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- Author or Editor: Hisashi Hukuda x
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Abstract
The two-layer baroclinic instability model of the California undercurrent (CU) off Vancouver Island due to Mysak (1977) is modified to include the effects of nonlinearity. As a simple model to examine nonlinear effects, nonlinear interaction between two unstable baroclinic waves of frequencies ω1 = k 1 c 1, ω2 = k 2 c 2 (k 1 = 2k 2) is studied. The assumptions that c 1 − c 2, Im(c 1), Im(c 2) = O(ε) (ε ≪ 1) lead to the amplitude equations describing the resonant interaction of two modes within a limited time scale.
A linear dispersion tendency of these waves is weak in the parameter range appropriate to the CU regime. However, the coupling of the two modes vanishes if the mean potential vorticity gradient is constant. This difficulty can be removed, however, if the horizontal shear of the current and/or the curvature of the bottom slope are introduced. Here the coupling due to nonlinearity is modeled by invoking the latter effect. For the CU model off Vancouver Island, we found strong subharmonic instability owes its existence to horizontal Reynolds stress induced into the linearly unstable mode by the presence of bottom slope curvature. The instability has such a structure that the cross-stream mode function has a phase shift of π across the center of the channel. The present theory can explain well the discrepancy noted by Emery and Mysak (1980) between the waves observed in satellite images and those predicted by a linear theory.
Abstract
The two-layer baroclinic instability model of the California undercurrent (CU) off Vancouver Island due to Mysak (1977) is modified to include the effects of nonlinearity. As a simple model to examine nonlinear effects, nonlinear interaction between two unstable baroclinic waves of frequencies ω1 = k 1 c 1, ω2 = k 2 c 2 (k 1 = 2k 2) is studied. The assumptions that c 1 − c 2, Im(c 1), Im(c 2) = O(ε) (ε ≪ 1) lead to the amplitude equations describing the resonant interaction of two modes within a limited time scale.
A linear dispersion tendency of these waves is weak in the parameter range appropriate to the CU regime. However, the coupling of the two modes vanishes if the mean potential vorticity gradient is constant. This difficulty can be removed, however, if the horizontal shear of the current and/or the curvature of the bottom slope are introduced. Here the coupling due to nonlinearity is modeled by invoking the latter effect. For the CU model off Vancouver Island, we found strong subharmonic instability owes its existence to horizontal Reynolds stress induced into the linearly unstable mode by the presence of bottom slope curvature. The instability has such a structure that the cross-stream mode function has a phase shift of π across the center of the channel. The present theory can explain well the discrepancy noted by Emery and Mysak (1980) between the waves observed in satellite images and those predicted by a linear theory.
Abstract
A triply nested ocean general circulation model was used to examine how the model horizontal resolution influences the Kuroshio in the East China Sea (ECS) and the sea level variability. As the model resolution increases from 1/2° to 1/18° the path, current intensity, and vertical structure of the model Kuroshio and the variability of sea level become closer to observations. In general, the higher-resolution model improves the baroclinic as well as barotropic component of the Kuroshio and thus reproduces more realistic density and current fields. This improvement, in addition to better representation of topography, results in better reproduction of the interaction between baroclinicity and bottom topography, that is, JEBAR (joint effect of baroclinicity and bottom relief) in a high-resolution model. Modeling the Kuroshio in the ECS provides an ideal example of such improvement. In particular, the Kuroshio veering phenomenon at (30°N, 129°E) southwest of Kyushu is discussed, together with the seasonal meridional migration of the path. It is shown that JEBAR and advection of the geostrophic potential vorticity are two major contributions to the vorticity balance in this area. The summer intensification of JEBAR resulting from the intensified stratification yields a strong offshore volume transport across the shelf break, thereby leading to the southward shift of the veering latitude. In winter, the weakened JEBAR, combined with the increased wind stress curl, decreases the offshore volume transport in a considerable amount, explaining the northward shift of the veering latitude. The present reproduction of seasonal migration of the Kuroshio axis at 129°E is in good agreement with observation.
Abstract
A triply nested ocean general circulation model was used to examine how the model horizontal resolution influences the Kuroshio in the East China Sea (ECS) and the sea level variability. As the model resolution increases from 1/2° to 1/18° the path, current intensity, and vertical structure of the model Kuroshio and the variability of sea level become closer to observations. In general, the higher-resolution model improves the baroclinic as well as barotropic component of the Kuroshio and thus reproduces more realistic density and current fields. This improvement, in addition to better representation of topography, results in better reproduction of the interaction between baroclinicity and bottom topography, that is, JEBAR (joint effect of baroclinicity and bottom relief) in a high-resolution model. Modeling the Kuroshio in the ECS provides an ideal example of such improvement. In particular, the Kuroshio veering phenomenon at (30°N, 129°E) southwest of Kyushu is discussed, together with the seasonal meridional migration of the path. It is shown that JEBAR and advection of the geostrophic potential vorticity are two major contributions to the vorticity balance in this area. The summer intensification of JEBAR resulting from the intensified stratification yields a strong offshore volume transport across the shelf break, thereby leading to the southward shift of the veering latitude. In winter, the weakened JEBAR, combined with the increased wind stress curl, decreases the offshore volume transport in a considerable amount, explaining the northward shift of the veering latitude. The present reproduction of seasonal migration of the Kuroshio axis at 129°E is in good agreement with observation.
