Mechanisms for climate Oscillations and Retroactions at Decadal tImesCale : Uncertainties and Sensitivity – MORDICUS

Climate variability mechanisms from interannual to multidecadal timescales: implication for climate change over the next 30 years.

Global warming over the last century, which has been mostly attributed to anthropogenic greenhouse gazes emissions, is modulated by internal modes of variability driven by oceanic processes at decadal timescale. Associated teleconnections over the continents through the atmosphere, can mask temporaly the anthropogenic climate imprints at regional scale.

Respective role of anthropogenic forcings (greenhouse gazes, aerosols, stratospheric ozone) and internal modes of variability in the observed trends over the last century.

MORDICUS aims at : <br />1. Understand the physical/dynamical coupled ocean-atmosphere mechanisms leading to internal variability at decadal timescale. Emphasis is laid on processes connecting subsurface ocean, acting as a source of long-term memory for climate, to surface temperature anomalies possibly associated with large-scale atmospheric changes when interactions occur. A special focus is devoted to Atlantic Multidecadal Variability (AMV) modes and Pacific (PDV) as well as their interconnections. <br />2. Extract, characterize the fingerprint of the various external forcings (greenhouse gazes, stratospheric ozone depletion, solar+volcanoes and aerosols) on regional climate and explore the physical/dynamical processes (direct radiative effect, ocean-atmosphere coupling etc.) by which the external forcings may contribute to the decadal spectral band of variability. A special attention is devoted to the interactions between those forcings and the PDV and AMV internal modes (phase reversal or lock etc.). <br />3. Isolate the respective role of decadal ocean modes of variability and anthropogenic factors upon atmospheric and continental changes with a special attention to the hydrological cycle (focus on droughts) and extreme events such as midlatitude storms and tropical cyclones. The overall arching goal of MORDICUS is thus to better understand and contrast the accelerated warming periods, such as in 1980-1990 or 1920-1940, to the hiatus ones like in 1960-1970 and 2000-2015. <br />

MORDICUS is a process-oriented project built on the lessons drawn from the latest CMIP5 intercomparison project at the core of the 5th IPCC report in 2013-2014. MORDICUS relies on climate models and the design of targeted and dedicated simulations carried out in a coordinated way between the partners, aiming at investigate the origins of the climate decadal variability , better interpret the observed signal over the last century or so, and provide a robust estimate of the range of outcomes for regional climate. The ultimate goal is to go beyond current decadal predictions simulations through the development of an upstream modelling strategy useful to interpret and improve them. Two global circulation models, namely IPSL and CNRM-Cerfacs, are used and intercompared as much as possible, to better evaluate the associated uncertainties.

Evidences are provided in MORDICUS for :
1. The volcanic eruptions crucial role in triggering El Nino events one year after the eruption though the trade-winds relaxation in the tropical Pacific and the slackening of the tropical Walker circulation. Changes in trade-winds are due a «mega-breeze« type of mechnism due to the differentiated response in temperature between the continents (strong cooling induced through direct radiative effect) and oceans of greater inertia, leading to a reduction of land-sea constrast, especially over the western part of the Pacific.
2. The volcanic eruptions crucial role in phasing the ~20yr intrinsic cycle of the Atlantic Meridional Overturning Circulation (AMOC). This indirect and delayed volcanic effect stands out in the observed fluctuations over the last 50 yr or so. This result helps interprete the paleoclimate record over Greenland where such a signal is also clearly present.
3. The detection of anthropogenic greenhouse gazes imprint in the observed change of the seasonal cycle of temperature and atmospheric dynamics over western Europe since 1950 (earlier onset of summertime dynamics in line with phenological indicators) and the role of the anthropogenic aerosols in mitigating the greenhouse gazes effects, especially over 1950-1970.

