OceaniC Chaos - ImPacts, strUcture, predicTability – OCCIPUT

The PRACE allocation of 16 million computing hours allowed us to perform an ensemble of 50 global ocean/sea-ice turbulent simulations (1/4° resolution), perturbed initially then driven by the same atmospheric variability between 1960 and 2015. The NEMO ocean model has been adapted for (i) the simultaneous integration of the members on the CURIE supercomputer, (ii) the generation of physically-consistent perturbations, (iii) the monitoring of the ensemble run during integration, (iv) the production of new physical and statistical outputs for ongoing analyses. Once the ensemble dispersion has converged, we have derived the atmospherically-forced variability from the time-dependent ensemble mean, and the chaotic variability from the time-dependent ensemble dispersion. Advanced statistical techniques allow us to probabilistically describe the full ocean variability, compare it with satellite and in-situ observations, and extract the space-time structure of both variability components.

The OCCIPUT global ocean ensemble simulation (50 members, 1960-2015) reveals the complex spatio-temporal structure of the low-frequency chaotic ocean variability, its dominance over the atmospherically-forced variability in turbulent regions, its marked imprint on major oceanic climate indices, and the strength of this probabilistic approach for the interpretation of observations. Partnerships are engaged in France and abroad for the analysis of this pioneering simulation.

The NEMO model was adapted and calibrated for ensemble simulations. Three (two 20-year regional, one 56-year global) simulations of this kind have been performed on the CURIE supercomputer and are currently being analyzed. Our present results confirm the strength of the ensemblist approach in oceanography, which opens new perspectives for the study of physical processes, of the atmospheric constraint on the oceanic chaos, and for the interpretation of observations.

peer-reviewed papers (3 published, 3 submitted/in preparation), 4 invited talks, 31 communications, 2 newsletters
4 scientific meetings + 1 HDR largely focused on OCCIPUT + 1 PhD + 2 Master theses on this theme.
Outreach : OSUG Science Museum, LGGE website (http://lgge.osug.fr/article998.html)

The OCCIPUT project aims at separating and characterizing, through an ensemble of realistic, eddying ocean simulations, the intrinsic and atmospherically-forced components of the global ocean variability at low frequency (1-10 years). This self-sustained intrinsic component is unrealistically weak when mesoscale eddies are not explicitly simulated, especially in the laminar ocean models presently used in IPCC-class climate prediction systems. However, this intrinsic component contributes substantially to the low-frequency variability simulated by « eddy-permitting » ocean models, such as those being currently implemented into the coming generation of operational climate prediction systems.

This intrinsic component is poorly known in the global ocean, despite [i] its recently acknowledged important contribution to the oceanic variability; [ii] its chaotic character (hence possibly questioning our interpretation of low-frequency oceanic variability, which is still largely based on deterministic concepts); [iii] its expected interactions with the atmospherically-forced variability (on which most studies are focused); and [iv] its possible impacts on the atmosphere, and on climate variability in the upcoming generation of coupled models (with eddying oceans). One barely knows, in particular, the spatial and temporal structure of the intrinsic variability, its chaotic character, its precise footprint upon the surface and subsurface ocean (and on observations), and its weight on the variance of the main oceanic climate indices.

Two research teams (MEOM-LGGE and GLOBC-SUC CERFACS/CNRS) propose to address these questions. We first propose to analyze 4 global eddying ocean simulations at ¼° and 1/12° resolution, and then to extend our results by carefully designing, performing and analyzing an innovative 30/50-member ensemble of 50-year global ocean simulations at ¼°. This numerical experiment and planned analyses will be the first of their kind, hence challenging. They are however mandatory to robustly separate the deterministic (atmospherically-forced) and chaotic (intrinsic) parts of the low-frequency ocean variability, to study their spatio-temporal structures and possible interactions. Our results about the imprints of the intrinsic component on the upper ocean thermal variability (and turbulent air-sea fluxes) will also be key to anticipate and eventually interpret future coupled climate simulations with eddying oceans. The ensemble experiment and ensemble statistics will be made available to the scientific community to foster collaborative investigations.

OCCIPUT will put an effort on outreach in collaboration with the OSUG Communication Department, in line with institutionally planned educational projects. A 30/45-minute multimedia show will be prepared on the basis of our simulations: model-based animations illustrating multi-scale oceanic features and their chaotic variability will be broadcast on a spherical projection device in which we propose to invest. This presentation will first settle in the showroom of the OSUG building dedicated to scientific outreach. It will be also itinerant and will be proposed to scientific venues in France.