A probabilistic description of the forced and intrinsic oceanic variability: SSH, SST, MOC, water masses.

Thierry Penduff (CNRS - LGGE, France)

CoAuthors

Stéphanie Leroux (CNRS - LGGE, France); Guillaume Sérazin (CNRS - LGGE, CERFACS, France); Jean-Marc Molines (CNRS - LGGE, France); Laurent Bessières (CERFACS, France); Bernard Barnier (CNRS - LGGE, France); Laurent Terray (CERFACS, France)

Event: 2015 Ocean Surface Topography Science Team Meeting

Session: Science III: Large scale and global change ocean processes: the ocean's role in climate

Presentation type: Type Poster

Laminar Ocean General Circulation Models (2° to 1° resolution) used in recent climate projections are being progressively replaced by turbulent ocean models (about 1/4° resolution) in the perspective of the next CMIP exercise. Atmospherically-forced ocean simulations show that this resolution increase improves the physical consistency of simulations, but also allows the ocean to spontaneously generate a large variability up to multi-decadal timescales. Consistently with idealized studies, this low-frequency intrinsic variability (LFIV) is negligible in the laminar regime.

This oceanic LFIV has a stochastic character, a marked signature on sea-level variability and on the upper ocean temperatures in mid-latitude regions where air-sea heat fluxes are maximum in Nature. Global eddying ocean simulations analyzed in the framework of the current CHAOCEAN OST/ST project exhibit the strong, small- and large-scale imprints of this stochastic LFIV on several climate-relevant oceanic indices: sea-surface height (SSH) and temperature (SST) in western boundary current systems and the Antarctic Circumpolar Current, water masses at various depths, Meridional Overturning Circulation (MOC), etc.

How these low-frequency intrinsic variability modes are impacted, and may be paced, by the interannually-varying atmosphere is an important question for attributing and interpreting the observed low-frequency variability and long-term trends. The ongoing OCCIPUT ANR/PRACE project aims at investigating these questions probabilistically through a 50-member ensemble of 1/4° global ocean/sea-ice 57-year hindcasts, driven by the same 1958-present atmospheric forcing. Present results demonstrate that initial state perturbations spontaneously grow, cascade toward long space and time scales, and non-linearly saturate. The resulting ensemble spread is then modulated by the atmospheric forcing and oceanic nonlinearities ; it describes the atmospherically-paced stochastic LFIV (uncertainty), with marked imprints on oceanic variables at large space and time scales both at the surface (SST, SSH) and below (AMOC, mode/intermediate/deep water mass properties and depths, etc).

This ensemble experiment provides the first probabilistic description of the ocean state and evolution over the last decades, and a measure of the actual constraint exerted by the atmosphere on low-frequency ocean variability. Besides SSH, the imprint of this stochastic LFIV on upper-ocean thermal fields and AMOC will then provide insights into how this eddy-driven low-frequency oceanic “noise” might ultimately impact the atmosphere and climate predictability in future coupled climate projections.
 
A probabilistic description of the forced and intrinsic oceanic variability: SSH, SST, MOC, water masses.

Poster show times:

RoomStart DateEnd Date
Grand Ballroom Foyer Thu, Oct 22 2015,11:00 Thu, Oct 22 2015,18:00
Thierry Penduff
CNRS - LGGE
France
Thierry.Penduff@legi.grenoble-inp.fr