HR: 0830h
AN: OS41C-0815    [PDF]
TI: Three-Dimensional Structure of the Circulation Induced by a Shoaling Topographic Wave
AU: * Mizuta, G
EM: mizuta@ees.hokudai.ac.jp
AF: Division of Ocean and Atmospheric Science,School of Environmental Earth Science, Hokkaido University,, Kita-10, Nishi-5, Sapporo, 060-0810 Japan
AU: Hogg, N G
AF: Physical Oceanography Department, Wood Hole Oceanographic Instutution, MS #21, 360 Woods Hole Road, Woods Hole, MA 02543 United States
AB: Rectification of Rossby wave energy has been proposed as a mechanism for the maintenance of the recirculation cell of the Gulf Stream (Hogg 1988; Rizzoli et al. 1995). We investigated the three-dimensional structure of potential-vorticity flux and a mean flow induced by a topographic wave incident over a bottom slope analytically and numerically, focusing on the limit that bottom friction is the dominant dissipation process. In this limit it is shown that the topographic wave cannot be a steady source of the potential vorticity outside the bottom Ekman layer. Instead, the distribution of potential vorticity is determined from the initial transient of the topographic wave. This potential vorticity and the heat flux by the topographic wave at the bottom determine the mean flow, and give a relation between the horizontal and vertical scales of the mean flow. When the horizontal scale of the mean flow is larger than the internal deformation radius, the mean flow is almost constant with depth independent of whether or not the topographic wave is trapped near the bottom. Then the mean flow at the bottom is proportional to the divergence of vertically integrated Reynolds stress $\int_{-D}^0 \overline{u'v'}\, dz$. This divergence, which is caused by bottom friction, is large when the group velocity, $c_g$ and the vertical scale, $\mu^{-1}$ of the wave motion are small. Thus the mean flow tends to be large where $c_g$ and $\mu^{-1}$ become small, and decreases as the topographic wave is dissipated by bottom friction. Since bottom friction also dissipates the mean flow, the mean flow asymptotes to a constant value as the friction becomes zero. These features of the potential-vorticity flux and the mean flow are reproduced in numerical experiments. It is also shown from the numerical experiment that the distribution of the mean flow depends on the amplitude of the wave because of the Doppler shift of the wave by the mean flow. These feature of the mean flow are preserved when we used stratification and bottom topography resembling to those over the continental slope near the Gulf Stream. The transport of the mean flow is about 20 Sv when the wave amplitude is about 2 cm/s. These numbers are similiar to those observed in the Gulf Stream region.
DE: 4512 Currents
DE: 4520 Eddies and mesoscale processes
DE: 4532 General circulation
DE: 4576 Western boundary currents
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting