conditions as in the no-slip case except an additional background stratification (Fig.1). Figure 5 shows horizontal velocity fields in the homogeneous and the stratified case at three vertical levels on day 35, till which the instability fully develops. In the homogeneous case, an anticyclonic eddy, appearing at the center of the basin is most intense in the bottom layer with a size of 10 km and a velocity of 0.2 ms-1. Further, a cyclonic eddy with the same size and velocity scales appears in the upper layer, forming a pair with the anticyclonic eddy, which is typical to the baroclinic instability,.
In the stratified case, on the other hand, two anticy.clonic eddies are distinct just above the top of stratification and their size and velocity are 5 km and 0.1 ms-1, respectively, which are half of those in the free-slip case. However, no distinct cyclonic eddy is detected to form a pair with these eddies at any level. This means that they are no longer forced by the instability. The anticyclonic eddies above the stratification have a life time of a few days.
4, SUMMARY
Executing experiments with a three-dimensional model, we can conclude about the effects of no-slip surface condition on the descent of dense water along slope, as follows:
(1) Before the onset of instability, the bottom Ekman transport increases up to two times of the free-slip case because of the decreased upslope pressure gradient force associated with the depth-mean flow in the alongshore direction.
(2) After the onset of instability, changes of stratification and the vertical shear of the mean flow causes the growth rate to be 1.4 times larger than in the free-slip case.
Thus, the downslope transport of dense water is more effective in the no-slip case than in the free slip case. As for the effect of stratification, the following points are concluded.
(3) Eddies formed by the instability are 5 km in size and 0.1 m/s in velocity, which are half of those in the fee-slip case (10 km and 0.2 m/s).
(4) anticyclonic eddies are predominant at the top of stratification while cyclonic eddies are too feasible to form a pair with anticyclonic eddies. This is a contrast to the free-slip case that anticyclonic and cyclonic eddies with an equal strength appear in the surface and the bottom layer, respectively, forming a pair.
REFERENCE
Gawarkiewicz, G. and D. C. Chapman, 1995: A numerical study of dense water formation and transport on a shallow, sloping continental shell J.Geophys.Res., 100, 4489-4507.
Jiang, L. and R. W. Garwood, 1995: A numerical study of three-dimensional dense bottom plumes on a Southern Ocean continental slope, J.Geophys.Res., 100, 18471-18488.
Tanaka, K., K. Akitomo and T. Awaji, 1998a: A numerical study on density current descending along continental slope. Proc. International Workshop on Exchange Processes Between the Arctic Shelves and Basins.
Tanaka, K., K. Akitomo, T. Awaji and N. Imasato, 1998b: Density current descending along continental slope and associated deep water formation: two-dimensional numerical experiments with a nonhydrostatic model. Deep-Sea Res., submitted.
