Ensemble prediction study of the East Australian Current

Abstract

We examine the role of dynamical instabilities and flow dependent, growing errors in an ensemble prediction study of the East Australian Current. In the region of the Tasman front where the East Australian Current separates from the coast, the circulation is characterised by meandering and significant mesoscale eddy variability, and is dominated by nonlinear dynamics, thereby representing a severe challenge for operational forecasting. We explore the mechanisms for and structures of forecast errors over a month using the Australian operational ocean forecast system. To examine the role of dynamical instabilities, forecast ensemble perturbations are generated as bred vectors allowing the identification of those perturbations to a given initial state that grow most rapidly. We consider a period spanning the Austral autumn/summer corresponding to the seasons of maximum eddy variability. We find that the bred vectors span a low dimensional subspace whose structures correspond to regions of large forecast error. Further, these structures typically occur in regions of instability and in particular in the vicinity of the Tasman front and the East Australian Current extension. Our results suggest that even a very few bred vectors will be useful in adaptive sampling and targeted observations. References

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