Juliette Mignot

Decadal climate prediction: An update from the trenches

This paper provides an update on research in the relatively new and fast-moving field of decadal climate prediction, and addresses the use of decadal climate predictions not only for potential users of such information but also for improving our understanding of processes in the climate system. External forcing influences the predictions throughout, but their contributions to predictive skill become dominant after most of the improved skill from initialization with observations vanishes after about 6–9 years. Recent multimodel results suggest that there is relatively more decadal predictive skill in the North Atlantic, western Pacific, and Indian Oceans than in other regions of the world oceans. Aspects of decadal variability of SSTs, like the mid-1970s shift in the Pacific, the mid-1990s shift in the northern North Atlantic and western Pacific, and the early-2000s hiatus, are better represented in initialized hindcasts compared to uninitialized simulations. There is evidence of higher skill in initialize...

Impact of Freshwater Release in the North Atlantic under Different Climate Conditions in an OAGCM

Abstract The response of climate to freshwater input in the North Atlantic (NA) has raised a lot of concern about the issue of climate stability since the discovery of abrupt coolings during the last glacial period. Such coolings have usually been related to a weakening of the Atlantic meridional overturning circulation (AMOC), probably associated with massive iceberg surges or meltwater pulses. Additionally, the recent increase in greenhouse gases in the atmosphere has also raised the possibility of a melting of the Greenland ice sheet, which may impact the future AMOC, and thereby the climate. In this study, the extent to which the mean climate influences the freshwater release linked to ice sheet melting in the NA and the associated climatic response is explored. For this purpose the simulations of several climatic states [last interglacial, Last Glacial Maximum, mid-Holocene, preindustrial, and future (2 × CO2)] are considered, and the climatic response to a freshwater input computed interactively acc...

Estimating Changes in Global Temperature since the Preindustrial Period

The United Nations Framework Convention on Climate Change (UNFCCC) process agreed in Paris to limit global surface temperature rise to “well below 2°C above pre-industrial levels.” But what period is preindustrial? Somewhat remarkably, this is not defined within the UNFCCC’s many agreements and protocols. Nor is it defined in the IPCC’s Fifth Assessment Report (AR5) in the evaluation of when particular temperature levels might be reached because no robust definition of the period exists. Here we discuss the important factors to consider when defining a preindustrial period, based on estimates of historical radiative forcings and the availability of climate observations. There is no perfect period, but we suggest that 1720–1800 is the most suitable choice when discussing global temperature limits. We then estimate the change in global average temperature since preindustrial using a range of approaches based on observations, radiative forcings, global climate model simulations, and proxy evidence. Our assessment is that this preindustrial period was likely 0.55°–0.80°C cooler than 1986–2005 and that 2015 was likely the first year in which global average temperature was more than 1°C above preindustrial levels. We provide some recommendations for how this assessment might be improved in the future and suggest that reframing temperature limits with a modern baseline would be inherently less uncertain and more policy relevant.

Bidecadal North Atlantic ocean circulation variability controlled by timing of volcanic eruptions

While bidecadal climate variability has been evidenced in several North Atlantic paleoclimate records, its drivers remain poorly understood. Here we show that the subset of CMIP5 historical climate simulations that produce such bidecadal variability exhibits a robust synchronization, with a maximum in Atlantic Meridional Overturning Circulation (AMOC) 15 years after the 1963 Agung eruption. The mechanisms at play involve salinity advection from the Arctic and explain the timing of Great Salinity Anomalies observed in the 1970s and the 1990s. Simulations, as well as Greenland and Iceland paleoclimate records, indicate that coherent bidecadal cycles were excited following five Agung-like volcanic eruptions of the last millennium. Climate simulations and a conceptual model reveal that destructive interference caused by the Pinatubo 1991 eruption may have damped the observed decreasing trend of the AMOC in the 2000s. Our results imply a long-lasting climatic impact and predictability following the next Agung-like eruption.

