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Ocean Surface Topography from Space
CHAOCEAN (low-frequency intrinsic variability in the eddying ocean : observations, simulations and processes)


Thierry Penduff - (LGGE)

  Laurent Terray
Bernard Barnier
Sandy Gregorio
Jean-Marc Molines
Guillaume Serazin
Thierry Huck

  Pavel Berloff
Joël Hirschi
Henk Dijkstra
(Imperial College, UK)
(Utrecht Univ, NL)


CHAOCEAN (low-frequency intrinsic variability in the eddying ocean : observations, simulations and processes)
Contribution (in %) of intrinsic/chaotic ocean processes in the interannual variance of Seal Level Anomaly (a) and SST (b), as estimated from global eddying ocean simulation
The advent of satellites has led to an explosion of ocean observations at unprecedented resolution and has illuminated the rich spatial and temporal character of upper ocean variability. There has also been recent, concomitant growth in subsurface observations due to the Argo and Rapid arrays and other major observational programs. Ocean General Circulation Models (OGCMs, i.e. primitive equation models implemented in realistic settings and forced by comprehensive atmospheric products) have contributed importantly to the synthesis and interpretation of this information. In turn, the observations show that realistic models have grown in skill. Dynamical understanding of ocean variability has also grown over the last few decades through a number of insightful process-oriented analyses. Accurate model-based climate projections constitute a significant societal need, and the above elements naturally benefit this goal.

The aim of this proposal is to exploit the above advances with a view to low-frequency ocean variability, motivated by evidence that both external (atmospheric) and intrinsic sources contribute importantly to the ocean variability observed by altimetry and other means, that the latter is poorly known (and is indeed absent from current climate models), and that analyses and projections of climate variability are likely sensitive to it. The foundations for these statements will be outlined below, where we argue that distinguishing the internal (intrinsic) and external (atmospherically-forced) components of the ocean variability is central to understanding and using observational (altimeter and other) timeseries for ocean/climate science: climate monitoring, climate reconstruction/projection, and detection of climate change in the ocean.

The improvement of OGCMs is evidenced in the convergence of model statistics towards observations, in particular sea level variability (Penduff et al., 2010). In order to analyze the features and origin of the observed signals, we propose to simulate and extract the low-frequency intrinsic ocean variability from ensemble-like OGCM simulations, and study its dynamics using dynamical concepts developed in process-oriented settings. The objectives of this French project are to

  1. categorize interannual-to-decadal ocean variability according to its forced/intrinsic origin,
  2. quantify the imprint of both contributions in observational datasets,
  3. explore the dynamics of this variability, and
  4. assess the sensitivity of these results to model construction.

Accordingly, this proposal, along with its American counterpart that was just submitted to the partnered NASA/ROSES OST-ST program, gathers French, European and American scientists/engineers who work on the ocean variability from complementary perspectives: namely 'realistic ' ocean modelers, making routine use of altimeter/satellite/in-situ observations, specialists in non-linear dynamics and dynamical systems theory with insights into the foundations of ocean variability, and experts in statistical analysis of large datasets applied to climate. Our international group will produce, share and analyze an ensemble of observed/simulated fields and will take advantage of the complementarity between their own approaches to address several scientific questions with operational relevance (ocean/climate observation, monitoring, reconstruction, modeling) on climatic timescales. The PIs will coordinate the collaboration between French and American members, who will meet at least once every year at OSTST meetings.

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