Nonlinear and dissipative dynamics in the connection region between western boundary currents and equatorial currents
Nonlinear and dissipative dynamics in the connection region between western boundary currents and equatorial currents
复制标题
西边界流与赤道流连接区的非线性和耗散动力学
DOI：
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复制
发表时间：
1993
期刊：
影响因子：
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通讯作者：
M. Kawase
中科院分区：
文献类型：
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作者：
S. Springer;M. Kawase
The roles of nonlinearity and dissipation in connecting a western boundary current to an equatorial current are explored with a 1½ -layer numerical model. A permanent western boundary current - equatorial current system is maintained between a mass source located near the western boundary several deformation radii from the equator and a sponge layer located along the eastern boundary. When a no-slip boundary condition is used, the western boundary current separates within two deformation radii after crossing the equator to feed the eastward equatorial current. Because of equatorial scalings, flow within the connection region between these two currents is nearly nondivergent, and similarities with barotropic models of western boundary current extensions are emphasized by potential vorticity analysis. A series of experiments explores the relative importance of inertia to dissipation, measured by the Reynolds number, by varying the forcing strength. For small Reynolds numbers advection of relative vorticity out of the western boundary layer eastward results in a meandering equatorial current, which is interpreted as a damped, stationary Rossby wave. At higher Reynolds numbers, the boundary current is susceptible to horizontal shear instability, resulting in eddies which translate along the western boundary. At yet higher Reynolds numbers, eddy variability is not confined to the western boundary. Disturbances are advected out of the boundary layer, producing large (300 km diameter) eddies in the connection region. Variability also radiates eastward along the equator as free Yanai wave packets with periods between 30 and 70 days. Even in the presence of energetic eddies, net flow across the equator is due mostly to vorticity dissipation in the western boundary current. The effect of different boundary conditions is also examined. When a free-slip rather than no-slip boundary condition is used, the boundary current overshoots well outside the equatorial waveguide, but all flow returns to the equatorial region in an offshore countercurrent, and the solution is otherwise similar to solutions with a no-slip boundary condition.
