Boundary Mixing Processes by Internal Waves in the North Pacific
Toshiyuki Awaji, Tomohiro Nakamura, Takaki Hatayama, and Kazunori Akitomo
(Department of Geophysics, Kyoto University, Kyoto 606-8502, Japan)
Takatoshi Takizawa
(Japan Marine Science and Technology Center, Yokosuka 237, Japan)
e-mail: awaji@kugi.kyoto-u.ac.jp
ABSTRACT
Numerical experiments with a 2-dirnensional nonhydrostatic model are performed to investigate the vertical mixing induced by tidally generated internal waves in the Kuril Straits. The results show that sill-scale internal waves at the K1 tidal frequency are confined to the sill slopes because the K1 tide is subinertial in the Kuril Straits. On the other hand, unlike in previous theories, intense short internal waves generated at the sill breaks by the subinertial K1 tidal current can propagate upstream as the tidal current slackens. Our theoretical consideration identifies these short waves as unsteady lee waves. The unsteady lee waves tend to be trapped at the generation region to grow into large-amplitude waves, eventually inducing vigorous mixing along their ray paths. In particular, superposition of a propagating unsteady lee wave and a newly generated lee wave over a sill causes significant wave breaking leading to a maximum vertical diffusivity of 〜 103cm2s-1. This quite intense mixing reaches down to the density layer of the North Pacific Intermediate Water (NPIW). In contrast, the M2 tidal current does not cause such strong vertical mixing, because most of generated internal waves propagate away as 1st-mode internal tides and because the barotropic flow amplitude is small. Thus, a major cause for the observed modification of the Okhotsk Sea water required for the formation of the NPIW is the generation of lee waves by interactions between the K1 current and the topography in the Kuril Straits.
1. Introduction
Since the density range of the NPIW (〜 26.8 δθ) does not outcrop in the open North Pacific Ocean, a driving mechanism is required for downward motion of fresh subarctic surface water, which in turn may induce the effective subduction of other materials such as carbon dioxide (CO2). Since the NPIW is distributed in the entire subtropical gyre and even extends to the tropics (Talley 1993), the formation mechanism of the NPIW has important implications for the dynamics of the mid-depth circulation in the North Pacific and the sources and paths through which anomalies and materials in the surface layer are conveyed into the intermediate layer.
