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Predictive Understanding of the Oceans' Wind-Driven Circulation on Interdecadal Time Scales

PI:
Michael Ghil, UCLA
Co-PI
Roger Temam, Indiana U

Yizhak Feliks, IIBR, Eric Simonnet, INLN, Theodore Tachim-Medjo, FIU, Collaborators

Summary

The goal of this project is to obtain a predictive understanding of a major component of the climate system's interdecadal variability: the oceans' wind-driven circulation. To do so, we develop and apply advanced computational and statistical methods to the problems of climate variability and climate change. We concentrate at first on the wind-driven, near-surface flow in mid-latitude ocean basins.

Climatological and oceanographic setting

A large, subtropical, anticyclonic gyre and a smaller, subpolar, cyclonic gyre dominate the large-scale flow of the mid-latitude ocean basins. These two gyres are induced by the shear in the winds that cross the respective ocean basins. They share the eastward extension of western boundary currents, such as the Gulf Stream or Kuroshio. The boundary currents and eastward jets carry large amounts of mass, heat, momentum and trace substances. They substantially affect the surface temperatures and precipitation patterns over the adjacent landmasses.

To understand the oceanic circulation's interdecadal variability, it is essential to (i) carry out long integrations of high-resolution, realistic ocean models; and (ii) obtain a complete picture of how the oceanic flows' behavior changes as various climate parameters change. So far the low-frequency variability of the double-gyre circulation has been studied mostly in simple-to-intermediate models by using the analytical and numerical methods of dynamical systems theory. Two types of oscillatory instabilities present in these models have periods of a few months and a few years, respectively. Numerical evidence points to solutions that switch between more or less vigorous circulations and exhibit interdecadal variability. We aim to understand and predict as much of this variability as possible.

Dynamical aspects

Major progress has been made this year in understanding the origin of the wind-driven circulation’s interdecadal variability. We have used a hierarchy of simple and intermediate, high-resolution models (15-km [3,4] and 10-km resolution [5]) to explore symmetry-breaking bifurcations, from steady to periodic and aperiodic flows, as wind stress increases or dissipation decreases. The physical mechanisms and symmetry properties of the previously found pitchfork and Hopf bifurcations, with periods of a few months and a few years, have been clarified further. Key results have been obtained on the connection between these two local bifurcations, due to linear instabilities with given spatial pattern, and the subsequent global, homoclinic bifurcations. The latter lead to orbits that connect high- and low-energy flow regimes and exhibit interdecadal variability, along with great irregularity, even in the limit of very small lateral viscosity and at the eddy-resolving grid sizes we used.

The modeling results are compared with decade-long in situ and more recent, satellite observations of three ocean basins, the North and South Atlantic, and the North Pacific, using advanced spectral methods [2]. The significance of these interannual and interdecadal oscillations for mid-latitude climate variability is under investigation. In particular, we are studying the effect of the oceanic fronts associated with the eastward jets on atmospheric low-frequency variability [1].

Numerical aspects

The numerical study of these problems stretches the capabilities of today's most powerful computing devices. We address this problem in three complementary ways, by developing: (i) novel, highly efficient and accurate numerical methods for the fluid-flow equations; (ii) numerical continuation packages designed to explore systematically parameter dependence for very large fluid dynamics problems; and (iii) advanced statistical tools that can be used for the systematic comparison of interdecadal variability across a full hierarchy of models [2], up to and including ocean and coupled GCMs, as well as climatic data sets.

Numerical simulations of the primitive equations (PEs) for the double-gyre problem have been conducted. The PEs provide the next tier in a model hierarchy that starts with the quasi-geostrophic (QG) and shallow-water (SW) models, and are widely used in ocean GCMs. We have shown that, with our novel barotropic–baroclinic decomposition of the flow fields, the PEs are numerically manageable with a reasonable amount of computing power. Appropriate numerical methods are developed as part of the project and the preliminary results are in good qualitative agreement with those previously obtained with the QG and SW models [6, 7].

This research is being carried out in close collaboration between the climate dynamics group at UCLA and the numerical mathematics group at Indiana University. The methods will be tested first on small and intermediate models at Indiana and UCLA. The knowledge so obtained will then be applied to full-blown SciDAC models and used to improve the latter.

References

1. Feliks, Y., M. Ghil, and E. Simonnet, 2003: Low-frequency variability in the mid-latitude atmosphere induced by an oceanic thermal front, J. Atmos. Sci., sub judice.
2. Ghil, M., M. R. Allen, M. D. Dettinger, K. Ide, D. Kondrashov, M. E. Mann, A. W. Robertson, A. Saunders, Y. Tian, F. Varadi, and P. Yiou, 2002: Advanced spectral methods for climatic time series, Rev. Geophys., 40(1), pp. 3.1–3.41, 10.1029/2000GR000092.
3. Simonnet, E., M. Ghil, K. Ide, R. Temam, and S. Wang, 2003a: Low-frequency variability in shallow-water models of the wind-driven ocean circulation. Part I: Steady-state solutions. J. Phys. Oceanogr., 33, 712–728.
4. Simonnet, E., M. Ghil, K. Ide, R. Temam, and S. Wang, 2002b: Low-frequency variability in shallow-water models of the wind-driven ocean circulation. Part II: Time-dependent solutions. J. Phys. Oceanogr., 33, 729–752.
5. Simonnet, E., M. Ghil, and H.A. Dijkstra, 2003: Quasi-homoclinic behavior of the barotropic quasi-geostrophic double-gyre circulation. J. Phys. Oceanogr., submitted.
6. Simonnet, E., T. Tachim-Medjo, R. Temam, Barotropic–baroclinic formulation of the primitive equations of the ocean, Applicable Analysis, to appear.
7. Simonnet, E., T. Tachim-Medjo, R. Temam, in preparation.

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