EFFECTS OF SEA SURFACE DRAG AND STRATIFICATION ON SPREADING PROCESS OF DENSE WATER ALONG CONTINENTAL SLOPE
Kazunori Akitomo*, Kiyoshi Tanaka, Ken'ichi Yamazaki and Toshiyuki Awaji
Kyoto University, Kyoto, Japan
1. INTRODUCTION
As for a spreading process of dense water along continental slope, recent studies ( e.g. Gawarkiewicz and Chapman,1995; Jiang and GarWood,1995 ), using a three-dimensional numerical model with primitive equations, have revealed that eddies due bo baroclinic instability play an important role. Further, Tanaka et al.(1998a) showed that the magnitude of bottom slope is a crucial factor to the activities of such eddies and largely affects the spreading process of dense water along continental slope.
In this study, we have investigated the effects of additional two factors on the process, i.e. ice-cover and stratification, using the same model as in Tanaka et al.(1998a). These factors are common to the polar oceans, especially the cold halocline characterizes the stratification in the Arctic Ocean.
2. MODEL
To facilitate the study, the two factors are introduced in the model experiment, as follows (Fig.1).
That is,
(1) no-slip condition at the sea surface,
and
(2) background stratification below 200-m depth.
Ice-cover has various effects on the ocean flow in a dynamical or a thermodynamical sense. In this study, however, only the drag effect is considered by the simple (rough) condition (l). The strength of stratification is 0.lkgm-3 over the vertical scale of 100 m, which is only a tenth of the cold halocline in the Arc-tic Ocean, 〜 1 kgm-3. This is for the investigation of its basic effect on the process. The other experimental conditions are the same as in the experiment with the bottom slope of 0.0l by Tanaka et al. (1998a).
3. RESULT
efect of no-slip surface
Figure 2 shows the vertical section of density averaged over the y-direction before the onset of instability (day 15) and after it (day 30), compared with the result in the free-slip surface case by Tanaka et al.(1998a). As easily seen, the transport of dense water is more rapid in the no-slip case than in the frees-lip case whether the instability occurs or not. To investigate the reason for this, it should be noted that the transport is done by the bottom Ekman current and by eddies before and after the onset of instability, respectively.
According to Tanaka et al.(1998b), the volume transport in the bottom Ekman layer is determined by the pressure gradient due to the depth-mean flow v in the alongshore direction as well as due to the density deviation. That is, the surface pressure gradient is induced by the depth-mean flow v through the geostrophic adjustment. Thus, the pressure gradient force is downslope when v is negative in the northern hemisphere, and vice versa. Figure 3 shows the vertical section of the alongshore velocity averaged over the y-direction on day 15 in the two cases. The positive region is predominant over the shelf break in the free-slip case (v = 4.8 × 10-2ms-1 at × = 6.6 km), whereas the positive and negative regions equally appears in the no-slip case (v = 0.6 × l0-2ms-1). That is, the depth-mean flow v is generated by the non-linear effect under the asymmetry of the boundary condition between surface and bottom (Tanaka et al., 1998b). As a result, the bottom Ekman transport in the non-slip case is 0.37m3s-1 at × =6.6 km (per unit width in the alongshore direction), which is two times of that, 0.18m3s-1, in the free-slip case.
In the later stage (e.g. day 30), the instability becomes another factor affecting the efficiency of the transport. Figure 4 shows time evolutions of mean and eddy kinetic energies in the two cases. As easily seen, the growth rate of the instability is larger in the no-slip case (1.1 × l0-5s-1) than in the free-slip case (0.77 × l0-5s-1). Unstable eddies becomes of finite amplitude on day 20 in the former whereas on day 25 in the latter. Then, dense water is transported to the offshore end of the model basin in the no-slip case till day 30 while not in the free-slip case (Fig.2).
effect of stratification
Next, we executed the experiment with the same
*Corresponcing to author address, Kazunori Akitomo: Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan; e-mail:akitomo｡?ugi.kyoto-u.ac.jp
