Geophysical Fluid
Dynamics Laboratory

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CHiMES: Decadal predictability studies


Weather — that is, the day-to-day fluctuations of the local atmospheric state — is largely unpredictable beyond a week or two: i.e, a small perturbation of the initial conditions will over that period lead to two solutions that are essentially uncorrelated. However, the presence of persistent long-term circulation anomalies in the oceanic state suggests that the climate — the year-to-year fluctuations of the global climate state superimposed on the secular trend - is predictable at timescales of decades or longer.

The longevity of these circulation anomalies is dependent on dissipation by small-scale eddies. A key result that may be obtained at the resolutions proposed for this project is an answer to the "decadal predictability" conundrum, which is principally driven by the ocean state: are there modes of variability of the coupled ocean-atmosphere system that are predictable on timescales of a decade or more; and to what extent is this dependent on ocean resolution? We propose long-term simulations of the climate under constant external forcing, with a very high-resolution ocean capable of permitting direct simulation of the time and space scales associated with ocean eddies.

Model configuration

The model used for these studies - CM2.4 - is based on the same model codes as GFDL's flagship model CM2.1 used for the IPCC AR4 simulations. However, the resolution of the atmospheric model has been increased over CM2.1 by a factor of 2, to 288x180, and the ocean model grid has become finer by a factor of 4 to 10 (the model resolution varies in latitude, and use of a square projection further enhances resolution at high latitudes).

Current status

Several years of a control run of CM2.4 have been completed. The results are being analyzed and validated against similar runs undertaken at GFDL (to ascertain if differences at the bit level between runs on the different platforms are acceptably small). Preliminary results are very encouraging. The Visualization Group at LBNL has prepared several animations from these runs which illustrate the tremendous potential of these runs: many thanks to Prabhat of LBL for his role in preparing these visualizations. For instance, see the beautiful "Agulhas rings": vortices shed by flow around the Cape.

CM2.4 Sea Surface Temperature dataset for Agulhas current.

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last modified: 18 January 2010