The Variability of the Atlantic Meridional Overturning Circulation, the North Atlantic Oscillation, and the El Niño-Southern Oscillation in the Bergen Climate Model

Abstract The link between the interannual to interdecadal variability of the Atlantic meridional overturning circulation (AMOC) and the atmospheric forcing is investigated using 200 yr of a control simulation of the Bergen Climate Model, where the mean circulation cell is rather realistic, as is also the location of deep convection in the northern North Atlantic. The AMOC variability has a slightly red frequency spectrum and is primarily forced by the atmosphere. The maximum value of the AMOC is mostly sensitive to the deep convection in the Irminger Sea, which it lags by about 5 yr. The latter is mostly forced by a succession of atmospheric patterns that induce anomalous northerly winds over the area. The impact of the North Atlantic Oscillation on deep convection in the Labrador and Greenland Seas is represented realistically, but its influence on the AMOC is limited to the interannual time scale and is primarily associated with wind forcing. The tropical Pacific shows a strong variability in the model,...

The Role of Northern Sea Ice Cover for the Weakening of the Thermohaline Circulation under Global Warming

Abstract An increase in atmospheric CO2 concentration and the resulting global warming are typically associated with a weakening of the thermohaline circulation (THC) in model scenarios. For the models participating in the Coupled Model Intercomparison Project (CMIP), this weakening shows a significant (r = 0.62) dependence on the initial THC strength; it is stronger for initially strong overturning. The authors propose a physical mechanism for this phenomenon based on an analysis of additional simulations with the coupled climate models CLIMBER-2 and CLIMBER-3α. The mechanism is based on the fact that sea ice cover greatly reduces heat loss from the ocean. The extent of sea ice is strongly influenced by the near-surface atmospheric temperature (SAT) in the North Atlantic but also by the strength of the THC itself, which transports heat to the convection sites. Consequently, sea ice tends to extend farther south for weaker THC. Initially larger sea ice cover responds more strongly to atmospheric warming; ...

Multiyear predictability of tropical marine productivity

Significance Phytoplankton is at the base of the marine food web. Its carbon fixation, the net primary productivity (NPP), sustains most living marine resources. In regions like the tropical Pacific (30°N–30°S), natural fluctuations of NPP have large impacts on marine ecosystems including fisheries. The capacity to predict these natural variations would provide an important asset to science-based management approaches but remains unexplored yet. In this paper, we demonstrate that natural variations of NPP in the tropical Pacific can be forecasted several years in advance beyond the physical environment, whereas those of sea surface temperature are limited to 1 y. These results open previously unidentified perspectives for the future development of science-based management techniques of marine ecosystems based on multiyear forecasts of NPP. With the emergence of decadal predictability simulations, research toward forecasting variations of the climate system now covers a large range of timescales. However, assessment of the capacity to predict natural variations of relevant biogeochemical variables like carbon fluxes, pH, or marine primary productivity remains unexplored. Among these, the net primary productivity (NPP) is of particular relevance in a forecasting perspective. Indeed, in regions like the tropical Pacific (30°N–30°S), NPP exhibits natural fluctuations at interannual to decadal timescales that have large impacts on marine ecosystems and fisheries. Here, we investigate predictions of NPP variations over the last decades (i.e., from 1997 to 2011) with an Earth system model within the tropical Pacific. Results suggest a predictive skill for NPP of 3 y, which is higher than that of sea surface temperature (1 y). We attribute the higher predictability of NPP to the poleward advection of nutrient anomalies (nitrate and iron), which sustain fluctuations in phytoplankton productivity over several years. These results open previously unidentified perspectives to the development of science-based management approaches to marine resources relying on integrated physical-biogeochemical forecasting systems.

