On the predictability of regional oceanic jet stream: The impact of model errors at the inflow boundary
Abstract:When a jet is simulated in a fine-resolution regional model with inflow and outflow boundaries, its dynamical behavior depends crucially on the specified inflow boundary condition (IBC). Our primary goals are (1) to explore the model's ability to simulate the jet when the IBC is approximately known, (2) to examine the dynamical propagation and growth of the IBC errors in both space and time, and (3) to assess the relative usefulness of different types of data used to specify the IBC.
This paper represents the results of an ensemble of 30 IBC-perturbed experiments derived from the Semi-Spectral Primitive Equation Model (SPEM). It is found that the system is very sensitive to the IBC perturbation, particularly in the velocity component. Evaluation of the evolution of both ensemble-mean error variance and anomaly correlation shows that the system's predictability limits are reached at 64 days for the streamfunction () field, 43 days for the density () field at the surface, and 50 days for the field at the thermocline level. The longer predictability in the field is mainly due to the use of the rigid-lid approximation, which filters out small-scale and hence unpredictable barotropic waves. A much shorter local predictability for all fields occurs in the domain's central area, where the jet is most unstable and where the richest dynamical activity is concentrated.
We also analyze the ensemble mean fields and the ensemble distribution of the jet-axis positions from the 30 IBC-perturbed simulations. The major results are: (1) the ensemble mean field exhibits a better representation of the reference jet than a single simulation, because the averaging procedure smooths out small-scale fluctuations for which there is little simulation skill left; (2) the least number of realizations necessary to constitute a usable ensemble lies between 12 and 20; (3) the distribution of jet-axis positions is Gaussian within 60 days and then becomes non-Gaussian and excursive. In addition, the difference of jet axis positions between the ensemble mean and the reference is not statistically distinguishable until 30 days. However, it deviates with time and becomes statistically significant at 60 days.
Additional experiments are performed to help clarify (1) the difference between the present and traditional predictability known as the sensitivity to initial conditions, (2) the relative impact of velocity and density perturbations on the jet simulation, and (3) the minimum strength of the IBC perturbation that cause the jet system to be unpredictable.
Document Type: Research Article
Publication date: July 1, 1999
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- The Journal of Marine Research publishes peer-reviewed research articles covering a broad array of topics in physical, biological and chemical oceanography. Articles that deal with processes, as well as those that report significant observations, are welcome. In the area of biology, studies involving coupling between ecological and physical processes are preferred over those that report systematics. Authors benefit from thorough reviews of their manuscripts, where an attempt is made to maximize clarity. The time between submission and publication is kept to a minimum; there is no page charge.
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