The PIRATE OST/ST project

Thierry Penduff (CNRS - IGE, France)


Bernard Barnier (IGE, CNRS, France); Jean-Marc Molines (IGE, CNRS, France); Sabrina Speich (ENS-LMD, France); Guillaume Maze (LOPS, IFREMER, France); Benoit Meyssignac (LEGOS, CNES, France); Laurent Terray (CERFACS, France); Sally Close (LOPS, UBO , France)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Salient results from the 2017-2020 OSTST PIs

Presentation type: Type Forum

The main goal of the PIRATE (Probabilistic InteRpretation of Altimeter & in-siTu obsErvations) OST/ST project is to study the intrinsic vs extrinsic sources of the ocean variability observed by altimeter and in-situ. We consider real observations and the OCCIPUT ensemble of ocean/sea-ice simulations to disentangle and characterize 2 components of the oceanic variability for several variables, focusing on interannual-to-decadal timescales: the first component is the Atmospherically Forced Variability (AFV) that is directly driven by the fluctuating atmospheric forcing; the second component is the Chaotic Intrinsic Variability (CIV) that is spontaneously generated by the eddying ocean, whose phase is random, and is independent of the phase of the atmospheric variability.

In PIRATE, we have disentangled the extrinsic and intrinsic sources of ocean variability in 2 ways. [1] we have first split the total model variability into AFV and CIV from simple and classical ensemble statistics: the fluctuations of the ensemble mean were used to estimate and characterize the AFV, and the deviations of each member around this ensemble mean provided us with the CIV. [2] Then we have acknowledged that this pragmatic, usual splitting of the variability into both components is not consistent with the Dynamical Systems Theory (DST): we have thus developed more mathematically-consistent diagnostics to characterize the sea level variability, now properly seen as an ocean-driven CIV modulated by the atmospheric variability throughout the integration.

The first method allowed us to confirm that the imprint of the interannual-to-decadal CIV on many observed variables (sea level, Ocean Heat Content, Atlantic Overturning, etc) can be as strong as the imprint of the AFV in several eddy-active regions: these CIV-related random low-frequency fluctuations cannot be neglected when interpreting the global ocean simulated or observed variability. We also showed that CIV may hamper in several regions the detection and attribution of externally-driven long-term trends (i.e. anthropogenic sea level, Ocean Heat Content). We have recently shown that the second method is compatible with the first one, while providing additional views on the determinism of the ocean variability (connection with DST, access to the temporal evolution of the atmospheric contraint on the oceanic chaos, validity in non-Gaussian cases, etc).
Thierry Penduff