MORDICUS clearly shows the interaction between the internal modes of variability (El Nino, atmospheric circulation, AMOC) and the external forcings (volcanoes in particular). This evidence highlights the importance to account for potential volcanic eruptions to provide an estimate of the climate possible outcomes over the next 20 years or so. MORDICUS also clearly emphasizes the important role of the aerosols whose imprints are significant over Europe in terms of both temperature and hydrological cycle. Aerosols are shown to have acted as a strong modulator in the trends induced by the increase concentration of greenhouse gazes in the atmosphere over the last century. These resultst are crucial in the interpretation of decadal forecasts, not only for physical undestandings but also in terms of skill.

Selection of 3 publications, one per partner:
1. The influence of separate teleconnections from the Pacific and Indian Oceans on the Northern Annular Mode (2015). Fletcher and Cassou, Journal of Climate, in press.
2. Douville H., A. Voldoire,

The scientific understanding of climate change is now sufficiently clear to show that anthropogenic global warming is already upon us, with a projected rate of change that exceeds anything seen in nature in the past 10,000 years. Uncertainties remain, however, especially regarding how climate will change in the next decades, in particular at regional and local scales for which natural (both internal and external) variability is large. Furthermore, a complete understanding and attribution of 20th century climate changes is still missing. This lack of knowledge could delay needed adaptation strategies as decision makers in diverse arenas, from water managers to public health experts, need to know if the climate events they are seeing are the result of natural variability, and hence can be expected to reverse at some point, or are the result of potentially irreversible anthropogenic climate change. This epistemic uncertainty has two main causes: the first one is the poor understanding of decadal natural variability and the degree to which it modulates anthropogenic climate change at continental to regional scales. The second concerns uncertainties related to past and future emissions of radiatively important trace gases (including greenhouse gases, stratospheric ozone, and stratospheric ozone depleting substances) and pollutants affecting atmospheric aerosol composition as well as our capability to accurately simulate the response of the climate system to that altered radiative forcing.

In that context, MORDICUS has three main scientific objectives. The first one is to improve understanding of the key physical and dynamical mechanisms leading to decadal internal variability. The main focus is on the two leading decadal modes of variability, namely the Interdecadal Pacific Variability (IPV) and the Atlantic Multidecadal Variability (AMV). Research will focus on inter-basin connections and processes connecting the subsurface ocean to surface temperature anomalies, which in turn force and respond to large-scale atmospheric changes. The second goal is to extract the fingerprint of the various external forcings on regional climate and to mechanistically understand their contribution to decadal variability. A special focus will be on aerosols and interactions between all external forcings and IPV/AMV internal modes. The third goal is to quantify the respective influence of IPV/AMV internal modes and anthropogenic factors on both observed (1920-2010) atmospheric and continental changes and near-future change estimates (2010-2035). A specific focus will be put on the hydrological cycle and extreme events, namely droughts and tropical cyclones.

MORDICUS is a process-oriented project motivated by the first key lessons drawn from the analysis of simulations from the 5th Coupled Model Intercomparison Project (CMIP5). It follows the recommendations and needs for mechanistic approaches to go beyond the initial CMIP5 decadal initiative, which was primarily focused on predictability issues. MORDICUS is an ambitious modeling-oriented project proposed by a national consortium of the teams involved in CMIP5. MORDICUS is based on a suite of innovative model experiments and configurations designed to tackle the above key scientific questions that will contribute to anticipate the next CMIP6 exercise. Together with multi-model climate prediction exercises that aim to exploit the full predictive potential of forced and free climate variations, MORDICUS results will form the raw material for the decision making needed to enable societies to adapt to the climate changes of the next decades.

Project coordinator

Monsieur Christophe Cassou (Sciences de l'Univers au CERFACS) –

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


SUC - DR14 CNRS Sciences de l'Univers au CERFACS
LOCEAN - DR2 CNRS Laboratoire d'Oceanographie et du Climat: Experimentations et Approches Numeriques
LMD - UPMC Laboratoire de Meteorologie Dynamique
CNRM-GAME / DR14 CNRS Centre National de Recherches Météorologiques - Groupe d'étude de l'atmosphère météorologique

Help of the ANR 1,068,080 euros
Beginning and duration of the scientific project: December 2013 - 48 Months

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