| 概要 |
Kuroshio frontal eddies observed west of Ryukyu are supposed to be generated by baroclinic instability, just as the frontal eddies of the Gulf Stream. The former、eddiesare propagated downstream at a h...alf speed of the latter, though their wavelenghs are almost the same~ 200 km. As regardsthe current and temperature distributions of frontal eddies, Lee et al. already proposed a schematic picture, suggesting the important role of frontal eddies in the meterial transport across the western boundary current. Details of frontal eddies, however, still remain ambiguous. This article is concerned therefore with some basic properties of frontal eddies under the assumption that _Kuroshio frontal eddies are baroclinic unstable waves in the linear regime. For that purpose we first formulate the baroclinic instability in a simplest manner: an inviscid twolayer quasi-geostrophic model with a uniform surface current over a quiet lower layer disturbed by waves independent of the inshore coordinate. In spite of, or, by virtue of, this idealization, our model proves quite useful in clarifying some basic aspects of frontal eddies as baroclinic unstable waves. In particular, it yields the most unstable wave with a wavelength and a propagation speed which agree well with those observed so far and those based on a much more realistic numerical model by Xue et al. Then the current and density fields are compared between the previous scheme of Lee et al. and our model of baroclinic unstable waves. According to Lee et al., offshore bottom water seems to shoal and upwell near the cyclonic center of a frontal eddy. Our model, to the contrary, suggests that inshore surface water downwells (subducts) across the front near the cyclonic center of the baroclinic wave. Attention must be paid for this discrepancy in discussing the role and properties of frontal eddies. The phase of various quantities such as upwelling or inshore displacement are investigated relative to the cold core or the maximum elevation of the interface between the upper and lower layers. Finally a shingle structure (backward breaking) accompanying a frontal eddy is argued to be associated with horizontal shear of the basic current, which is confirmed by a numerical experiment based on a two-layer quasi-geostrophic experiment and supported by a visualization of a flow behind a cylinder that started abruptly.続きを見る
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