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Initialization and Predictability of a Coupled ENSO Forecast Model

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  • 1 Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York
  • | 2 Laboratory for Hydrospheric Processes, NASA/GSFC, Greenbelt, Maryland
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Abstract

The skill of a coupled ocean–atmosphere model in predicting ENSO has recently been improved using a new initialization procedure in which initial conditions are obtained from the coupled model, nudged toward observations of wind stress. The previous procedure involved direct insertion of wind stress observations, ignoring model feedback from ocean to atmosphere. The success of the new scheme is attributed to its explicit consideration of ocean–atmosphere coupling and the associated reduction of “initialization shock” and random noise. The so-called spring predictability barrier is eliminated, suggesting that such a barrier is not intrinsic to the real climate system. Initial attempts to generalize the nudging procedure to include SST were not successful; possible explanations are offered. In all experiments forecast skill is found to be much higher for the 1980s than for the 1970s and 1990s, suggesting decadal variations in predictability.

Corresponding author address: Dr. Dake Chen, Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, RT 9W, Palisades, NY 10964-8000.

Email: dchen@ldeo.columbia.edu

Abstract

The skill of a coupled ocean–atmosphere model in predicting ENSO has recently been improved using a new initialization procedure in which initial conditions are obtained from the coupled model, nudged toward observations of wind stress. The previous procedure involved direct insertion of wind stress observations, ignoring model feedback from ocean to atmosphere. The success of the new scheme is attributed to its explicit consideration of ocean–atmosphere coupling and the associated reduction of “initialization shock” and random noise. The so-called spring predictability barrier is eliminated, suggesting that such a barrier is not intrinsic to the real climate system. Initial attempts to generalize the nudging procedure to include SST were not successful; possible explanations are offered. In all experiments forecast skill is found to be much higher for the 1980s than for the 1970s and 1990s, suggesting decadal variations in predictability.

Corresponding author address: Dr. Dake Chen, Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, RT 9W, Palisades, NY 10964-8000.

Email: dchen@ldeo.columbia.edu

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