The Role of Stratification-Dependent Mixing for the Stability of the Atlantic Overturning in a Global Climate Model*

Using the “CLIMBER-3” coupled climate model of intermediate complexity, the effect of a stratification-dependent vertical diffusivity on the sensitivity of the Atlantic Ocean meridional overturning circulation to perturbations in freshwater forcing is investigated. The vertical diffusivity is calculated as N, where N is the local buoyancy frequency and the parameter is a measure of the sensitivity of the vertical diffusivity to changes in stratification. Independent of , the stratification of the deep ocean is weakly increased as a response to an anomalous freshwater flux in the North Atlantic in these experiments. In the region of freshwater forcing and north of it this is caused by the freshwater anomaly itself, but farther south it is associated with anomalously warm surface waters caused by a reduction of the northward oceanic heat transport. Subsequently, and in opposition to results from previous studies, the overturning is reduced by the anomalous freshwater flux, independent of the choice of . However, the amount of reduction in overturning following a freshwater perturbation is found to depend critically on the choice of the mixing sensitivity .I f cr, the response is similar to the model’s response using constant vertical diffusivity ( 0). For cr, a sharp increase of the sensitivity is found. The value of cr is found to be between 0.5 and 1. A general feedback is proposed explaining this threshold behavior: if is large, both positive and negative perturbations of stratification are amplified by associated changes in diffusivity. In the experiments presented here, this enhances the initial positive stratification anomaly in northern high latitudes, which is created by the anomalous freshwater flux. As a result, convection is strongly reduced, and the overturning is significantly weakened.

Presentation and analysis of the IPSL and CNRM climate models used in CMIP5

After several years of development, the French IPSL and CNRM-CERFACS modelling groups are presenting to the international scientific community the climate simulations they have prepared as part of the 5th Phase of the Coupled Model Intercomparison Project (CMIP5). The following set of manuscripts provides a detailed description and evaluation of the coupled models and of their individual components. It also presents an analysis of the simulations in terms of internal climate variability, climate response to natural and anthropogenic forcings, physical, chemical and biogeochemical feedbacks, and regional studies. This work benefitted tremendously from the national INSU/LEFE ‘‘MissTerre’’ project (P.I. P. Braconnot and S. Planton) which aims at strengthening collaborations among the French research teams involved in climate modeling. Several workshops and substantial data and expertise exchanges has been possible thanks to this project. This joint Special Issue analyzing simulations from the two models is one of the main results of this collaboration. This work was also supported by other national and international projects cited in the relevant articles. This work was made possible thanks to the High-Performance Computing resources of CCRT and IDRIS made available by GENCI (Grand Equipement National de Calcul Intensif), CEA (Commissariat a l’Energie Atomique et aux Energies Alternatives) and CNRS (Centre National de la Recherche Scientifique) and thanks to Meteo-France computing facility. More information about the IPSL-CM5 and CNRMCM5 climate models can be found at: http://icmc.ipsl.fr/ and at http://www.cnrm.meteo.fr/cmip5/, respectively. We are most grateful to the many people from the IPSL Climate Modelling Centre, CNRM and CERFACS who made possible the production, the preparation and the distribution of the IPSL-CM5 and CNRM-CM5 model results. This special collection was coordinated by Juliette Mignot and Sandrine Bony

Links between the Southern Annular Mode and the Atlantic Meridional Overturning Circulation in a Climate Model

Abstract The links between the atmospheric southern annular mode (SAM), the Southern Ocean, and the Atlantic meridional overturning circulation (AMOC) at interannual to multidecadal time scales are investigated in a 500-yr control integration of the L’Institut Pierre-Simon Laplace Coupled Model, version 4 (IPSL CM4) climate model. The Antarctic Circumpolar Current, as described by its transport through the Drake Passage, is well correlated with the SAM at the yearly time scale, reflecting that an intensification of the westerlies south of 45°S leads to its acceleration. Also in phase with a positive SAM, the global meridional overturning circulation is modified in the Southern Hemisphere, primarily reflecting a forced barotropic response. In the model, the AMOC and the SAM are linked at several time scales. An intensification of the AMOC lags a positive SAM by about 8 yr. This is due to a correlation between the SAM and the atmospheric circulation in the northern North Atlantic that reflects a symmetric E...