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

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, O. Geoffroy (2015) The recent global warming hiatus: What is the role of Pacific variability? Geophys. Res. Lett., doi: 10.1002/2014GL062775
3. Swingedouw, D., P. Ortega, J. Mignot, E. Guilyardi, V. Masson-Delmotte, P. G. Butler and M. Khodri (2015). Bidecadal North Atlantic ocean circulation variability controlled by timing of volcanic eruptions. Nature Communications, 6, 6545, doi: 10.1038/ncomms75

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.