due to the strong link to models being developed for the CMIP DECK experiments and other model intercomparison projects (MIPs). Increases in high-performance computing (HPC) resources, as well as the revised experimental design for CMIP6, now enable a detailed investigation of the impact of increased resolution up to synoptic weather scales on the simulated mean climate and its variability. The High Resolution Model Intercomparison Project (HighResMIP) presented in this paper applies, for the first time, a multi-model approach to the systematic investigation of the impact of horizontal resolution. A coordinated set of experiments has been designed to assess both a standard and an enhanced horizontal-resolution simulation in the atmosphere and ocean. The set of HighResMIP experiments is divided into three tiers consisting of atmosphere-only and coupled runs and spanning the period 1950–2050, with the possibility of extending to 2100, together with some additional targeted experiments. This paper describes the experimental set-up of HighResMIP, the analysis plan, the connection with the other CMIP6 endorsed MIPs, as well as the DECK and CMIP6 historical simulations. HighResMIP thereby focuses on one of the CMIP6 broad questions, “what are the origins and consequences of systematic model biases?”, but we also discuss how it addresses the World Climate Research Program (WCRP) grand challenges.

608 citations

Journal ArticleDOI
26 Jul 2018-Nature
TL;DR: A synopsis of the current understanding of the spatio-temporal complexity of this important climate mode and its influence on the Earth system is provided and a unifying framework that identifies the key factors for this complexity is proposed.
Abstract: El Nino events are characterized by surface warming of the tropical Pacific Ocean and weakening of equatorial trade winds that occur every few years Such conditions are accompanied by changes in atmospheric and oceanic circulation, affecting global climate, marine and terrestrial ecosystems, fisheries and human activities The alternation of warm El Nino and cold La Nina conditions, referred to as the El Nino–Southern Oscillation (ENSO), represents the strongest year-to-year fluctuation of the global climate system Here we provide a synopsis of our current understanding of the spatio-temporal complexity of this important climate mode and its influence on the Earth system

598 citations

01 Apr 2013
TL;DR: In this article, the ability of CMIP3 and CMIP5 coupled ocean-atmosphere general circulation models (CGCMs) to simulate the tropical Pacific mean state and El Nino-Southern Oscillation (ENSO) was analyzed.
Abstract: We analyse the ability of CMIP3 and CMIP5 coupled ocean–atmosphere general circulation models (CGCMs) to simulate the tropical Pacific mean state and El Nino-Southern Oscillation (ENSO). The CMIP5 multi-model ensemble displays an encouraging 30 % reduction of the pervasive cold bias in the western Pacific, but no quantum leap in ENSO performance compared to CMIP3. CMIP3 and CMIP5 can thus be considered as one large ensemble (CMIP3 + CMIP5) for multi-model ENSO analysis. The too large diversity in CMIP3 ENSO amplitude is however reduced by a factor of two in CMIP5 and the ENSO life cycle (location of surface temperature anomalies, seasonal phase locking) is modestly improved. Other fundamental ENSO characteristics such as central Pacific precipitation anomalies however remain poorly represented. The sea surface temperature (SST)-latent heat flux feedback is slightly improved in the CMIP5 ensemble but the wind-SST feedback is still underestimated by 20–50 % and the shortwave-SST feedbacks remain underestimated by a factor of two. The improvement in ENSO amplitudes might therefore result from error compensations. The ability of CMIP models to simulate the SST-shortwave feedback, a major source of erroneous ENSO in CGCMs, is further detailed. In observations, this feedback is strongly nonlinear because the real atmosphere switches from subsident (positive feedback) to convective (negative feedback) regimes under the effect of seasonal and interannual variations. Only one-third of CMIP3 + CMIP5 models reproduce this regime shift, with the other models remaining locked in one of the two regimes. The modelled shortwave feedback nonlinearity increases with ENSO amplitude and the amplitude of this feedback in the spring strongly relates with the models ability to simulate ENSO phase locking. In a final stage, a subset of metrics is proposed in order to synthesize the ability of each CMIP3 and CMIP5 models to simulate ENSO main characteristics and key atmospheric feedbacks.

571 citations