A NUMERICAL STUDY ON DENSITY CURRENT DESCENDING ALONG CONTINENTAL SLOPE

　

Kiyoshi Tanaka*, Kazunori Akitomo and Toshiyuki Awaji

Kyoto University, Kyoto

　

1. INTRODUCTION

　

Descent of dense water is an important process for bottom and deep water formation in polar oceans such as the Arctic Sea. The two recent studies (Gawarkiewicz and Chapman,1995; Jiang and Garwood,1995), using a three-dimensional numerical model with primitive equations, have indicated that eddies due to baroclinic instability play an important role in transporting dense water downslope. However, the eddies' activities appear to be somewhat different between the two experiments. In the experiment by Gawarkiewicz and Chapman(1995), the effective downslope transport is realized by the eddies which are detached to move downslope smoothly. On the other hand, in that by Jiang and Garwood(1995), the eddies appear to be trapped near the shelf break and not to move downslope smoothly.

The difference between the two experiments is probably attributed to the magnitude of bottom slope, since it ranges from 0 to 5 × l0^-3 in the former experiment while 1 × l0^-2 in the latter, and implies that the bottom slope is crucial to descending process of dense water. Then, we will investigate the effects of the bottom slope on three-dimensional response of the density current, using a nonhydrostatic numerical model for an extended range of the bottom slope from 0 to 3 × l0^-2.

　

2. MODEL

　

Experiments were executed in a three-dimensional basin consisting of shelf and slope regions (L_x = 29.7km,L_y = 51.2km,H_z = 0.1 〜 0.84km,L_s = 5.0km,H_s = 0.1km) in a rotatig frame ( Fig.1 ). The x and y axes are set in the horizontal plane and the z axis upward. Governing equations are the momentum equation for a Boussinesq fluid under the rigidlid approximation, the continuity equation and the advective-difusive equation for water density, which are given as follows:

3. RESULT

　

Figure 2 shows the distribution of the density deviation along the bottom at the onset time of the eddies after the developments of unstable waves for 2 cases: the gentle slope case with s = 5 × l0^-3 (Fig.2a) and the steep slope case with s=2 × l0^-2 (Fig.2b), where the onset of the eddies is defined as the time that the eddy-induced density flux at the shelf break is equal to the sum of the bottom Ekman and the diffusive fluxes. The onset time is on day 27.5 in the gentle slope case with s = 5 × 10^-3 and on day 18.3 in the steep slope case with s = 2 × 10^-2, respectively. The unstable waves in the steep slope case develops more rapidly than in the gentle slope. Figure 3 shows the

　

*Corresponding to author address. Kiyoshi Tanaka, Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan; e-mail: tanaka@kugi.kyoto-u.ac.jp

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