A long-term hindcast of the Kuroshio using a high resolution GCM
Abstract
The Kuroshio, the western boundary current of the North Pacific subtropical gyre, is well known to take two typical paths south of Japan: Large Meander (LM) and non-Large Meander (NLM). Recent advances in computing resource and numerical modeling technology allow the local dynamics of this path variation to be studied using high resolution numerical models. However, it is still considered extremely difficult to reproduce the long-term variability in a hindcast simulation forced by a time history of surface atmospheric state and thus to study the cause of the long-term variability. There are several facts causing this situation. Path variations involve highly stochastic phenomena such as mesoscale eddy and baroclinic instability. High resolution numerical models frequently exhibit unrealistic path of the Kuroshio south of Japan such as the separation from the coast at the Tokara Strait. Difficulties seem to be related to the fact that the simulated Kuroshio is too strong and that the surface eddy kinetic energy level is too high along the Kuroshio and its extension. Some modelers impose an additional horizontal viscosity along the southern coast of Japan without solid physical basis, which has nonetheless been successful to some extent. On the other hand, recent works suggest that the wind vector relative to the surface oceanic current instead of the absolute wind vector should be used to calculate the surface wind stress. By using this method, we may expect a modest Kuroshio and a reduced level of eddy kinetic energy. Incorporating this method of calculating wind stresses as well as various recent improvements in numerical ocean modeling techniques for a set of nested numerical ocean models, a long-term hindcast simulation is performed for the western North Pacific region from 1948 to 2007 using CORE-IAF-v2 without applying an ad hoc tuning for the horizontal viscosity. The modeled Kuroshio tends to take the NLM path, which seems to be due to the reduced level of eddy kinetic energy. But a notable feature of the long-term variability of the Kuroshio path south of Japan, LM in early 1960s and mid to late 1970s, is reproduced. This fact might indicate that the long-term behavior of the Kuroshio is forced by the surface wind field variability. Some analyses to confirm this hypothesis will be presented.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMOS21B1496T
- Keywords:
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- 4255 OCEANOGRAPHY: GENERAL / Numerical modeling